C++:Vector中push_back和emplace_back到底有什么区别？

push_back应该是我们最常使用的标准容器，而且对于LZ这种C++菜鸟，主要C++写的风格其实还是C，所以，理解不到位处还请小伙伴批评指正。

这是在阅读其他代码的时候发现了一个emplace_back的用法，但是LZ之前没遇到过，所以就很想知道emplace_back和push_back之间到底有什么区别呢？

先看下官方文件，emplace_back是从C++11开始的一个新特性，一直到后续的C++17，主要是将新元素添加到容器的末尾，元素呢是通过std :: allocator_traits :: construct构造的，通常使用placement-new在容器提供的位置就地构造元素。

以下是官网给出的一段示例代码：使用emplace_back将President类型的对象添加到std :: vector。它演示了emplace_back如何将参数转发到President构造函数，并演示了使用emplace_back如何避免使用push_back时所需的额外复制或移动操作。

#include <vector>
#include <string>
#include <iostream>
 
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(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) = default;
};
 
int main()
{
    std::vector<President> elections;
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
 
    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";
    }
}

output:

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.

从上面的例子中可以看到，使用emplace_back是不需要另外的copy或者移除操作的，而使用push_back则会存在额外的开销，但是在实际工程中也会存在一些问题，实际上并不是所有编译器都会支持c++11的，所以这个也要作为考虑的一个因素。

参考地址：
https://en.cppreference.com/w/cpp/container/vector/emplace_back