Is the following hack legal at all?

[Chris Thomasson]

I was wondering of the foo::m_fp_callback member will really have extern "C"
linkage...

simple code
____________________________
#include <cstdio>
extern "C" typedef void (fp_callback_c_t) (void*);
struct foo {
void operator()();
static fp_callback_c_t m_fp_callback;
};
void foo::operator()() {
printf("(%p) - foo::operator()\n", (void*)this);
}
extern "C" void foo::m_fp_callback(void *state) {
foo* const _this = reinterpret_cast<foo*>(state);
(*_this)();
}
int main() {
foo f;
f.m_fp_callback(&f);

printf("\n\n____________________\npress <ENTERto exit...\n");
getchar();
return 0;
}
--------

I think the answer is no way will it have C linkage... Although I can't seem
to make any compilers complain about it...

Humm...
--
Chris M. Thomasson
http://appcore.home.comcast.net

[Alf P. Steinbach]

* Chris Thomasson:

I was wondering of the foo::m_fp_callback member will really have extern
"C" linkage...

Not in standard C++.

Also, you can't declare a member function as

T f;

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Branimir Maksimovic]

On May 13, 6:29 pm, "Chris Thomasson" <[e-mail]:

I was wondering of the foo::m_fp_callback member will really have extern "C"
linkage...

No, member functions have to be always mangled. This is simply
ignored.
Just cast it to extern "C" function pointer before passing to library
function.
This will not work if compiler you use have different calling
conventions
for C and C++ linkage.
But then again, I guess, such compilers will have means to
explicitelly
specify calling convention anyway.

Greetings, Branimir.

[James Kanze]

On May 13, 6:29 pm, "Chris Thomasson" <[e-mail]:

I was wondering of the foo::m_fp_callback member will really have extern "C"
linkage...

A member function, even a static member, will always have C++
linkage, see §7.5/4: "A C language linkage is ignored for the
names of class members and the member function type of class
member functions."

simple code
____________________________
#include <cstdio>

extern "C" typedef void (fp_callback_c_t) (void*);

struct foo {
void operator()();
static fp_callback_c_t m_fp_callback;

};

void foo::operator()() {
printf("(%p) - foo::operator()\n", (void*)this);

}

extern "C" void foo::m_fp_callback(void *state) {
foo* const _this = reinterpret_cast<foo*>(state);
(*_this)();

}

int main() {
foo f;
f.m_fp_callback(&f);

printf("\n\n____________________\npress <ENTERto exit...\n");
getchar();
return 0;}
--------

I think the answer is no way will it have C linkage...
Although I can't seem to make any compilers complain about
it...

I'm not sure where you expect a complaint in the above. Despite
the `extern "C"' in the program text, there's not the slightest
`extern "C"' in the program itself, so there's nothing for the
compiler to complain about. (Perhaps a warning because of an
`extern "C"' which is ignored?)

Try passing &foo::m_fp_callback to a function which expects a
fp_callback_c_t as argument, and you should get an error. (Sun
CC complains in this case, for example.)

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[James Kanze]

On May 13, 7:13 pm, "Alf P. Steinbach" <[e-mail]:

* Chris Thomasson:

I was wondering of the foo::m_fp_callback member will really have extern
"C" linkage...

Not in standard C++.

Also, you can't declare a member function as

T f;

Are you sure? §8.3.5/7 says "A typedef of function type may be
used to declare a function but shall not be used to define a
function", and I don't find any additional limitations
concerning member functions (although I'm not quite sure what it
should mean in the case of non-static member functions).

--
James Kanze (GABI Software) email:ja*********@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[Alf P. Steinbach]

* Alf P. Steinbach:

* Chris Thomasson:

>I was wondering of the foo::m_fp_callback member will really have
extern "C" linkage...

Not in standard C++.

Right.

Also, you can't declare a member function as

T f;

Wrong, §8.3.5/7.

Sorry.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Alf P. Steinbach]

* James Kanze:

On May 13, 7:13 pm, "Alf P. Steinbach" <[e-mail]:

>* Chris Thomasson:

>>I was wondering of the foo::m_fp_callback member will really have extern
"C" linkage...

>Not in standard C++.

>Also, you can't declare a member function as

> T f;

Are you sure? §8.3.5/7 says "A typedef of function type may be
used to declare a function but shall not be used to define a
function", and I don't find any additional limitations
concerning member functions (although I'm not quite sure what it
should mean in the case of non-static member functions).

Well you beat me to it by several seconds (checking posting times). :-)

There's also a note later on specifically regarding member function,
with an example.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?