Re: Different results from different gcc versions

"Alf P. Steinbach" <>
Tue, 16 Mar 2010 04:57:04 +0100
* ks:


I have a problem with a piece of code. I'm in a situation where I have
to compile code using 2 different g++ compilers (2.95 and 4.12). I've
reduced the issue to a small, self contained program (given below)

#include <stdio.h>

struct A {
        int a;
        int b;
} ;

namespace NS {
        void func(struct A *, int);

void NS::func(struct A * a_ptr, int i)
        printf("%ld %d\n", a_ptr, i);

This should probably be a p-format.


namespace NS {
        struct A : public ::A {} ;

struct NS::A* get_A(void)
        return (struct NS::A *) 0;

int main()

                func(get_A(), 1);
This program compiles on g++ 4.12 but fails on g++ 2.95 with the
following error:

test.cpp: In function `int main()':
test.cpp:30: implicit declaration of function `int func(...)'

If I change the call from func(...) to NS::func(...), both the
compilers are happy. I can understand the reason for that. However,
how does the g++ 4.12 compiler build without explicitly specifying the

Argument dependent lookup, a.k.a. ADL a.k.a Koenig lookup (after Andrew Koenig).

Since the type of the argument is "pointer to T" where T is defined in NS, the
overload set for the function includes matching declarations from NS.

Which is the correct behavior?


It's great but it can get pretty tricky and yield unexpected results.

I would have liked for ADL to only apply to operators (where it's necessary),
but it applies to ordinarily named functions, sneaking in subtleties.

Cheers & hth.,

- Alf

PS: Comeau online, at <url:>, is great
for checking out such snippets. And I did, since I was a little unsure about
"pointer to T" as opposed to just T. ADL is that subtle. It's sort of like
database hinting. Great when it just works, which is most of the time, but
correspondingly very ungreat when it doesn't work or doesn't work as expected.

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