读书摘要：C++ Standard Library, The: A Tutorial and Reference

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**C++ Standard Library, The: A Tutorial and Reference**

**Chap 2 Introduction to C++ and the Standard Library**

**2.1 history**  
  
string classes are designed as a safe and convenient component. Thus, they provide an almost self-explanatory interface and check for many errors in the interface.  
  
The STL was designed to combine different data structures with different algorithms while achieving the best performance. Thus, the STL is not very convenient and it is not required to check for many logical errors. To benefit from the powerful framework and great performance of the STL, you must know the concepts and apply them carefully.  
  
A template is not compiled once to generate code usable for any type; instead, it is compiled for each type or combination of types for which it is used. This leads to an important problem in the handling of templates in practice: You must have the implementation of a template function available when you call it, so that you can compile the function for your specific type. Therefore, the only portable way of using templates at the moment is to implement them in header files by using inline functions.  
  
  
**2.2 New Language Features**  
  
  
**2.2.1 Templates**  
  
  
 **Non-type Template Parameters**  
  
In addition to type parameters, it is also possible to use nontype parameters. A nontype parameter is then considered as part of the type.  
  
 **Default Template Parameters**  
  
Templates classes may have default arguments  
  
 **Keyword Typename**  
  
Note that typename is always necessary to qualify an identifier of a template as being a type, even if an interpretation that is not a type would make no sense. Thus, the general rule in C++ is that any identifier of a template is considered to be a value, except it is qualified by typename.  
  
 **Member Templates**  
  
Member functions of classes may be templates. However, member templates may not be virtual, nor may they have default parameters.  
  
This feature is often used to support automatic type conversions for members in template classes. By providing a different template type for the member function, you relax the rule of exact match.  
  
A special form of a member template is a template constructor. Template constructors are usually provided to enable implicit type conversions when objects are copied. Note that a template constructor does not hide the implicit copy constructor. If the type matches exactly, the implicit copy constructor is generated and called.  
  
 **Nested Template Classes**  
  
  
**2.2.2 Explicit Initialization for Fundamental Types**  
  
If you use the syntax of an explicit constructor call without arguments, fundamental types are initialized with zero  
  
  
**2.2.3 Exception Handling**  
  
  
**2.2.4 Namespace**  
  
Unlike classes, namespaces are open for definitions and extensions in different modules. Thus you can use namespaces to define modules, libraries, or components even by using multiple files.  
  
A namespace defines logical modules instead of physical modules.  
  
using directives in header files is really bad design.  
  
**2.2.5 Type bool**  
  
  
**2.2.6 Keyword explicit**  
  
Note that explicit also rules out the initialization with type conversion by using the assignment syntax:  
  
  
 Stack s1(40); // OK  
 Stack s2 = 40; // **ERROR**  
  
This is because there is a minor difference between  
  
 X x;  
 Y y(x); // explicit conversion  
  
and  
  
 X x;  
 Y y = x; // implicit conversion  
  
The former creates a new object of type Y by using an explicit conversion from type X, whereas the latter creates a new object of type Y by using an implicit conversion.  
  
**2.2.8 Initialization of Constant Static Members**  
  
It is now possible to initialize integral constant static members inside the class structure. This is useful when the constant is used in the class structure after the initialization. For example:  
  
  
 class MyClass \{   
 static const int num = 100;  
 int elems\[num\];  
 ...  
 \};  
  
Note that you still have to to define space for a constant static member that is initialized within a class definition:  
  
 const int MyClass::num; // no initialization here

**Chap 3 General Concept**

**3.3 Error and Exception Handling**  
  
These standard exception classes can be divided into three groups:  
  
1. Exceptions for language support:  
  
 **bad\_alloc** (new)  
**bad\_cast**  (dynamic\_cast)  
 **bad\_typeid** (typeid)  
**bad\_exception** (unexpected())  
  
2. Exceptions for the C++ standard library:usually derived from class logic\_error  
  
**invalid\_argument  
length\_error  
out\_of\_range  
domain\_error  
ios\_base::failure**  
  
3. Exceptions for errors outside the scope of a program:derived from runtime\_error  
  
 **range\_error**  
 **overflow\_error**  
 **underflow\_error**  
  
  
**3.4 Allocators**  
  
An allocator represents a special memory model. It is used as abstraction to translate the need to use memory into a raw call for memory.  
  
It serves as a base for technical solutions that use certain memory models, such as shared memory, garbage collection, and object-oriented databases, without changing the interfaces.  
  
**default allocator:** Only occasionally does it make sense to program customized allocators.

**Chapter 4 Utilities**

**4.1 class pair**  
  
the container classes map and multimap use pairs to manage their elements  
  
**make\_pair()**  
  
makes it convenient to pass two values of a pair directly to a function that requires a pair as its argument  
  
  
**4.2 auto\_ptr**  
  
Note that class auto\_ptr<> does not allow you to initialize an object with an ordinary pointer by using the assignment syntax. Thus, you must initialize the auto\_ptr directly by using its value :  
  
 std::auto\_ptr<ClassA> ptr1(new ClassA); //OK  
 std::auto\_ptr<ClassA> ptr2 = new ClassA; **//****ERROR**  
  
The transfer of ownership implies a special use for auto\_ptrs\----that is, functions can use them to transfer ownership to other functions.  
  
constant auto\_ptrs reduce the danger of an unintended transfer of ownership.The const does not mean that you can't change the value of the object the auto\_ptr owns (if any). You can't change the ownership of a constant auto\_ptr; however, you can change the value of the object to which it refers.  
  
auto\_ptrs are not provided for arrays!  
  
auto\_ptrs don't meet the requirements for container elements.Fortunately, the design of the language and library prevents this misuse from compiling in a standard-conforming environment.  
  
we needs to find a mechanism to enable an rvalue to be converted to an lvalue.Thus, the auto\_ptr\_ref class was introduced to provide this convert-to-lvalue mechanism.  
  
**4.3 numeric\_limits<>**  
  
numeric\_limits is a typical example of specialization of a general template  
  
The specializations are provided for any fundamental type that can represent numeric values  
  
**4.4 Auxiliary Functions:min, max, swap**  
  
min() and max() require that both types match.  
  
comparison criterion as an additional argument.  
  
The big advantage of using swap()is that it enables to provide special implementations for more complex types by using template specialization or function overloading.These special implementations might save time by swapping internal members rather than by assigning the objects. This is the case, for example, for all standard containers and strings .  
  
**4.5 Supplementary Comparison Operators**  
  
Four template functions define the comparison operators !=, >, <=, and >= by calling the operators == and <.  
  
they are defined in a subnamespace of std, i.e, std::rel\_ops.  
  
**4.6 Header Files <cstddef> and <cstdlib>**  
  
In C++,NULL is simply value 0.Note that in C, NULL often is defined as (void\*)0. This is incorrect in C++ because there the type of NULL must be an integer type. Otherwise, you could not assign NULL to a pointer. This is because in C++ there is no automatic conversion from void\* to any other pointer type.

**Chapter 5 STL**

The STL's flexibility, however, has a price, chief of which is that it is not self-explanatory.  
  
**5.1 STL Components**  
  
The concept of the STL is based on a separation of data and operations.The data is managed by container classes, and the operations are defined by configurable algorithms. Iterators are the glue between these two components. They let any algorithm interact with any container.  
  
**5.1 Components**  
  
 In a way, the STL concept contradicts the original idea of object-oriented programming: The STL separates data and algorithms rather than combining them.  
  
  
**5.2 Containers**  
  
**Sequence containers**: element's position depends on the time and place of the insertion, but it is independent of the value of the element.  
  
**Associative containers**:actual position of an element depends on its value due to a certain sorting criterion.  
  
The key advantage of automatic sorting is better performance when you search elements. In particular, you can always use a **binary search**, which results in logarithmic complexity rather than linear complexity.  
  
Strictly speaking, the particular implementation of any container is not defined inside the C++ standard library. However, the behavior and complexity specified by the standard do not leave much room for variation. So, in practice, the implementations differ only in minor details.  
  
Usually, the STL containers provide only those special member functions that in general have "good" timing.This prevents a programmer from calling a function that might cause bad performance.  
  
Note that list.pop\_front() does not return the element it removed.  
  
By default,  associative containers  compare the elements or the keys with operator <. However, you can supply your own comparison function to define another ordering criterion.  
  
Associative containers are typically implemented as Red-Black trees.  
  
All of these associative container classes have an optional template argument for the sorting criterion. The default sorting criterion is the operator <. The sorting criterion is also used as the test for equality; that is, two elements are equal if neither is less than the other.  
  
**Container Adapters**: Stack,Queue,Priority Queue.  
  
  
**5.3 Iterators**  
  
fundamental operations of an iterator:  
  
 Operator \*  
 Operator ++ and --  
 Operator == and !=  
 Operator =  
  
In fact, each container class defines its iterator type as a nested class.As a result, iterators share the same interface but have different types.Because the iterator is defined by the container, it surely does the right thing.  
  
begin() and end() define a half-open range.It has 2 advantage:  
  
1.You have a simple end criterion for loops that iterate over the elements: They simply continue as long as end() is not reached.  
  
2. It avoids special handling for empty ranges. For empty ranges, begin() is equal to end().  
  
  
Every container defines two iterator types:  
  
 container:: iterator  
 container:: const\_iterator  
  
You can't use the subscript operator(\[\]) for multimaps.  
  
Random access iterator provide operators for iterator arithmetic(in accordance with "pointer arithmetic" of an ordinary pointer).The iterators of the container classes vector and deque, and iterators of strings are random access iterators.  
  
  
**5.4 Algorithms**  
  
 Algorithms are not member functions of the container classes. Instead, they are global functions that operate with iterators.Instead of each algorithm being implemented for each container type, all are implemented only once for any container type.  
  
Note that this is not an object-oriented programming paradigm; it is a generic functional programming paradigm.  
  
Ensuring the ranges are valid is not always as easy as it sounds.If this is not the case, the behavior is undefined  
  
Every algorithm processes half-open ranges.  
  
Multiple Ranges：Several algorithms process more than one range. In this case you usually must define both the beginning and the end only for the first range. For all other ranges you need to pass only their beginnings.  
  
  
**5.5 Iterator Adapters**  
  
 Iterators are pure abstractions: Anything that behaves like an iterator is an iterator.  
  
**5.5.1 Insert Iterators**  
  
Insert iterators, or inserters,are used to let algorithms operate in insert mode rather than in overwrite mode.In particular, they solve the problem of algorithms that write to a destination that does not have enough room: they let the destination grow accordingly.  
  
As for Inserter Iterator,A call to step forward is a "no-op"  
  
**Three predefined insert iterators:**  
  
Back inserters:insert the elements at the back of their container (appends them) by calling push\_back()  
  
Front inserters: insert the elements at the front of their container by calling push\_front()  
  
General inserters:inserts elements directly in front of the position that is passed as the second argument of its initialization. It calls the insert() member function with the new value and the new position as arguments.For associative containers, the position is taken as a hint to start the search for the correct position. If the position is not correct, however, the timing may be worse than if there was no hint  
  
**5.5.2 Stream Iterators**  
  
Stream iterators are iterators that read from and write to a stream.  
  
**5.5.3 Reverse Iterators**  
  
The advantage of using reverse iterators is that all algorithms are able to operate in the opposite direction without special code.  
  
**5.6 Manipulating Algorithms**  
  
**5.6.1 "Removing" Elements**  
  
**The price of the flexibility of the STL:**  
  
In general, iterators do not know their containers(knowing its type ,though). Thus, the algorithms, which use the iterators to access the elements of the container, can't call any member function of the container.  
  
**5.6.2 Manipulating Algorithms and Associative Containers**  
  
Manipulation algorithms can't use associative containers as a destination;it will results in a failure at compile time  
  
**5.6.3 Algorithms versus Member Functions**  
  
Even if you are able to use an algorithm, it might be a bad idea to do so. A container might have member functions that provide much better performance.  
  
**5.8 Functions as Algorithm Arguments**  
  
**5.8.2 Predicates**  
  
Predicates are functions that return a Boolean value.The STL requires that predicates always yield the same result for the same parameters.  
  
**5.9 Function Objects**  
  
There are rarely things that are not possible in C++!  
  
It is true that functors do complicate code. However, function objects are more than functions, and they have some advantages:  
  
1. Function objects are "smart functions."  
2. Each function object has its own type  
3. Function objects are usually faster than ordinary functions.  
  
 **5.9.2 Predefined Function Objects**  
  
A typical example is a function object used as a sorting criterion.  
  
What is interesting is that all these function objects are usually declared inline. Thus, you use a function-like notation or abstraction, but you get good performance.  
  
  
**5.10 Container Elements**  
  
Elements of containers must meet certain requirements because containers handle them in a special way.  
  
**5.10.1 Requirements for Container Elements**  
  
1.An element must be copyable by a copy constructor. The generated copy should be equivalent to the source.  
  
2.An element must be assignable by the assignment operator.  
  
3.An element must be destroyable by a destructor  
  
4.For some member functions of sequence containers, the default constructor must be available.  
  
5.For several operations, the test of equality with operator == must be defined.  
  
6.**For associative containers** the operations of the sorting criterion must be provided by the elements. By default, this is the operator <, which is called by the less<> function object.  
  
  
**5.10.2 Value Semantics or Reference Semantics**  
  
The approach of the STL, only to support value semantics, has strengths and weaknesses  
  
  
**5.11 Errors and Exceptions Inside the STL**  
  
  
**5.11.1 Error Handling**  
  
The design goal of the STL was the best performance rather than the most security.Error checking wastes time, so almost none is done.  
  
The C++ standard library states that any use of the STL that violates preconditions results in undefined behavior  
  
**5.11.2 Exception Handling**  
  
The STL almost never checks for logical errors. Therefore, almost no exceptions are generated by the STL itself due to a logical problem.In fact, there is only one function call for which the standard requires that it might cause an exception directly: the at() member function for vectors and deques.  
  
The C++ standard library provides the following basic guarantee for exception safety: will not leak resources or violate container invariants in the face of exceptions.  
  
For all node-based containers (lists, sets, multisets, maps and multimaps), any failure to construct a node simply leaves the container as it was.  
  
All array-based containers (vectors and deques) do not fully recover when an element gets inserted.

**Chapter 6. STL Containers**

**6.1 Common Container Abilities and Operations**  
  
Container's constructors are member templates  
  
Assignments VS swap()：swap() offers much better efficiency  
  
  
**6.2 Vectors**  
  
**6.2.1 Abilities of Vectors**  
  
vector's iterators are random access iterators, so you can use any algorithm of the STL  
  
The capacity of a vector is important for two reasons:  
  
1. Reallocation invalidates all references, pointers, and iterators for elements of the vector.  
  
2. Reallocation takes time.  
  
 std::vector<int> v; // create an empty vector  
 v.reserve (80); // reserve memory for 80 elements  
  
 std::vector<T> v(5); // creates a vector and initializes it with five values  
 // (calls five times the default constructor of type T)  
  
 It is not possible to call reserve() for vectors to shrink the capacity.Calling reserve() with an argument that is less than the current capacity is a no-op.  
  
Only at() performs range checking  
  
**6.2.3 Using Vectors as Ordinary Arrays**  
  
You can expect that for any valid index i in vector v, the following yields true:  
  
 &v\[i\] == &v\[0\] + i  
  
It simply means that you can use a vector in all cases in which you could use a dynamic array.  
  
For Boolean elements of a vector, the C++ standard library provides a specialization of vector：vector<bool>.  
  
Better not use vector<bool>!  
  
**6.3 Deques**  
  
It manages its elements with a dynamic array, provides random access, and has almost the same interface as a vector.  
  
To provide this ability, the deque is implemented typically as a bunch of individual blocks, with the first block growing in one direction and the last block growing in the opposite direction.  
  
**6.4 Lists**  
  
Lists provide many special member functions for moving elements. These member functions are faster versions of general algorithms that have the same names. They are faster because they only redirect pointers rather than copy and move the values.  
  
List only provide bidirectional iterators.  
  
For sorting the elements, lists provide the special member function sort().  
  
  
**6.5 Sets and Multisets**  
  
If a special sorting criterion is not passed, the default criterion less<> is used.The function object less sorts the elements by comparing them with operator <.  
  
The sorting criterion must define "**strict weak ordering**." Strict weak ordering is defined by the following three properties:  
  
1. It has to be **antisymmetric**.(非对称)  
2. It has to be **transitive**.(传递性)  
3. It has to be **irreflexive**.(非自反)  
  
Like all standardized associative container classes, sets and multisets are usually implemented as balanced binary trees . The standard does not specify this, but it follows from the complexity of set and multiset operations.In fact, sets and multisets are typically implemented as "red-black trees."  
  
Automatic sorting imposes an important constraint on sets and multisets: You may not change the value of an element directly because this might compromise the correct order.  
  
The interface reflects this behavior:  
  
\* Sets and multisets don't provide operations for direct element access.  
  
\* Indirect access via iterators has the constraint that, from the iterator's point of view, the element value is constant.  
  
  
You can define the sorting criterion in two ways:  
  
1.As a **template parameter**  
  
std::set<int,std::greater<int> > coll;  
  
This is the usual way to specify the sorting criterion.In this case, the sorting criterion is part of the type.  
  
2.As a constructor parameter  
  
In this case, you might have a type with several sorting criteria.  
  
Sets and multisets are optimized for fast searching of elements, so they provide special search functions.  
  
  
 **Assignments**  
  
For these operations both containers must have the same type. In particular, the type of the comparison criteria must be the same, although the comparison criteria themselves may be different.  
  
 **Inserting and removing**  
  
\* Sets provide the following interface:  
  
 pair<iterator,bool> insert(const value\_type& elem);  
 iterator insert(iterator pos\_hint,const value\_type& elem);  
  
\* Multisets provide the following interface:  
  
 iterator insert(const value\_type& elem);  
 iterator insert(iterator pos\_hint, const value\_type& elem);  
  
  
**6.6 Maps and Multimaps**  
  
The elements of a map or multimap may have any types Key and T that meet the following two requirements:  
  
1. The key/value pair must be assignable and copyable.  
  
2. The key must be comparable with the sorting criterion.  
  
In fact, sets, multisets, maps, and multimaps typically use the same internal data type.  
  
maps can be used as associative arrays.  
  
To check whether a map container is less than another map container is done by a lexicographical comparison.  
  
More important is the constraint that the key of all elements inside a map and a multimap is considered to be constant.So,you can't call any modifying algorithm if the destination is a map or multimap.  
  
  
**6.6.3 Using Maps as Associative Arrays**  
  
Associative containers don't typically provide abilities for direct element access. Instead, you must use iterators. For maps, however, there is an exception to this rule.  
  
This behavior of an associative array has both advantages and disadvantages:  
  
The advantage is that you can insert new elements into a map with a more convenient interface.  
  
The disadvantage is that you might insert new elements by accident or mistake.  
  
multimap is the typical container for dictionaries.  
  
**6.7 Other STL Containers**  
  
There are three different approaches to making containers "STL-able"  
  
invasive approach: provide the interface that ah STL container requires  
  
noninvasive approach: provide special iterators  
  
wrapper approach: combining the two previous approaches  
  
  
The string classes of the C++ standard library are an example of the invasive approach of writing STL containers  
  
string can be considered containers of characters.  
  
  
**6.8 Implementing Reference Semantics**  
  
STL container classes provide value semantics but not reference semantics.if you want reference semantics in STL containers,you should use a smart pointer class that avoids possible errors.

**Chapter 7. STL Iterators**

**7.2 Iterator Categories**  
  
sorting algorithms require iterators that can perform random access because otherwise the runtime would be poor  
  
Two input iterators are equal if they occupy the same position. However, as stated previously, this does not mean that they return the same value on element access.  
  
Output iterators are the counterparts of input iterators.You can't check whether an output iterator is valid or whether a "writing" was successful.  
  
Note: input iterators are comparable while ouptut iterator are not  
  
Another typical example of output iterators are inserters. Inserters are iterators that insert values into containers  
  
**Forward iterators** are combinations of input and output iterators.Unlike input iterators and output iterators, forward iterators can refer to the same element in the same collection and process the same element more than once.  
  
**Bidirectional iterators** are forward iterators that provide the additional ability to iterate backward over the  elements.  
  
**Random access iterators** are bidirectional iterators that are bidirectional iterators that support "iterator arithmetic". That is ,hey can add and subtract offsets, process differences, and compare iterators with relational operators such as < and >  
  
**7.2.6 The Increment and Decrement Problem of Vector Iterators**  
  
In general, you can increment and decrement temporary iterators. However, for vectors and strings, you typically can't**.**  
  
The reason for this strange problem lies in the fact that vector iterators are typically implemented as ordinary pointers**.**  
  
**7.3 Auxiliary Iterator Functions**  
  
Three auxiliary functions for iterators: **advance()****,** **distance()****,** **and iter\_swap()**  
  
void advance (InputIterator& pos, Dist n)  
  
Note that advance() does not check whether it crosses the end() of a sequence (it can't check because iterators in general do not know the containers on which they operate). Thus, calling this function might result in undefined behavior because calling operator ++ for the end of a sequence is not defined.  
  
Dist distance (InputIterator pos1, InputIterator pos2)  
  
Dist's type : iterator\_traits<InputIterator>::difference\_type  
  
The first iterator must refer to an element that is not after the element of the second iterator. Otherwise, the behavior is undefined.  
  
distance() has bad performance for other than random access iterators.  
  
 void iter\_swap (ForwardIterator1 pos1, ForwardIterator2 pos2)  
  
The iterators don't need to have the same type. However, the values must be assignable.  
  
  
**7.4 Iterator Adapters**  
  
**7.4.1 Reverse Iterators**  
  
Reverse iterators are adapters that redefine increment and decrement operators so that they behave in reverse  
  
You can convert normal iterators to reverse iterators. Naturally, the iterators must be bidirectional iterators, but note that the logical position of an iterator is moved during the conversion .This is not a bug; it's a feature!  
  
**logical position (the value returned )** VS  **physical position (the element pointed to )**  
  
Within a conversion from an iterator to a reverse iterator,physical position stays the same**.**  
  
You can convert reverse iterators back to normal iterators using the base() member function provided by reverse iterators.  
  
Again, physical position (the element the iterator pointer to) is retained, but the logical position (the value the iterator returns ) is changed.  
  
  
  
**7.4.2 Insert Iterators**  
  
The C++ standard library provides three kinds of insert iterators: back inserters, front inserters, and general inserters.Each uses a different member function, which it calls for the container to which it belongs  
  
  
**7.4.3 Stream Iterators**  
  
The implementation of an ostream iterator uses the same concept as the implementation of insert iterators .The only difference is that they transform the assignment of a new value into an output operation by using operator >>.  
  
Istream iterators are the counterparts of ostream iterators.However, istream iterators are a bit more complicated than ostream iterators (as usual, reading is more complicated than writing).  
  
**end-of-stream iterator**:created with the default constructor for istream iterators.If a read fails, every istream iterator becomes an end-of-stream iterator.  
  
**7.5 Iterator Traits**  
  
For each iterator category, the C++ standard library provides an iterator tag that can be used as a "label" for iterators  
  
C++ standard library provides a special template structure to define the iterator traits.  
  
template <class T> struct iterator\_traits \{...\}

**Chapter 8. STL Function Objects**

**8.1 The Concept of Function Objects**  
  
3 important advantages of functors:  
  
1. A function object might be smarter because it may have a state.  
2. Each function object has its own type.  
3. A function object is usually faster than a function pointer.  
  
There are 2 ways to get a "result" or "feedback" from using function objects with algorithms:  
  
1. You can pass the function objects by reference.To pass a function object by reference you simply have to explicitly qualify the call of the algorithm so that the function object type is a reference  
  
2. You can use the return value of the for\_each() algorithm.for\_each() has the unique ability to return its function object (no other algorithm can do this)  
  
  
**8.1.4 Predicates versus Function Objects**  
  
Not every function that returns a Boolean value is a valid predicate for the STL.  
  
 A predicate must be "pure function".  
  
**8.2 Predefined Function Objects**  
  
less<> is the default criterion whenever objects are sorted or compared, so it is used often. Default sorting operations always produce an ascending order (element < nextElement).  
  
**8.2.1 Function Adapters**  
  
bind1st,bind2rd,not1,not2  
  
A function adapter is a function object that enables the combining of function objects with each other, with certain values, or with special functions.  
  
Function adapters are function objects themselves.  
  
By using function adapters you can combine different function objects to form very powerful expressions. This kind of programming is called **functional composition**.  
  
**8.2.2 Function Adapters for Member Functions**  
  
mem\_fun\_ref,mem\_fun  
  
The adapter is necessary because you can't pass a member function directly to an algorithm. Doing so would cause a compile-time error  
  
mem\_fun adapters are for sequences that contain pointers to elements. Probably mem\_fun\_ptr would have been a less confusing name .  
  
Both mem\_fun\_ref and mem\_fun can call member functions with zero or one argument. However, you can't call member functions with two or more arguments in this way.  
  
Note that the member functions called by mem\_fun\_ref and mem\_fun must be constant member functions. Unfortunately, the C++ standard library does not provide function adapters for nonconstant member functions  
  
  
**8.2.3 Function Adapters for Ordinary Functions**  
  
 ptr\_fun  
  
**8.2.4 User-Defined Function Objects for Function Adapters**  
  
You can write your own function objects, but to use them in combination with function adapters they must meet certain requirements: They must provide type members for the type of their arguments and the result. The C++ standard library provides structures to make this more convenient:  
  
unary\_function,binary\_function  
  
**8.3 Supplementary Composing Function Objects**  
  
In general it should be possible to define almost every functional behavior as a combination of function objects. However, the C++ standard library does not provide enough adapters to support this**.**

**Chapter 9. STL Algorithms**

**9.2 Algorithm Overview**  
  
Algorithms work in overwrite mode rather than in insert mode. Thus, the caller must ensure that destination ranges have enough elements. You can use special insert iterators (see Section 7.4.2) to switch from overwrite to insert mode.  
  
To increase their flexibility and power, several algorithms allow the user to pass user-defined operations, which they call internally. These operations might be ordinary functions or function objects. If these functions return a Boolean value , they are called predicates  
  
**The \_if suffix**  
  
Generally, if there is a version of an algorithm which takes a predicate, it gets the name of the algorithm with the suffix \_if.  
  
**The \_copy suffix**  
  
The \_copy suffix is used as an indication that elements are not only manipulated but also copied into a   destination range  
  
**Non-modifying Algorithms**  
  
One of the most important algorithms is for\_each()  
  
**Modifying Algorithms**  
  
The fundamental modifying algorithms are for\_each() and transform()  
  
Nevertheless, to be safe you should call merge() only for sorted ranges.  
  
You can't use associative containers as a destination for modifying algorithms.  
  
 **Removing Algorithms**  
  
As with modifying algorithms, you can't use an associative container as a destination  
  
Note that removing algorithms remove elements logically only by overwriting them with the following elements that were not removed. Thus, they do not change the number of elements in the ranges on which they operate. Instead, they return the position of the new "end" of the range  
  
**Mutating Algorithms**  
  
Mutating algorithms are algorithms that change the order of elements (and not their values) by assigning and swapping their values.As with modifying algorithms, you can't use an associative container as a destination  
  
**Sorting Algorithms**  
  
Sorting algorithms are a special kind of mutating algorithm.However, sorting is more complicated and therefore usually takes more time than simple mutating operations  
  
As with modifying algorithms, you can't use an associative container as a destination  
  
In fact, these algorithms usually have worse than linear complexity and require random access iterators (for the destination).  
  
sort() is based historically on quicksort  
  
partial\_sort() is based historically on heapsort  
  
stable\_sort() is also based historically on heapsort  
  
The standard guarantees complexity, but not how it is implemented  
  
  
 **Sorted Range Algorithms**  
  
Sorted range algorithms require that the ranges on which they operate are sorted according to their sorting criterion.  
  
  
**9.4 The for\_each() Algorithm**  
  
**9.5 Nonmodifying Algorithms**

**Chapter 10. Special Containers**

Container Adapters:Stack,Queue,Priority queue  
  
**10.1 Stack**  
  
 Stack's low-level default container is deque;however,You can use any sequence container class that provides the member functions back(), push\_back(), and pop\_back().  
  
The core interface of stack:  
  
**push()  
pop()  
top()**  
  
The standard class stack<> prefers speed over convenience and safety  
  
**10.2 Queue**  
  
queue 's low-level default container is also deque;  however You can use any sequence container class that provides the member functions front(), back()**,** push\_back(), and pop\_front().  
  
The core interface of queues:  
  
 **push()**  
 **front()**  
 **back()**  
 **pop()**  
  
**10.3 Priority Queues**  
  
 The class priority\_queue<> implements a queue from which elements are read according to their priority.  
  
As usual ,you can provide the sorting criterion as a template parameter. By default, the elements are sorted by using operator < in descending order.  
  
Unlike queue, priority\_queue use vector as its default low-level container; however,You can use any sequence container class that provides random access iterators and the member functions front(), push\_back(), and pop\_back().  
  
Random access is necessary for sorting the elements, which is performed by the heap algorithms of the STL  
  
The core interface of priority queues:  
  
 **push()**  
 **top()**  
 **pop(**)  
  
queue's member function push() and pop() will call STL algorithm push\_heap() internally.  
  
Note that, unlike other container adapters, no comparison operators are defined.  
  
  
**10.4 Bitsets**  
  
 Bitsets model fixed-sized arrays of bits or Boolean values.  
  
You can't change the number of bits in a bitset. The number of bits is the template parameter.  
  
The class bitset is defined as a template class with the number of bits as the template parameter  
  
For bitsets, some special constructors are defined. However, there is no special copy constructor, assignment operator, and destructor defined  
  
some useful member functions provided by bitset  
  
bitset<bits>::bitset( ) : Creates a bitset with all bits initialized with zero.  
  
bitset<bits>::bitset(unsigned long value) Creates a bitset that is initialized according to the bits of the integral value value.  
  
explicit bitset<bits>::bitset (const string& str) Create a bitset that is initialized by the string str which may contain only the characters '0' and '1'.  
  
size\_t bitset<bits>::size() const Returns the number of bits  
  
size\_t bitset<bits>::count() const Returns the number of set bits  
  
bool bitset<bits>::any() const Returns whether any bit is set.  
  
bool bitset<bits>::none() const Returns whether no bit is set.  
  
bitset<bits>& bitset<bits>::set(size\_t idx) Sets the bit at position idx to true.  
  
bitset<bits>& bitset<bits>::reset() Resets all bits to false  
  
bitset<bits>& bitset<bits>::flip() Toggles all bits (sets unset bits and vice versa)  
  
Operator \[ \] returns a special temporary object of type bitset<>::reference when it is called for non-constant bitsets.  
  
unsigned long bitset<bits>::to\_ulong( ) const Returns the integral value that the bits of the bitset represent  
  
string bitset<bits>::to\_string () const Returns a string that contains the value of the bitset as a binary representation written with characters '0' for unset bits and '1' for set bits

**Chapter 11. Strings**

**11.1 Motivation**   
  
Modern data processing is mostly string processing  
  
**find()**  
  
All these find functions return an index of the first matching position.Yes, the  return  value is an integer and not an iterator.  
  
The return type of all find functions is  string::size\_type **.**  
  
  Unlike C-strings, objects of class string have no special character '/0' at the end of the string.   
   
If the search fails, a special value is needed to return the failure. That value is npos, which is also defined by the string class.  
  
The type and value of npos are a big pitfall for the use of strings.  
  
Be very careful that you always use string::size\_type and not int or unsigned for the return type when you want to check the return value of a find function  
  
At all places where an index and a length are used as arguments, strings behave according to the following two rules:  
  
1. index must have a valid value；otherwise it throws out\_of \_range.  
2. number of characters could have any value；In particular, string::npos always works as a synonym for "all remaining characters."  
  
  
Both size() and length() return the numberof characters. In particular, size() has nothing to do with the memory that the string uses.(length()这个名字体现了典型的C函数strlen的风格，size()这个名字则体现了典型STL成员函数的风格)  
  
The function getline() is a special function to read input from streams into a string. It reads every character up to the next end-of-line, which by default is the newline character.  
  
string::size\_type is an unsigned integral type  
  
  
**11.2 Description of the String Classes**  
  
In most cases ,character '/0' is not treated as a special character that terminate the string  
  
In general, a string might contain any character. In particular, a string might contain the contents of a binary file  
  
Operations that Are Not Provided  
  
 1. Regular expressions  
2. Word processing (capitalization, case-insensitive comparisons)  
  
  
**11.2.3 Constructors and Destructors**  
  
You can't initialize a string with a single character.This means that there is an automatic type conversion from type const char\* but not from type char to type string.  
  
**11.2.4 Strings and C-Strings**  
  
You can use ordinary C-strings in almost every situation where strings are  combined  with other string-like objects  
  
However, there is no automatic type conversion from a string object to a C-string. This is for safety reasons.  
  
Note that string do not provide a special meaning for the character '/0', which is used as special character in an ordinary C-string to mark the end of the string. The character '/0' may be part of a string just like every other character.  
  
There are three possible ways to convert the contents of the string into a raw array of characters or C-string:  
  
1.data( )  
  
Returns the contents of the string as an array of characters. Note that the return type is not a valid C-string because no '/0' character gets appended  
  
2.c\_str( )  
  
Returns the contents of the string as a C-string. Thus, the '/0' character is appended  
  
3.copy( )  
  
Copies the contents of the string into a character array provided by the caller. An '/0' character is not appended.  
  
Note that data() and c\_str() return an array that is owned by the string. Thus, the caller must not modify or free the memory. The return value of c\_str() and data() is valid only until the next call of a non-constant member function for the same string  
  
**11.2.5 Size and Capacity**  
  
For strings, three "sizes" exist:  
  
1. size() and length()  
  
Return the actual number of characters of the string. Both functions are  equivalent  
   
2. max\_size()  
  
Returns the maximum number of characters that a string may contain.this value usually is the maximum value of the type of the index less one.  
  
3.**capacity()**  
  
Returns the number of characters that a string could contain without having to reallocate its internal memory  
  
  
Member function reserve() is provided to avoid reallocations.Unlike vectors, you can call reserve() for strings to shrink the capacity.If the argument is less than the current number of characters, it is a nonbinding shrink-to-fit request.The default value of reserve() for string is 0. So, a call of reserve() without any argument is always a nonbinding shrink-to-fit request  
  
The standard, however, specifies that capacity may shrink only because of a call of reserve().  
  
  
**11.2.7 Comparisons**  
  
If strings are compared by <, <=, >, or >=, their characters are compared lexicographically according to the current character traits.  
  
**11.2.8 Modifiers**  
  
 The specialization of swap() for strings guarantees constant complexity.  
  
**11.2.11 Searching and Finding**  
  
 All search functions have the word find inside their name.  
  
If the search fails, they return npos  
  
**11.2.12 The Value npos**  
  
 Be very careful when using the string value npos and its type. When you want to check the return value always use string::size\_type and not int or unsigned for the type of the return value; otherwise, the comparison of the return value with string::npos might not work.  
  
This behavior is the result of the design decision that npos is defined as -1  
  
 static const size\_type npos = -1;  
  
Unfortunately, size\_type (which is defined by the allocator of the string) must be an unsigned integral type.Because -1 is converted into an unsigned integral type, npos is the maximum unsigned value of its type  
  
To write portable code, however, you should always use string::size\_type for any index of your string type  
  
**11.2.13 Iterator Support for Strings**  
  
String iterators are random access iterators. This means that tyou can use all algorithms  
  
To support the use of **back inserters** for string, the push\_back() function is defined  
  
**11.2.14 Internationalization**  
  
 The character traits are provided to specify the details of how to deal with aspects depending on the representation of a character type  
  
**11.2.15 Performance**  
  
The standard does not specify how the string class is to be implemented. It only specifies the interface. There may be important differences in speed and memory usage depending on the concept and priorities of the implementation.  
  
If you prefer better speed, make sure that your string class uses a concept such as reference counting  
  
  
**11.2.16 Strings and Vectors**  
  
 Primary difference between string and vector is their goal:  
  
The primary goal of vectors is to handle and to manipulate the elements of the container, not the container as a whole.Thus, vectors implementations are optimized to operate on elements inside the container  
  
The primary goal of strings is to handle and to manipulate the container (the string) as a whole.Thus, strings are optimized to reduce the costs of assigning and passing the whole container

**Chapter 13. Input/Output Using Stream Classes**

IOStream library were made templates during standardization.As a side effect, this renders simple forward declarations of stream classes illegal. A header was introduced to provide the appropriate declarations: <iosfwd>  
  
**13.1 Common Background of I/O Streams**  
  
 class istream and ostream are instantiations of class template basic\_istream<> and basic\_ostream<>, with char as the template parameter.  
  
**13.1.3 Global Stream Objects**  
  
 cin (of class istream)  
  
cout (of class ostream)  
  
cerr (of class ostream) :No buffer  
  
clog (of class ostream)**:**By default, this stream is connected to the same destination as cerr, with the difference that output to clog is buffered.  
  
**13.1.5 Manipulators**  
  
 Manipulators are special objects that are used to manipulate a stream.  
  
endl Outputs '/n' and flushes the output buffer  
ends Outputs '/0'  
flush Flushes the output buffer  
ws Reads and discards whitespaces  
  
**13.2 Fundamental Stream Classes and Objects  
  
13.2.1 Classes and Class Hierarchy**  
  
 The class templates basic\_istream<> and basic\_ostream<> **derive virtually** from basic\_ios<>,  
  
class template basic\_streambuf<> is the heart of the IOStream library.  
  
The IOStream library is designed with a rigid separation of responsibilities(严格分工).  
  
 The classes derived from basic\_ios "only" handle formatting of the data. Actually, they don't even do the formatting! The actual formatting is delegated to corresponding **facets** in the locale library.  
  
The actual reading and writing of characters is performed by the stream buffers maintained by the basic\_ios sub-objects.  
  
In addition, an abstraction from the external representation (for example files or strings or socket) is formed by the stream buffers. By using stream buffers it is quite easy to define access to a new "external representation" like a new storage device ——derive and custoimize.  
  
  
 **Detailed Class Definitions**  
  
  
Here are two instantiations of the class basic\_ios<> for the two character types used most often:  
  
 namespace std \{   
 typedef basic\_ios<char> ios;  
 typedef basic\_ios<wchar\_t> wios;  
 \}  
  
the class templates basic\_istream<>, basic\_ostream<>, and basic\_iostream<> are also parameterized with the character type and a traits class.  
  
  
**13.2.3 Header Files**  
  
 <iosfwd>  
  
Contains forward declarations for the stream classes. This header file is necessary because it is no longer permissible to use a simple forward declaration such as "class ostream;"  
  
<istream>  
  
Contains the definitions for the classes that **support input only** (basic\_istream<>) and for the classes that **support both input and output** (basic\_iostream<>).  
  
<iostream>  
  
Contains declarations of the global stream objects.  
  
  
In general, only those headers defining necessary stuff should be included. In particular, header files should only include <iosfwd>, and the corresponding **implementation files** should then include the header with the complete definition.  
  
**13.3 Standard Stream Operators << and >>  
  
Type bool**  
  
 By default, Boolean values are printed and read numerically: false is converted from and to 0, and true is converted from and to 1.  
  
**Types char and wchar\_t**  
  
When a char or wchar\_t is being read with operator >>, leading whitespace is skipped by default.  
  
 **Type char\***  
  
 A C-string (that is, a char\*) is read word-wise. That is, when a C-string is being read, leading whitespace is skipped by default and the string is read until another whitespace character or end-of-file is encountered  
  
 **Type void\***  
  
 Operators << and >> also provide the possibility of printing a pointer and reading it back in again.  
  
**13.4 State of Streams**  
  
For the general state of streams, several constants of type iostate are defined to be used as flags .The type iostate is a member of the class ios\_base  
  
 goodbit eofbit failbit badbit  
  
The exact type of the constants is an implementation detail .it is not defined whether iostate is an enumeration, a type definition for an integral type, or an instantiation of the class bitset  
  
**eofbit** normally happens with **failbit** because the end-of-file condition is checked and detected when an attempt is made to read beyond end-of-file. After reading the last character, the flag eofbit is not yet set. The next attempt to read a character sets eofbit and failbit, because the read fails.  
  
These constants are not defined globally. Instead, they are defined within the class  
  
  
**13.4.2 Member Functions Accessing the State of Streams**  
  
The current state of the flags can be determined by the member functions  
  
Note that you always have to clear error bits explicitly.If failbit is set, each following stream operation is a no-op until failbit is cleared explicitly.  
  
  
**13.4.3 Stream State and Boolean Conditions**  
  
 operator void\*()  
 operator !()  
  
**13.4.4 Stream State and Exceptions**  
  
To stay backward compatible, by default, streams throw no exceptions. However, for the standardized streams, it is possible to define, for every state flag, whether setting that flag will trigger an exception. This definition is done by the exceptions() member function  
  
The exceptions thrown are objects of the class std::ios\_base::failure,which is derived from class exception  
  
**13.5 Standard Input/Output Functions**  
  
The member functions in this section read or write "unformatted" data.  
  
These functions use type streamsize to specify counts, which is defined in <ios>:  
  
**13.5.1 Member Functions for Input**  
  
When C-strings are read it is safer to use the functions from this section than to use operator >>.  
  
It is often better to use the stream buffer directly instead of using istream member functions  
  
**13.5.2 Member Functions for Output**  
  
Like the input functions, it may also be reasonable to use the stream buffer directly or to use the template class ostreambuf\_iterator for unformatted writing.  
  
  
**13.6 Manipulators**  
  
  
**13.6.1 How Manipulators Work**  
  
Manipulators are nothing more than functions that are passed to the I/O operators as arguments. The functions are then called by the operator.

![None.gif][]ostream& ostream::operator<< ( ostream& (\*op) (ostream&))  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][]...\{  
![InBlock.gif][]// call the function passed as parameter with this stream as the argument  
![InBlock.gif][]return (\*op) (\*this);  
![ExpandedBlockEnd.gif][]\}

The manipulator endl() for ostream is implemented basically like this

![None.gif][]std::ostream& std::endl (std::ostream& strm)  
![ExpandedBlockStart.gif][]![ContractedBlock.gif][]...\{  
![InBlock.gif][]// write newline  
![InBlock.gif][]strm.put(' ');  
  
![InBlock.gif][]// flush the output buffer  
![InBlock.gif][]strm.flush();  
  
![InBlock.gif][]// return strm to allow chaining  
![InBlock.gif][]return strm;  
![ExpandedBlockEnd.gif][]\}

**13.7 Formatting**  
  
 Two concepts influence the definition of I/O formats: format flag and locale  
  
**13.7.1 Format Flags**  
  
You can also use manipulators to set and clear format flags  
  
The manipulators setiosflags() and resetiosflags() provide the possibility of setting or clearing, respectively, one or more flags in a write or read statement with operator << or >> respectively  
  
**13.7.3 Field Width, Fill Character, and Adjustment**  
  
Two member functions are used to define the field width and the fill character: width() and fill()  
  
For the output ,width() defines a minimum field. This definition applies only to the next formatted field written  
  
fill() defines the fill character that is used to fill the difference between the formatted representation of a value and the minimum field width. The default fill character is a space.  
  
**13.7.4 Positive Sign and Uppercase Letters**  
  
Two format flags are defined to influence the general appearance of numeric values: showpos and uppercase  
  
**13.7.7 General Formatting Definitions**  
  
 ios::skipws is set by default, meaning that by default leading whitespaces are skipped by certain read operations.  
  
ios::unitbuf controls the buffering of the output. With ios::unitbuf set, output is basically unbuffered.By default, this flag is not set. However, for the streams cerr and wcerr this flag is set initially.  
  
  
 **13.8 Internationalization**  
  
Each stream uses an associated locale object. The initial default locale object is a copy of the global locale object at the construction time of the stream  
  
  imbue (loc) Sets the locale object  
getloc() Returns the current locale object  
widen (c) Converts the char character c to a character of the stream's character set  
narrow (c,def) Converts character c from the stream's character set to a char (if there is no such char, def is returned)  
  
  
 **13.9 File Access**  
  
 For streams that are both read and written , it is not possible to switch arbitrarily between reading and writing .This is a restriction inherited from C.  
  
**13.9.1 File Flags**  
  
A set of flags is defined in the class ios\_base .These flags are of type openmode.  
  
 ios\_base::in ios\_base::out ios\_base::binary ios\_base::append ios\_base::trunc ios\_base::ate  
  
ios\_base::binary configures the stream to suppress conversion of special characters or character sequences such as end-of-line or end-of-file  
  
Whether a file is opened for reading and/or for writing is**independent**of the corresponding stream object's class. The class only determines the default open mode if no second argument is used.  
  
The open mode is passed to the corresponding stream buffer class, which opens the file. However, the operations possible for the object are determined by the stream's class  
  
The member function open() does not clear the stream's state flags  
  
**13.9.2 Random Access**  
  
Not all stream classes support positioning. For example, positioning the streams cin, cout, and cerr is not defined  
  
The position's value is type pos\_type .The C++ standard library defines a global template class **fpos<>** for file positions. Class fpos<> is used to define types streampos for char and wstreampos for wchar\_t streams  
  
**13.9.3 Using File Descriptors**  
  
The C++ standard library unfortunately does not provide this possibility of attaching a stream to an I/O channel using file descriptors. This is because the language is supposed to be independent of any operating system.  
  
  
**13.10 Connecting Input and Output Streams**  
  
**13.10.1 Loose Coupling Using tie()**  
  
You can tie a stream to an output stream. This means the buffers of both streams are synchronized in a way that the buffer of the output stream is flushed before each input or output of the other stream.  
  
For each stream, you can only have one output stream that is tied to this stream.However, you can tie an output stream to different streams.  
  
By default, the standard input is connected to the standard output using this mechanism. This ensures that a message asking for input is flushed before requesting the input.

![None.gif][]// predefined connections:  
![None.gif][]std::cin.tie (&std::cout);  
![None.gif][]std::wcin.tie (&std::wcout);

You can also tie one output stream to another output stream  
  
**13.10.2 Tight Coupling Using Stream Buffers**  
  
Using the function rdbuf(), you can couple streams tightly by using a common stream buffer  
  
Note that the destructor of the classes basic\_istream<> and basic\_ostream<> does not delete the corresponding stream buffer (it was not opened by these classes anyway).  
  
The advantage of this approach is that the format does not need to be restored to its original state after it is modified because the format applies to the stream object, not to the stream buffer.  
  
Also note that the destruction of a stream object does not flush the buffer. To make sure that an output buffer is flushed, it has to be flushed manually  
  
The fact that stream buffer is not destroyed applies only to basic\_istream<> and basic\_ostream<>. The other stream classes destroy the stream buffers they allocated originally, but they do not destroy stream buffers set with rdbuf()  
  
**13.10.3 Redirecting Standard Streams**  
  
A stream can be redirected by setting a new stream buffer.  
  
Stream's member function copyfmt() can be used to assign all format information of a given stream to another stream object  
  
File streams allocate their stream buffer objects at construction time and destroy them on destruction.  
  
  
**13.10.4 Streams for Reading and Writing  
  
  
13.11 Stream Classes for Strings**  
  
 String streams provide a buffer but don't have an I/O channel  
  
Stream classes have a similar relationship to the stream base classes, as do the file stream classes.  
  
The major function in the interface of the string stream classes is the member function str(). This function is used to manipulate the buffer of the string stream classes.  
  
A typical programming error when dealing with string streams is to forget to extract the string with the function str(), and instead to write to the file stream directly.  
  
  
**13.11.2 char\* Stream Classes**  
  
The char\* stream classes are retained only for backward compatibility.Their interface is error prone and they are rarely used correctly.  
  
  
**13.12 Input/Output Operators for User-Defined Types**  
  
**13.12.4 User-Defined Operators Using Unformatted Functions**  
  
The I/O operators defined in the C++ standard library are defined in a **common scheme** : First, with some pre-processing the stream is prepared for actual I/O. Then the actual I/O is done, followed by some post-processing.  
  
The classes basic\_istream and basic\_ostream each define an auxiliary class sentry. The constructor of these classes does the preprocessing, and the destructor does the corresponding pos-tprocessing.  
  
If an I/O operator uses a function for unformatted I/O or operates directly on the stream buffer, the first thing to be done should be the construction of a corresponding sentry object.  
  
Its state can be checked using the conversion of the sentry object to bool  
  
The sentry object takes the stream strm, on which the preprocessing and post-processing should be done, as the constructor argument.  
  
Of course, it is also possible to use this framework even if functions do not use unformatted functions for their implementation but use I/O operators instead. However, using basic\_istream or basic\_ostream members for reading or writing characters within code guarded by sentry objects is unnecessarily expensive. Whenever possible, the corresponding basic\_streambuf should be used instead.  
  
**13.13 The Stream Buffer Classes**  
  
**13.13.1 User's View of Stream Buffers**  
  
Output Member Function  
  
 sputc(c)  
 sputn(s, n)  
  
The interface to reading characters from a stream buffer is a little bit more complex. This is because for input it is necessary to have a look at a character without consuming it. Also, it is desirable that characters can be put back into the stream buffer when parsing.  
  
pubimbue() installs a new locale object in the stream buffer ,returning the previously installed locale object.   
  
getloc()returns the currently installed locale object.  
  
**13.14 Performance Issues**  
  
**13.14.1 Synchronization with C's Standard Streams**  
  
By default, the C++ standard streams are synchronized with the corresponding files from the C standard library  
  
**13.14.2 Buffering in Stream Buffers**  
  
The functions for formatted I/O use stream buffer iterators to access the streams, and operating on stream buffer iterators is slower than operating on pointers.  
  
**13.14.3 Using Stream Buffers Directly**  
  
All member functions of the class basic\_istream and basic\_ostream that read or write characters operate according to the same schema: First, a corresponding sentry object is constructed, then the actual operation is performed.The construction of the sentry object results in flushing of potentially tied objects, skipping of whitespace (for input only), and implementation-specific operations like locking in multithreaded environments  
  
Note that a stream buffer has no error state of its own. It also has no knowledge of the input or ouput stream that might connect to it.

**Chapter 14. Internationalization**

For the internationalization of programs, two related aspects are important  
  
1. Different character sets have different properties  
2. The user of a program expects to see national or cultural conventions obeyed  
  
 The major approach toward internationalization is to use locale objects to represent an extensible collection of aspects to be adapted to specific local conventions  
  
Actually, the C++ locale mechanism can be used to address all kinds of customization, depending on the user's environment or preferences.  
  
Most of the mechanisms of internationalization involve no or only minimal additional work for the programmer  
  
Some internationalized aspects supported by the C++ standard library are not used by the C++ standard library itself, and to use them the programmer has to call those functions manually.  
  
Strings and streams use another concept for internationalization: character traits  
  
**14.1 Different Character Encodings**  
  
**14.1.1 Wide-Character and Multibyte Text**  
  
  
 Two different approaches are common to address character sets that have more than 256 characters: multibyte representation and wide-character representation  
  
The multibyte representation is more compact than the wide-character representation.Thus, the multibyte representation is normally used to store data outside of programs.  
  
Conversely, it is much easier to process characters of fixed size, so the wide-character representation is usually used inside programs.  
  
Like ISO C, ISO C++ uses the type wchar\_t to represent wide characters. However in C++, wchar\_t is a keyword rather than a type definition  
  
During iteration through a multibyte string, each byte is interpreted according to a current **"shift state****"**. Depending on the value of the byte and the current shift state, a byte may represent a certain character or a change of the current shift state.  
  
**14.1.2 Character Traits**  
  
C established the convention to return a character as int instead of as char from functions reading characters.This technique was extended in C++. The character traits define char\_type as the type to represent all characters, and int\_type as the type to represent all characters plus EOF.  
  
The C++ standard library provides specializations of char\_traits<> for types char and wchar\_t  
  
The specialization for char is usually implemented by using the global string functions of C that are defined in <cstring> or <string.h>.  
  
  
**14.2 The Concept of Locales  
  
14.2.1 Using Locales**  
  
 According to X/Open conventions, the environment variable LANG is used to define the locale to be used.  
  
C++ provides more library support than C, because basically everything available for char is also available for wchar\_t.  
  
The name "C" is a special name, and actually is the only locale a C++ implementation is required to support.  
  
If the name used to construct a locale object is unknown to the implementation, an exception of type runtime\_error is thrown.  
  
 Passing an empty string as the name of the locale has a special meaning: The default locale from the user's environment is used (this is often determined by the environment variable LANG).  
  
The static member function global()of the class locale can be used to install a global locale object. This object is used as the default value for functions that take an optional locale object as an argument.  
  
However, setting the global locale does not replace locales already stored in objects. It only modifies the locale object copied when a locale is created with a default constructor.  
  
**14.2.2 Locale Facets**  
  
 The actual dependencies on national conventions are separated into several aspects that are handled by corresponding objects. An object dealing with a specific aspect of internationalization is called a facet.  
  
A locale object is used as a container of different facets  
  
The type of the facet is passed explicitly as a template argument to the template function use\_facet(), accessing the desired facet.  
  
It is possible to define your own versions of the facets to create specialized locales.  
  
To use this facet in a locale, you need to create a new locale using a special constructor. This constructor takes a locale object as its first argument and a pointer to a facet as its second argument. The created locale is identical to the first argument except for the facet that is passed as the second argument.  
  
**14.3 Locales in Detail**  
  
A C++ locale is an immutable container for facets.  
  
The strange thing about locales is how the objects stored in the container are accessed. A facet in a locale is accessed using the type of the facet as the index.  
  
Using the facet's type as an index has the additional advantage of having a type-safe interface.  
  
Merely copying a locale is considered to be a cheap operation. Creating a modified locale is more expensive  
  
The standard basically requires that two locales consisting of the same set of named facets be considered identical  
  
**14.4 Facets in Detail**  
  
 Every class F that conforms to the following two requirements can be used as a facet:  
  
1. F derives publically from class locale::facet. It declares the copy constructor and the assignment operator to be private, thereby making it infeasible to copy or to assign facets  
  
2. F has a publically accessible static member named id of type locale::id.  
  
**Some special implementation guidelines**  
  
1. All member functions are declared to be const  
2. All public functions are nonvirtual and delegate each request to a protected virtual function.

**Chapter 15. Allocators**

**15.1 Using Allocators as an Application Programmer**

Beware that you don't mix elements with different allocators; otherwise, the behavior is undefined.  
  
You can check whether two allocators use the same memory model by using operator ==.  
  
To access the allocator, all types that are parameterized by an allocator provide the member function get\_allocator().  
  
  
**15.2 Using Allocators as a Library Programmer**  
  
Allocators provide an interface to allocate, create, destroy, and deallocate objects  
  
For the initialization of uninitialized memory the C++ standard library provides some convenience functions:  
  
 uninitialized\_fill(beg,end,val)  
 uninitialized\_fill\_n(beg,num,val)  
 uninitialized\_copy(beg,end,mem)  
  
**Raw Storage Iterators**  
  
 class raw\_storage\_iterator is provided to iterate over uninitialized memory to initialize it.  
  
**Temporary Buffers**  
  
get\_temporary\_buffer() and return\_temporary\_buffer().  
  
They are provided to handle uninitialized memory that is provided for short, temporary use inside a function  
  
However, it is rather complicated to write exception-safe code with get\_temporary\_buffer() and return\_temporary\_buffer(), so they are usually no longer used in library implementations  
  
  
**15.3 The Default Allocator**  
  
 The default allocator uses the global operators new and delete to allocate and deallocate memory  
  
There is a strange definition of a template structure inside the allocator, called rebind. This template structure provides the ability that any allocator may allocate storage of another type indirectly.  
  
rebind<> is useful if you implement a container and you have to allocate memory for a type that differs from the element's type  
  
  
**15.4 A User-Defined Allocator**  
  
Writing your own allocator is not very hard. The most important issue is how you allocate or deallocate the storage. The rest is more or less obvious.  
  
Typically, the only things that differ from typical implementation are max\_size(),allocate()**,** and deallocate().  
  
  
**15.6 Utilities for Uninitialized Memory in Detail**  
  
 void uninitialized\_fill (ForwardIterator beg, ForwardIterator end, const T& value)  
  
 void uninitialized\_fill\_n (ForwardIterator beg, Size num, const T& value)  
  
ForwardIterator uninitialized\_copy (InputIterator sourceBeg, InputIterator sourceEnd, ForwardIterator destBeg)  
  
  
  These function either succeeds or has no effect( strong exceptional safe!)

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