C++ 中＂emplace_back＂ 与 ＂push_back＂ 的区别

#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";   } }