C++ 中"emplace_back" 与 "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):
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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);
};
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";
}
}
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