emplace_back和push_back的区别

按照cppreference上的代码，简单测试了下

首先emplace_back和push_back区别在于：

• push_back会先在新的内存中构造临时对象，再调用拷贝构造函数将该对象拷贝到vector的末尾
• emplace_back则直接在vector相应的内存上构造对象，不产生临时对象

下面是cppreference的例子：

#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";
    }
}

输出为：

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.

可以看到，push_back调用了两个构造函数

---

再多添加几个元素：

#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::cout << std::endl;
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::cout << std::endl;
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::cout << std::endl;

 
    for (President const& president: elections) {
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
    }
}

输出为：

emplace_back:
I am being constructed.

I am being constructed.
I am being moved.

I am being constructed.
I am being moved.
I am being moved.

Nelson Mandela was re-elected president of South Africa in 1994.
Nelson Mandela was re-elected president of South Africa in 1994.
Nelson Mandela was re-elected president of South Africa in 1994.

为什么也调用了多次拷贝构造呢？因为和原本vector中剩下元素的数量对应，猜测因为发生了整体的vector内存搬移

先进行内存reverse，再进行测试看看

#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;
    
    elections.reserve(3);
    
    std::cout << "emplace_back:\n";
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::cout << std::endl;
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::cout << std::endl;
    elections.emplace_back("Nelson Mandela", "South Africa", 1994);
    std::cout << std::endl;

 
    for (President const& president: elections) {
        std::cout << president.name << " was re-elected president of "
                  << president.country << " in " << president.year << ".\n";
    }
}

则可看到输出只调用了普通构造函数

emplace_back:
I am being constructed.

I am being constructed.

I am being constructed.

Nelson Mandela was re-elected president of South Africa in 1994.
Nelson Mandela was re-elected president of South Africa in 1994.
Nelson Mandela was re-elected president of South Africa in 1994.

---

结论：

• emplace_back并不能完全代替push_back，失效场景之后待补
• 尽量使用reserve先请求好够用的空间，对性能的提升很重要
• 要善用cppreference网站，介绍权威，同时示例代码设计很典型，并且可以在线修改编译测试，很有收获