push_back vs emplace_back

Question

I\'m a bit confused regarding the difference between push_back and emplace_back.

void emplace_back(Type&& _Val);
void push_         


        

Answer 1

emplace_back shouldn't take an argument of type vector::value_type, but instead variadic arguments that are forwarded to the constructor of the appended item.

template  void emplace_back(Args&&... args); 

It is possible to pass a value_type which will be forwarded to the copy constructor.

Because it forwards the arguments, this means that if you don't have rvalue, this still means that the container will store a "copied" copy, not a moved copy.

 std::vector vec;
 vec.emplace_back(std::string("Hello")); // moves
 std::string s;
 vec.emplace_back(s); //copies

But the above should be identical to what push_back does. It is probably rather meant for use cases like:

 std::vector > vec;
 vec.emplace_back(std::string("Hello"), std::string("world")); 
 // should end up invoking this constructor:
 //template pair(U&& x, V&& y);
 //without making any copies of the strings