C++ - "emplace_back" 和 "push_back" 的区别

"emplace_back" 和 "push_back" 的区别

emplace_back和push_back都是向容器内添加数据.

对于在容器中添加类的对象时, 相比于push_back,emplace_back可以避免额外类的复制和移动操作.

"emplace_back avoids the extra copy or move operation required when using push_back."

参见: http://en.cppreference.com/w/cpp/container/vector/emplace_back

注意下面代码中的emplace_back和push_back的添加方式(VS2012下编译通过):

 

#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);
};

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";
	}
}