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"; 
  } 
}