Is it legal to pass a this pointer returned from a temporary object to a
function?  Here's an example:

#include <iostream>

struct A {
 A() {i = 123456;}
 const A* Ptr() {return this;}
 int i;
};

void fn(const A* a) {
 std::cout << a->i << std::endl;
}

int main() {
 fn(A().Ptr());
 return 0;
}

I expected that the temporary object of class A would have been overwritten
on the stack by the parameters to fn.  However, upon trying it, I found that
this code worked when compiling under egcs-2.91.57 and Microsoft Visual C++
6 SP2 targeting Win32.

Is this legal C++?  Or did I just get "lucky" that it worked?

(The reason I'm doing this is that I am writing some glue between two fixed
interfaces that take pointers to different structures, say Source and Dest.
My function gets passed a Source pointer and must call a function passing
Dest pointer.  There are many calls that do this, so I wanted to write a
class to inherit from Dest that would be equivalent to A above, but take a
Source* in its constructor.)
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Ed Brey wrote:

> Is it legal to pass a this pointer returned from a temporary object to a
> function?  Here's an example:
>
> #include <iostream>
>
> struct A {
>  A() {i = 123456;}
>  const A* Ptr() {return this;}
>  int i;
> };
>
> void fn(const A* a) {
>  std::cout << a->i << std::endl;
> }
>
> int main() {
>  fn(A().Ptr());
>  return 0;
> }

The temporary is not destroyed until the end of the expression, so IMHO, this
should be legal.

Bruce Visscher



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