emplace_back()

函数原型：

template <class... Args>void emplace_back (Args&&... args);

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。
给出一个示例，这个示例很有用。

#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(const President& other): name(std::move(other.name)), country(std::move(other.country)), year(other.year){std::cout << "I am being copy 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);
};  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.
Franklin Delano Roosevelt was re-elected president of the USA in 1936.

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