Re: rvalues and lvalues

On Dec 9, 6:56 am, Frank Birbacher wrote:


If you want to be sure about it, post an example. ;-)

I'm responding because I'd like to stress that std::move should only
be used when necessary because in some cases it's actually a bit
counter productive. Example:

  unique_ptr<int> source() {
    unique_ptr<int> p (new int(42));
    return std::move(p);
  }

Here, the std::move call is not needed. Even though, p is an lvalue
expression, the compiler knows that it refers to a local object which
is about to be destroyed right on exiting the function. So, in this
case, the compiler will automatically consider a move construction.
There is something even better than a move construction: a copy/move
elision (more specifically in this case: NRVO = named return value
optimization). The use of std::move actually inhibits this
optimization. So, prefer to write

  unique_ptr<int> source() {
    unique_ptr<int> p (new int(42));
    return p;
  }

instead (without std::move). :-)

Cheers!
SG

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