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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

main()
{
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.

Comments

Anonymous said…
Good insightful content... Cool use of a blog
Wed Mar 15, 03:24:00 AM PST
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()
{
__if(sunshine)
____return Obj("blue");
__else
____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.
[towi@geocities.com]
Wed Jun 04, 12:53:00 AM PDT
Anonymous said…
wrong. both forms WILL BE optimized.
Sun Sep 27, 08:50:00 PM PDT
Anonymous said…
Following code calls constructor only twice.

Chk Test()
{
Chk p;
p.Set(1);

return p;

}

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

q = Test();
}





http://vijayvcplusplus.blogspot.com/
Thu Jul 14, 02:34:00 AM PDT
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.
Wed May 22, 03:54:00 AM PDT
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. Most of the codes are copied from that book and if you read from book, you will get much more insight about these topics.
Wed May 22, 03:55:00 AM PDT
Anonymous said…
Striking blog. I liked the site I will be back
again! Websurfing is a good way to find blogs like
yours.
Everyd
thanks you!
gclub casino
Fri Jun 23, 09:24:00 AM PDT
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