c++11 vector中push_back和emplace_back的区别

原文：https://blog.csdn.net/xiaolewennofollow/article/details/52559364

1、两者区别

    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，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。 

2、代码实例

#include <vector>
#include <string>
#include <iostream>


struct President
{

    std::string name;
    std::string country;
    int year;

    //构造函数
    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";
    }


    //复制构造函数
    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";
    }

    //转移构造函数
    President(President&& other)
        : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
    {
        std::cout << "I am being moved.\n";
    }

    //赋值运算符函数
    President& operator=(const President& other);
};


int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994); //没有类的创建

    std::vector<President> 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";
    }
}

输出：

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.

总结：

push_back()右值时就会调用构造函数和转移构造函数

emplace_back()函数向容器中中加入临时对象， 临时对象原地构造，没有赋值或移动的操作。

emplace_back()函数要比push_back()函数要快一倍