TITLE: returning objects by value efficiently

(Newsgroups: comp.lang.c++.moderated)

[ This is a well-known technique for improving efficiency when
  you have to return an object by value. -adc ]


AUTHOR: clamage@Eng.Sun.COM (Steve Clamage), 2 May 96


[...]

If you write code like this:
	T f()
	{
		T val;
		...
		return val;
	}
the compiler is obliged to create and destroy the local variable "val"
and return a copy. (Actually, there is currently some discussion in
the C++ Committee about relaxing that rule, but I don't know what the
outcome will be. The difficulty is the violence it would do to the
C++ object model and to the meaning of sequence points.)

But if you can write the code like this:
	T f()
	{
		...
		return T(...);
	}
the compiler is allowed to construct the returned value directly in the
target, possibly avoiding an extra copy.