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原文：

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()，使用emplace_back()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。

测试代码：

#include 
   
    #include 
    
     #include 
     
      struct President  {    std::string name;    std::string country;    int year;    President(std::string p_name, std::string p_country, int p_year)        : name(std::move(p_name)), country(std::move(p_country)), year(p_year)  {        std::cout << "I am being constructed.\n";    }    President(const President& other)        : name(std::move(other.name)), country(std::move(other.country)), year(other.year){        std::cout << "I am being copy constructed.\n";    }    President(President&& other)        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)  {        std::cout << "I am being moved.\n";    }    President& operator=(const President& other);};int main()  {    std::vector
      
        elections;    std::cout << "emplace_back:\n";    elections.emplace_back("Nelson Mandela", "South Africa", 1994); //没有类的创建    std::vector
       
         reElections;    std::cout << "\npush_back:\n";    reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));    std::cout << "\nContents:\n";    for (President const& president: elections) {       std::cout << president.name << " was elected president of "            << president.country << " in " << president.year << ".\n";    }    for (President const& president: reElections) {        std::cout << president.name << " was re-elected president of "            << president.country << " in " << president.year << ".\n";    }}
       
      
     
    
   

测试结果：

emplace_back:I am being constructed.push_back:I am being constructed.I am being moved.Contents:Nelson Mandela was elected president of South Africa in 1994.Franklin Delano Roosevelt was re-elected president of the USA in 1936.

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