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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