why visual studio does not optimize constructor in this case

[George2]

Hello everyone,


Why visual studio does not optimize constructor in this case? I do not understand what the MSDN mentioned,

if use different named object, compiler can not optimize. Why?

http://msdn2.microsoft .com/en-us/library/ms364057(vs.80) .aspx

#include <stdio.h>
class RVO
{
public:
 
            RVO(){printf("I am in constructor\n");}
            RVO (const RVO& c_RVO) {printf ("I am in copy constructor\n");}
            int mem_var;       
};
RVO MyMethod (int i)
{
            RVO rvo;
            rvo.mem_var = i;
      if (rvo.mem_var == 10)
         return (RVO());
            return (rvo); 
}
int main()
{
            RVO rvo;
            rvo=MyMethod(5);
}
 

Output is,

I am in constructor
I am in constructor
I am in copy constructor

My expected output is,

I am in constructor
I am in constructor


thanks in advance,
George

[George2]

Hi weaknessforcats ,


I agree it is not the same object instance, but each different object is doing similar things, making a temporary object on the function return stack and assign it to the return value, then destruct the temporary object on function stack. This is the area which NRVO covers, why it could not optimize, I do not know.

It is appreciated if you could share some more perpectives. :-)

NRVO does not optimize if there are multiple return paths because it's not the same object every time.

regards,
George

[weaknessforcats]

You said it yourself: each different object.

NRVO is Named Returned Value Object. Not Objects.

You have to have only one object be the named return value.

[George2]

Hi weaknessforcats ,


I am confused. Named Return Value Optimization, is not the same as only one named return value optimization.

You can test that if you have two different variable instances, different names for different return paths, NRVO still does not work.

I am not sure why there is such limitation. I am not sure whether I have missed or mis-understood some important aspects of NRVO, which blocks my understanding of why multiple return names with multiple return paths blocks compiler from optimization. Any ideas?

You said it yourself: each different object.

NRVO is Named Returned Value Object. Not Objects.

You have to have only one object be the named return value.

regards,
George

[weaknessforcats]

Any ideas?

You can research the C++ language spec. Once you find that multiple return pathes prevent NRVO, then you can try to find the ANSI committee C++ design jnotes where that conclusion was reached.

You will never get to the bottom of this and what you think about it doesn't count. You need to find hard factual documentation to support your position. Nothing I can say is going to help.

[George2]

Thanks all the same, I appreciate your help all the way.

You can research the C++ language spec. Once you find that multiple return pathes prevent NRVO, then you can try to find the ANSI committee C++ design jnotes where that conclusion was reached.

You will never get to the bottom of this and what you think about it doesn't count. You need to find hard factual documentation to support your position. Nothing I can say is going to help.

regards,
George