Re: friend 'operator new' in inheritance as a template
James Kanze <james.kanze@gmail.com> kirjutas:
On Jun 14, 9:36 pm, Paavo Helde <nob...@ebi.ee> wrote:
"wo3...@gmail.com" <wo3...@gmail.com> kirjutas:
#include <iostream>
#include <map>
#include <utility>
//
// Base
// / | \
// Derived1 Derived2 \
// \ | /
// Derived3
//
template< typename _T >
struct Base {
friend void * operator new ( size_t _size ){
std::cout << typeid( _T ).name() << std::endl;
As far as I understand, operator new cannot be a template, and
a friend declaration does not make it into a template
automatically anyway, so the _T symbol should not be visible
inside the function. If the compiler still compiles this, I
think this is a compiler bug.
The whole thing is fishy. This basically says that 1) the
global operator new is a friend of Base, and 2) provides an
implementation of the global operator new. Which is, I guess,
legal. Once... because this is a template, it's going to
provide a new implementation of the global operator new each
time the template is instantiated. Which is a violation of the
one definition rule, and so undefined behavior. (Since the
function is defined *in* the template, I'm pretty sure that the
use of _T is legal.)
Yes, I somehow assumed that the definition of the friend function should
not depend on whether it is "inline" in the friend declaration or not.
And if it is not, then _T can't be visible in the function body. And _T
is not participating in the function signature, so it would be impossible
to forward this to the non-inline non-template function definition.
right?
But anyway, as Comeau online does not complain about use of _T (renamed
to T to be sure), it seems my assumption was wrong. So the meaning of the
function body can differ depending on if it is defined inline in the
friend declaration, or separately. You learn something new every day...
There's also the problem that because the function is defined in
a class definition, it is inline; the global operator new
function is not allowed to be inline (for the obvious reason
that it's likely to have been used in some library code
somewhere, and that code won't see your inline definition).
[...]
struct Derived1 : Base< Derived1 > {
};
struct Derived2 : Base< Derived2 >{
};
struct Derived3 : Derived1, Derived2, Base< Derived3 >{
};
Note that the code doesn't correspond to the diagram in comments
at the top.
How is that? I can't see anything wrong here.
int main(){
Derived1 * d1 = new Derived1; // prints 8Derived3
Derived2 * d2 = new Derived2; // prints 8Derived3
}
You have provided illegal source to the compiler; the results
can be whatever. I am not sure if the compiler is obliged to
produce a diagnostic; I suspect it is. Maybe you should file a
bug report to the compiler provider.
It's undefined behavior, but I can guess what is happening.
He's instantiated the template three times, which provides three
different implementations of the global operator new. The
compiler just happened to use the last one it saw.
Yes, you are right, this appears to be what gcc is doing. It seems VC++
9.0 detects the multiple definitions error:
error C2084: function 'void *Base<T>::operator new(size_t)' already has a
body
It seems Comeau online has the same opinion:
"ComeauTest.c", line 15: error: function "operator new(size_t)" has
already been
defined
friend void * operator new ( size_t _size ){
^
detected during instantiation of class "Base<T> [with
T=Derived2]"
(In addition to mention an error regarding the incompatible throw
specification).
Thanks, and regards
Paavo