emplace_back

参考

push_back和emplace_back

emplace_back

函数定义

//c++11 -- c++17
template< class... Args >
void emplace_back( Args&&... args );

//since c++17
template< class... Args >
reference emplace_back( Args&&... args );

void push_back( const T& value );
//since c++11
void push_back( T&& value );

// vector::emplace
#include <iostream>
#include <vector>

int main ()
{
  std::vector<int> myvector = {10,20,30};

  auto it = myvector.emplace ( myvector.begin()+1, 100 );
  myvector.emplace ( it, 200 );
  myvector.emplace ( myvector.end(), 300 );

  std::cout << "myvector contains:";
  for (auto& x: myvector)
    std::cout << ' ' << x;
  std::cout << '\n';

  return 0;
}

example

input:

#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.

总结

c++11以后push_back也支持右值引用了，但和emplace_back的区别是：emplace_back支持可变参数列表的形式，可以在括号内直接赋值：

elections.emplace_back("Nelson Mandela", "South Africa", 1994);

而push_back必须有一个具体的实现(临时对象)，看得到的参数来匹配对应的拷贝构造函数，operator=函数：

reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));

push_back创建了一个临时对象President，然后通过移动构造函数把临时对象移动过来。而emplace直接通过括号的值来初始化对象。一步到位。

ending