C++ 中"emplace_back" 与 "push_back&

#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"; 
  } 
}