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

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我是靠谱客的博主懵懂黑米，这篇文章主要介绍C++:Vector中push_back和emplace_back到底有什么区别？，现在分享给大家，希望可以做个参考。

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。

复制代码

#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";
    }
}

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#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:

复制代码

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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.