std::vector::emplace_back and std::move

Question

Is there any advantage of using std::vector::emplace_back and std::move together? or it is just redundant since std::vector::emplace_back will do an inplace-construction?

Cases for clarification:

std::vector<std::string> bar;

First:

bar.emplace_back(std::move(std::string("some_string")));

Second:

std::string str("some_string");
bar.emplace_back(std::move(str));

Third:

bar.emplace_back(std::move("some_string"));

Answer 1

In the second version, there is an advantage. Calling emplace_back will call the move constructor of std::string when std::move is used, which could save on a copy (so long as that string isn't stored in a SSO buffer). Note that this is essentially the same as push_back in this case.

std::move in the first version is unnecessary, as the string is already a prvalue.

std::move in the third version is irrelevant, as a string literal cannot be moved from.

The simplest and most efficient method is this:

bar.emplace_back("some_string");

That requires no unnecessary std::string constructions as the literal is perfect-forwarded to the constructor.