C++11 的 5 个实用特性

柔光的暖阳◎ 2022-08-18 03:00 244阅读 0赞

Kernel Panic 总结了 C++11 中的 5 个实用特性：自动类型推导、强类型枚举、Unique 指针、static\_assert、Lambdas 表达式。

## Auto Type Deduction 自动类型推导 ##

auto 关键字让用户得以使用 C++ 内置的类型推导特性。

Auto 关键字会对上述自变量（something）进行自动推导，得出其应该是 string 类型的结论，并在 auto 出现的地方用正确的类型进行替换。这个特性对迭代器特别有用。

上述代码可以写成：

赞！ 代码看上去简洁多了！

## Strongly Typed Enums 强类型枚举 ##

这个特性可以有效避免枚举类型的命名冲突，从而消除很多潜在的 bug。在旧版本的 C++ 中，程序员必须为每一个枚举项设定一个全局唯一的名字。例如，如果你给一个枚举项取名叫 None，那么其他的枚举集合就不能再用这个名字了。但是现在，你可以这么做了！（译注：作者下面给的例子仿佛和上面的文字不是非常搭配//myEnum ::All 和 myEnum::All是不是存在差异？）

## Lambdas 表达式 ##

Lambda 表达式简单说就是个匿名函数（译注：原文为in-place function，意思是“用来镶嵌的函数”，但是匿名函数能更精确表达这个含义）。对于迭代器以及for 循环非常有用，这种函数你只需要在程序的某一处使用一次，所以没有必要专门在程序里明确定义它。Lambda 表达式并没有让 C++ 在逻辑表达上做到“及以往之不可及”的程度，它是一种受函数式编程思想影响而引入的语言特性，能够让程序更紧凑。Lambda 表达式的最简形式是下面这样的：

\[\]() \{ \}

加上所有可能的操作符，会是这样：

其中\[\]是捕获列表，()是参数列表，\{\}是函数体

Capture List 捕获列表

捕获列表定义了什么类型的东西可以从 Lambda 表达式之外匹配到函数体中来。可以包含以下这些：

1.  一个值：\[x\]
2.  一个引用 \[&x\]
3.  当前范围内任意变量的引用 \[&\]
4.  同3，但是通过变量的值

你可以对上面的各项进行任意混合，只要用逗号隔开即可 \[x, &y\]

Argument List 参数列表

参数列表和 C++ 函数的参数列表是一个概念。

Function Body 函数体

函数体是指在 Lambda 表达式被调用时真正执行的代码。

Return Type Deduction

返回值推断

如果 Lambda 表达式只有一个返回声明，那么返回值类型就可以省略，其类型就是隐式类型：decltype(return\_statement)

可变 Labmda

如果一个 Lambda 表达式被标记为 mutable（例如：\[\]() mutable\{ \}），那么对于按值捕获的数值来说，在函数体内就允许对这些值进行修改操作。

下面举个例子：

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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">Uppercase++;</code> 
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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">});</code> 
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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">&nbsp;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; background:none!important">cout&lt;&lt; Uppercase&lt;&lt;</code> 
      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:inherit!important; color:blue!important; background:none!important">" uppercase letters in: "</code> 
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     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

## Unique 指针 ##

Unique 指针是 C++11 版本的智能指针类。

一旦你用 unique\_ptr 关键字定义了一个对象，那么下列事件只要发生一个，对象就会被销毁并释放内存：

*  unique\_ptr 管理的对象被销毁。
 *  unique\_ptr 管理的对象通过赋值操作符指向另一个指针，或调用了reset()方法。

对于不想了解太多细节的用户来说，这就意味着如果你使用了 unique 指针的语义，那么在跳出作用域之前，你就不用手动回收对象的内存了。

以前，我们需要这么写代码：

然后在程序的最后你一定要记得释放内存：

否则你可就造成内存泄露了。而现在，

当 obj 跳出作用域范围之外的时候，内存将会被自动回收。

## static\_assert ##

static\_assert 简单说就是一个在编译期执行的断言。例如，你可以这么做：

假设由于系统的原因造成了上述的逻辑判断的失败，那么 static\_assert 就会断言失败。

它的另一种用途，是和 C++ 特征类型搭配使用。比如：

POD 是指“简单数据”（Plain Old Data)结构，也就是说，它是一个的类（你可以用struct关键字定义，也可以用class关键字定义），但没有构造函数，析构函数和虚成员函数。所以，如果一个愚蠢的菜鸟程序员企图给这种类型增加构造函数的话，static\_assert 就会在编译的时候阻止这种行为，并报错。这对代码维护来说可是非常有用的。