Karl Tomlinson wrote:
Aryeh Gregor writes:
The compiler is required to use the move constructor (if one exists) instead of
the copy constructor when constructing the return value of a function, and also
when initializing an object from the return value of a function, or assigning
the return value of a function. So if you have
Foo GetFoo() { Foo f(1, 2, 7); /* do lots of stuff to f */ return f; }
void MyFunction() { Foo f = GetFoo(); }
the copy constructor of Foo will not get called anywhere.
I guess that means that with
struct Bar {
Bar(Foo f) : mF(f) {}
Foo GetFoo() { return mF; }
Foo mF;
}
GetFoo() would give away what mF owns?
If so, can we require that be more explicit somehow?
No, Aryeh's comment only applies to local variables.
Foo GetFoo()
{
Foo f1, f2;
if (rand() % 2)
return f2;
return f1;
}
Here C++03 compilers may find it hard to do NRVO because we're returning
different locals. But when a C++11 compiler tries and fails to do NRVO,
it then tries the move constructor.
Note that using ? : in this case does not work, since that results in a
Foo& which therefore gets copy constructed. See
http://stackoverflow.com/a/19698477
--
Warning: May contain traces of nuts.
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