[c++] push_back vs emplace_back


2 Answers

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

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

void emplace_back(Type&& _Val);
void push_back(const Type& _Val);
void push_back(Type&& _Val);

As there is a push_back overload taking a rvalue reference I don't quite see what the purpose of emplace_back becomes?




emplace_back conforming implementation will forward arguments to the vector<Object>::value_typeconstructor when added to the vector. I recall Visual Studio didn't support variadic templates, but with variadic templates will be supported in Visual Studio 2013 RC, so I guess a conforming signature will be added.

With emplace_back, if you forward the arguments directly to vector<Object>::value_type constructor, you don't need a type to be movable or copyable for emplace_back function, strictly speaking. In the vector<NonCopyableNonMovableObject> case, this is not useful, since vector<Object>::value_type needs a copyable or movable type to grow.

But note that this could be useful for std::map<Key, NonCopyableNonMovableObject>, since once you allocate an entry in the map, it doesn't need to be moved or copied ever anymore, unlike with vector, meaning that you can use std::map effectively with a mapped type that is neither copyable nor movable.







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