(VC7) Basic Object Const/Dest question

[ibm]

I am running a simple test

/*******CODE************/
class A
{
int _x;
public:
A(int i)
{
_x = i;
printf("A %d\n", _x);
}
A(const A& a)
{
_x = a._x;
printf("CopyA %d\n", _x);
}
~A()
{
printf("~A %d\n", _x);
}
};

A& f1()
{
A a(1);
return a;
}
A& f2()
{
return A(2);
}
A f3()
{
A a(3);
return a;
}
A f4()
{
return A(4);
}

int _tmain(int argc, _TCHAR* argv[])
{
A a1 = f1();
A a2 = f2();
A a3 = f3();
A a4 = f4();
printf("destroying all\n");
return 0;
}
/*******OUTPUT**********/
A 1
~A 1
CopyA -85899346 //copycon for 1
A 2
~A 2
CopyA -85899346 //copycon for 2
A 3
CopyA 3 //copycon for 3
~A 3
A 4 //copycon for 4
destroying all
~A 4
~A 3
~A -858993460
~A -858993460
-------------------------
This output is very much understandable bcz the rule is never return a
local reference from a function.

Now I remove the copy constructor from the code and get the following
ouput

/******OUTPUT with copycon ***********/
A 1
~A 1
A 2
~A 2
A 3
~A 3
A 4
destroying all
~A 4
~A 3
~A 2
~A 1
--------------------------------
Q1, if the rule "never return a local reference from a function"
remains intact than is this ( ~A 2, ~A 1 after destroying all )
something to do with how VC++ compiler is handling it. Also I think in
some compilers returning a reference from a function returns a compiler
error.
Q2, Why in both the codes a destructor for A 4 is not called before
destroying all statement that is how exactly is A a;return a;
differnt from return A;

thanks

 

[Victor Bazarov]

[...] --------------------------------
Q1, if the rule "never return a local reference from a function"
remains intact than is this ( ~A 2, ~A 1 after destroying all )
something to do with how VC++ compiler is handling it. Also I think in
some compilers returning a reference from a function returns a compiler
error.

I am sure I don't understand your question (if there is one). Returning
a reference to a local object is legal but causes undefined behaviour *if*
the reference is used. Compilers are not required to identify undefined
behaviour. If some do, that's fine. If some don't, that's just as OK.

Q2, Why in both the codes a destructor for A 4 is not called before
destroying all statement that is how exactly is A a;return a;
differnt from return A;

It's not different at all.

What you should also print in the destructor is the value of 'this'. You
cannot judge which object you're destroying and what led to its creation
if you can't tell if it was the one you constructed or it's a temporary to
help transfer some value from a function or during the initialisation.

V