Saturday, October 01, 2005

Return value optimization

In C++, writing a function with a compound return statement like this
const Rational function (void)
return Rational (a,b); // statement 1

can be more efficient than

const Rational function (void)
Rational r(a,b);
return r; // statement 2

when used in the surrounding context such as

Rational c = function (); // initializing c.

because compilers can avoid "invisible" creation and
destruction of temporaries when function returns an object
by value. This is known as "return value optimization".
In the optimized assembly code, object c is directly
initialized by statement 1. You save upto 2 temporaries (and
creation/destruction of them). One is the local object
r and other one is created and destroyed when the
function returns.


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towi said...

I am not absolutely sure, but I think, the rules for RVO are a bit wider. I think the compiler can also optimize if all "return"s directly call a constructor OR all "return"s return the same instance.
This could also be RVO'd:

Obj func()
____return Obj("blue");
____return Obj(42);

This would be, because the compiler can allocate the memory for the return value, then work through the function, and depending on which "return" call the appropriate constructor with the already allocated space.
It would not work if you mixed the two variants.

Anonymous said...

wrong. both forms WILL BE optimized.

Anonymous said...

Following code calls constructor only twice.

Chk Test()
Chk p;

return p;


int main(int argc, char* argv[])
{ Chk q ;

q = Test();

Anonymous said...

Please stop copying codes from famous books and put them in your blog to have fame. To all readers, instead reading these blogs, please read "Efficient C++" series from Scott Mayer.

Anonymous said...

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