emplace_back() 和 push_back 的区别（转）

在引入右值引用，转移构造函数，转移复制运算符之前，通常使用push_back()向容器中加入一个右值元素（临时对象）的时候，首先会调用构造函数构造这个临时对象，然后需要调用拷贝构造函数将这个临时对象放入容器中。原来的临时变量释放。这样造成的问题是临时变量申请的资源就浪费。
引入了右值引用，转移构造函数（请看这里）后，push_back()右值时就会调用构造函数和转移构造函数。
在这上面有进一步优化的空间就是使用emplace_back

emplace_back

函数原型：

1 template <class... Args>
2   void emplace_back (Args&&... args);

在容器尾部添加一个元素，这个元素原地构造，不需要触发拷贝构造和转移构造。而且调用形式更加简洁，直接根据参数初始化临时对象的成员。
给出一个示例，这个示例很有用。

 1 #include <vector>  
 2 #include <string>  
 3 #include <iostream>  
 4 
 5 struct President  
 6 {  
 7     std::string name;  
 8     std::string country;  
 9     int year;  
10 
11     President(std::string p_name, std::string p_country, int p_year)  
12         : name(std::move(p_name)), country(std::move(p_country)), year(p_year)  
13     {  
14         std::cout << "I am being constructed.\n";  
15     }
16     President(const President& other)
17         : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
18     {
19         std::cout << "I am being copy constructed.\n";
20     }
21     President(President&& other)  
22         : name(std::move(other.name)), country(std::move(other.country)), year(other.year)  
23     {  
24         std::cout << "I am being moved.\n";  
25     }  
26     President& operator=(const President& other);  
27 };  
28 
29 int main()  
30 {  
31     std::vector<President> elections;  
32     std::cout << "emplace_back:\n";  
33     elections.emplace_back("Nelson Mandela", "South Africa", 1994); //没有类的创建  
34 
35     std::vector<President> reElections;  
36     std::cout << "\npush_back:\n";  
37     reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));  
38 
39     std::cout << "\nContents:\n";  
40     for (President const& president: elections) {  
41        std::cout << president.name << " was elected president of "  
42             << president.country << " in " << president.year << ".\n";  
43     }  
44     for (President const& president: reElections) {  
45         std::cout << president.name << " was re-elected president of "  
46             << president.country << " in " << president.year << ".\n";  
47     }
48 
49 }

输出

1 emplace_back:
2 I am being constructed.
3 
4 push_back:
5 I am being constructed.
6 I am being moved.
7 
8 Contents:
9 Nelson Mandela was elected president of South Africa in 1994.