C++ 中"emplace_back" 與 "push_back" 的區別。本站提示廣大學習愛好者:(C++ 中"emplace_back" 與 "push_back" 的區別)文章只能為提供參考,不一定能成為您想要的結果。以下是C++ 中"emplace_back" 與 "push_back" 的區別正文
C++ 中"emplace_back" 與 "push_back" 的區別
emplace_back和push_back都是向容器內添加數據.
對於在容器中添加類的對象時, 相比於push_back,emplace_back可以避免額外類的復制和移動操作.
"emplace_back avoids the extra copy or move operation required when using push_back."
參見: http://en.cppreference.com/w/cpp/container/vector/emplace_back
注意下面代碼中的emplace_back和push_back的添加方式(VS2012下編譯通過):
#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); //沒有類的創建 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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