Using std::move when passing lvalue

Question

If in my code, I have declared a variable (for example string) and want to pass to some function so my vector would add that value. Do I always need to do this(using the std::move), considering I do not need that passed variable anymore?

Void add(string s){
    my_vector.push_back(move(s));
}

I do know that I can simply pass an rvalue but consider if I needed to declare the string first (making it an rvalue) and then adding it to the vector.

Because I don’t want to copy the variable and then push back to my vector

Your question is unclear. What do you think the code you posted does? What do you think it does if you leave out the `move`? What do you want it to do? Your formulation indicates that you are confused about move semantics in C++, but the way the question is asked, the only thing I can do is link you to a tutorial that explains them from the ground up. — Sebastian Redl, Jan 02 '18 at 09:15
Btw. please, consider, `std::vector::emplace_back()` as alternative to `std::vector::push_back()` which allows "in-place" construction to prevent one copy step. — Scheff's Cat, Jan 02 '18 at 09:20
For better understanding of this, I experimented with vector of an own class where I deleted copy constructor and copy assignment but had to add a move constructor and move assignment. — Scheff's Cat, Jan 02 '18 at 09:25
@Scheff Don't use `emplace_back` if you have a value of the right type already. Only use it when you want to construct a value in-place using constructor arguments. But not when doing so would use the copy or move constructor. `push_back` is the right function for those cases. — Sebastian Redl, Jan 02 '18 at 10:15
"*I don’t want to copy the variable and then push back to my vector*" - but that is exactly what your example is doing. `string s` is passed by value, so it makes a copy of whatever is passed to it. Then you are `move`'ing that copy into the vector — Remy Lebeau, Jan 02 '18 at 19:53

score 1 · Accepted Answer · answered Jan 02 '18 at 09:28

If you want the string to be moved, you have to say so. Otherwise it will be copied.

But unless this is a hot spot in your application, the difference might be small enough not to be noticed.

For a std::string in particular, the major implementations use a Small String Optimization, where short strings (typically 10-20 bytes) will be stored inside the string object to save on heap allocations. In that case moving and copying will do the same thing for the short strings.

score 0 · Answer 2 · answered Jan 02 '18 at 09:47

Depending on what you intend to do with your add method and how you plan to use it, you should consider using a universal reference: you declare the s argument as a rvalue reference and then:

if the calling argument is a lvalue, s will be a lvalue reference;
if the calling argument is a rvalue, s will be a rvalue reference.

Hence, the following code will benefit from move semantic optimisation whenever possible:

void add(string && s) {
    my_vector.push_back(s);
}

Using std::move when passing lvalue

2 Answers2