C++：如何高效的使用std::vector？

ゝ一世哀愁。 2022-05-04 08:30 700阅读 0赞

一、在std::vector尾部添加对象时应尽量使用emplace\_back，而不要使用push\_back

二、添加多个元素前应使用reserve设置容量，防止扩容时发生元素复制

三、删除元素时应从最后一个元素开始删除，不要从中间开始删除

四、添加新对象时应从结尾处添加，而不要从中间添加

五、使用std::vector(std::vector)和std::vector(std::initializer\_list)对std::vector赋初始值会将std::vector和std::initializer\_list中所有对象复制

六、需要将对象全部转移到另外一std::vector时，应使用std::vector.swap()、std::swap()、std::move(std::vector)

七、如果std::vector中在存放指针对象，即std::vector，则应使用智能指针

--------------------

先说一下关于std::vector的一些基本知识：

1.  std::vector<T>中存放的T对象是在堆中分配的。
2.  std::vector<T>会负责管理T对象的生命周期，所以当将T对象存放到std::vector<T>中时，std::vector<T>会构造一个自己能完全控制的T对象，构造方式可以是转移构造函数、拷贝构造函数还是普通构造函数。
3.  当std::vector<T>的生命周期结束时，std::vector<T>也同时会将其中存放的对象析构。

--------------------

对于如何高效使用std::vector，以下有几点建议。先给出一个类的代码，后面会用到它：

class Girl {
    
    public:
    	string name;
    	int age;
    
    
    	Girl() {
    		cout << "Girl()" << endl;
    	}
    
    	Girl(const string& _name, int _age) : name(_name), age(_age) {
    		cout << "Girl(string _name, int _age)" << name << endl;
    	}
    
    	Girl(const Girl& b) : name(b.name), age(b.age) {
    		cout << "Girl(const Girl&)" << name << endl;
    	}
    
    	Girl(Girl&& b) : name(move(b.name)), age(move(b.age)) {
    		cout << "Girl(Girl&&)" << name << endl;
    	}
    
    	Girl& operator=(const Girl& b) {
    		this->name = b.name;
    		this->age = b.age;
    
    		cout << "operator=(const Girl&)" << name << endl;
    
    		return *this;
    	}
    
    	~Girl() {
    		cout << "~Girl()" << name << endl;
    	}
    
    };

### 一、在std::vector<T>尾部添加对象时应尽量使用emplace\_back，而不要使用push\_back ###

int main() {
    	vector<Girl> vvv1;
    	vector<Girl> vvv2;
    
    	cout << "------------------------------------------------" << endl;
    
    	vvv1.push_back(Girl("dd", 90));
    
    	cout << "------------------------------------------------" << endl;
    
    	vvv2.emplace_back("dd", 90);
    
    	cout << "------------------------------------------------" << endl;
    
    	return 1;
    }

> \------------------------------------------------  
> Girl(string \_name, int \_age)dd  
> Girl(Girl&&)dd  
> ~Girl()  
> \------------------------------------------------  
> Girl(string \_name, int \_age)dd  
> \------------------------------------------------  
> ~Girl()dd  
> ~Girl()dd

从结果中可以看到，使用push\_back会比emplace\_back多调了转移构造函数和析构函数，因而性能偏低。

### 二、添加多个元素前应使用reserve设置容量，防止扩容时发生元素复制 ###

int main() {
    	vector<Girl> vvv1;
    
    	cout << vvv1.size() << endl;
    	cout << vvv1.capacity() << endl;
    
    	vvv1.emplace_back("ss", 12);
    	cout << "------------------------------------------------" << endl;
    	vvv1.emplace_back("xx", 33);
    	cout << "------------------------------------------------" << endl;
    	vvv1.emplace_back("wq", 123);
    
    	cout << "------------------------------------------------" << endl;
    
    	cout << vvv1.size() << endl;
    	cout << vvv1.capacity() << endl;
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv2;
    	vvv2.reserve(10);
    
    	cout << vvv2.size() << endl;
    	cout << vvv2.capacity() << endl;
    
    	vvv2.emplace_back("ss", 12);
    	cout << "------------------------------------------------" << endl;
    	vvv2.emplace_back("xx", 33);
    	cout << "------------------------------------------------" << endl;
    	vvv2.emplace_back("wq", 123);
    
    	cout << "------------------------------------------------" << endl;
    
    	cout << vvv2.size() << endl;
    	cout << vvv2.capacity() << endl;
    	cout << "------------------------------------------------" << endl;
    
    	return 1;
    }

> 0  
> 0  
> Girl(string \_name, int \_age)ss  
> \------------------------------------------------  
> Girl(string \_name, int \_age)xx  
> Girl(const Girl&)ss  
> ~Girl()ss  
> \------------------------------------------------  
> Girl(string \_name, int \_age)wq  
> Girl(const Girl&)ss  
> Girl(const Girl&)xx  
> ~Girl()ss  
> ~Girl()xx  
> \------------------------------------------------  
> 3  
> 3  
> \------------------------------------------------  
> 0  
> 10  
> Girl(string \_name, int \_age)ss  
> \------------------------------------------------  
> Girl(string \_name, int \_age)xx  
> \------------------------------------------------  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> 3  
> 10  
> \------------------------------------------------  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()wq  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()wq

从结果中可以看到，由于vvv1没有设置reserver，导致初始容量不足，每添加一个新的对象都要先扩容，然后将以前存放的对象复制过去，因而性能偏低。

### 三、删除元素时应从最后一个元素开始删除，不要从中间开始删除 ###

int main() {
    	vector<Girl> vvv1;
    	vvv1.reserve(10);
    
    	vvv1.emplace_back("ss", 12);
    	vvv1.emplace_back("xx", 33);
    	vvv1.emplace_back("wq", 123);
    
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv2;
    	vvv2.reserve(10);
    
    	vvv2.emplace_back("ss", 12);
    	vvv2.emplace_back("xx", 33);
    	vvv2.emplace_back("wq", 123);
    
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv3;
    	vvv3.reserve(10);
    
    	vvv3.emplace_back("ss", 12);
    	vvv3.emplace_back("xx", 33);
    	vvv3.emplace_back("wq", 123);
    
    	cout << "++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    	
    	vvv1.erase(vvv1.begin());
    	cout << "------------------------------------------------" << endl;
    	vvv1.erase(vvv1.begin());
    	cout << "++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    
    	vvv2.erase(vvv2.end() - 1);
    	cout << "------------------------------------------------" << endl;
    	vvv2.erase(vvv2.end() - 1);
    	cout << "++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    
    	vvv3.pop_back();
    	cout << "------------------------------------------------" << endl;
    	vvv3.pop_back();
    	cout << "++++++++++++++++++++++++++++++++++++++++++++++++" << endl;
    
    	return 1;
    }

> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \++++++++++++++++++++++++++++++++++++++++++++++++  
> operator=(const Girl&)xx  
> operator=(const Girl&)wq  
> ~Girl()wq  
> \------------------------------------------------  
> operator=(const Girl&)wq  
> ~Girl()wq  
> \++++++++++++++++++++++++++++++++++++++++++++++++  
> ~Girl()wq  
> \------------------------------------------------  
> ~Girl()xx  
> \++++++++++++++++++++++++++++++++++++++++++++++++  
> ~Girl()wq  
> \------------------------------------------------  
> ~Girl()xx  
> \++++++++++++++++++++++++++++++++++++++++++++++++  
> ~Girl()ss  
> ~Girl()ss  
> ~Girl()wq

从结果中可以看到，如果从中间或开头开始删除，后面的对象由于要向前移动而要调用赋值函数，运行性能偏低。这里移动的方式要注意，它是每二个对象赋值给第一个对象，第三个对象赋值给第二个对象，第三个对象析构销毁。

### 四、添加新对象时应从结尾处添加，而不要从中间添加 ###

int main() {
    	vector<Girl> vvv1;
    	vvv1.reserve(10);
    
    	vvv1.emplace_back("ss", 12);
    
    	cout << "------------------------------------------------" << endl;
    	vvv1.insert(vvv1.begin(), Girl("xx", 33));
    	
    	cout << "------------------------------------------------" << endl;
    	vvv1.insert(vvv1.begin(), Girl("wq", 123));
    
    	cout << "------------------------------------------------" << endl;
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv2;
    	vvv2.reserve(10);
    
    	vvv2.emplace_back("ss", 12);
    
    	cout << "------------------------------------------------" << endl;
    	vvv2.emplace(vvv2.begin(), "xx", 33);
    
    	cout << "------------------------------------------------" << endl;
    	vvv2.emplace(vvv2.begin(), "wq", 123);
    
    	cout << "------------------------------------------------" << endl;
    
    
    	return 1;
    }

> Girl(string \_name, int \_age)ss  
> \------------------------------------------------  
> Girl(string \_name, int \_age)xx  
> Girl(Girl&&)xx  
> Girl(Girl&&)ss  
> operator=(const Girl&)xx  
> ~Girl()xx  
> ~Girl()  
> \------------------------------------------------  
> Girl(string \_name, int \_age)wq  
> Girl(Girl&&)wq  
> Girl(Girl&&)ss  
> operator=(const Girl&)xx  
> operator=(const Girl&)wq  
> ~Girl()wq  
> ~Girl()  
> \------------------------------------------------  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> \------------------------------------------------  
> Girl(string \_name, int \_age)xx  
> Girl(Girl&&)ss  
> operator=(const Girl&)xx  
> ~Girl()xx  
> \------------------------------------------------  
> Girl(string \_name, int \_age)wq  
> Girl(Girl&&)ss  
> operator=(const Girl&)xx  
> operator=(const Girl&)wq  
> ~Girl()wq  
> \------------------------------------------------  
> ~Girl()wq  
> ~Girl()xx  
> ~Girl()ss  
> ~Girl()wq  
> ~Girl()xx  
> ~Girl()ss

从运行结果可以看到，无论是使用emplace还是insert，在插入位置后的对象都会发生移动，这样会影响性能。

### 五、使用std::vector(std::vector)和std::vector(std::initializer\_list<T>)对std::vector赋初始值会将std::vector和std::initializer\_list<T>中所有对象复制 ###

int main() {
    
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vv({ Girl("d",90), Girl("ll",80) });
    
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv1;
    	vvv1.reserve(10);
    
    	vvv1.emplace_back("ss", 12);
    	vvv1.emplace_back("xx", 33);
    
    	cout << "------------------------------------------------" << endl;
    
    	vector<Girl> vvv2(vvv1);
    
    	cout << "------------------------------------------------" << endl;
    
    
    	return 1;
    }

> \------------------------------------------------  
> Girl(string \_name, int \_age)d  
> Girl(string \_name, int \_age)ll  
> Girl(const Girl&)d  
> Girl(const Girl&)ll  
> ~Girl()ll  
> ~Girl()d  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> \------------------------------------------------  
> Girl(const Girl&)ss  
> Girl(const Girl&)xx  
> \------------------------------------------------  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()d  
> ~Girl()ll

通过运行结果可以看到，当初始化std::vector时，对象双被复制了一份。通过初始化的方式无法将对象在多个集合中共享。如果像让std::vector中的所有对象暴露出来，可以使用T\* std::vector.data()，它会返回std::vector中所包含的对象的指针。

### 六、需要将对象全部转移到另外一std::vector时，应使用std::vector.swap()、std::swap()、std::move(std::vector) ###

int main() {
    	{
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv1;
    		vvv1.reserve(10);
    
    		vvv1.emplace_back("ss", 12);
    		vvv1.emplace_back("xx", 33);
    		vvv1.emplace_back("wq", 123);
    
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv2 = move(vvv1);
    		cout << "------------------------------------------------" << endl;
    	}
    	{
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv1;
    		vvv1.reserve(10);
    
    		vvv1.emplace_back("ss", 12);
    		vvv1.emplace_back("xx", 33);
    		vvv1.emplace_back("wq", 123);
    
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv2;
    		vvv2.swap(vvv1);
    		cout << "------------------------------------------------" << endl;
    	}
    	{
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv1;
    		vvv1.reserve(10);
    
    		vvv1.emplace_back("ss", 12);
    		vvv1.emplace_back("xx", 33);
    		vvv1.emplace_back("wq", 123);
    
    		cout << "------------------------------------------------" << endl;
    
    		vector<Girl> vvv2;
    		swap(vvv1, vvv2);
    		cout << "------------------------------------------------" << endl;
    	}
    
    	return 1;
    }

> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> \------------------------------------------------  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()wq  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> \------------------------------------------------  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()wq  
> \------------------------------------------------  
> Girl(string \_name, int \_age)ss  
> Girl(string \_name, int \_age)xx  
> Girl(string \_name, int \_age)wq  
> \------------------------------------------------  
> \------------------------------------------------  
> ~Girl()ss  
> ~Girl()xx  
> ~Girl()wq

通过运行结果可以看到，转移的过程只是更改了指针的指向，没有发生任何复制或拷贝。

### 七、如果std::vector中在存放指针对象，即std::vector<T\*>，则应使用智能指针 ###

*  std::vector<std::unique\_ptr<T>>
 *  std::vector<std::shared\_ptr<T>>

因为如果std::vector中存放指针，指针指向的对象并不受std::vector管理，所以需要智能指针帮助管理这些对象。