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.
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
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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.
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Chk Test()
{
Chk p;
p.Set(1);
return p;
}
int main(int argc, char* argv[])
{ Chk q ;
q = Test();
}
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