std::move before assignment in the same expression

user_2604492 0
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Consider the following snippet:

std::string foo(std::string x) {
    x += "!";
    return x;
}
// ...
std::string y = "Hello";
// ...
y = foo(y);

My questions:

  • Does the compiler recognize that y can be moved when passed to foo, since it's going to be assigned into right after that.
  • If not, is it valid for me to add std::move for passing y to foo: y = foo(std::move(y))? Does it eliminate all copies?
move-semantics c++
1 Answers

Does the compiler recognize that y can be moved when passed to foo, since it's going to be assigned into right after that.

No, it is not allowed to do it. std::move() is merely a cast, in this case into std::string&&. Without the cast, std::string::string(const std::string&) constructor signature will be chosen, which is a copy constructor.

If not, is it valid for me to add std::move for passing y to foo: y = foo(std::move(y))? Does it eliminate all copies?

Sure, you can, and it does eliminate all copies. First, argument y is moved into parameter x. On the return, x is moved into y.

user_4595650
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