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Ian
Guest
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 Posted: Wed Jul 27, 2005 9:38 pm Post subject: why can't a template function have extern "C" linkage? |
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The title says it all.
I can see the case where a function is to be called directly from C, the
name mangling will stuff this up.
But I can't see a reason why a template function can't be given extern
"C" linkage where it is to be assigned to a C function pointer.
Ian
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Andre Kostur
Guest
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| Posted: Wed Jul 27, 2005 9:50 pm Post subject: Re: why can't a template function have extern "C" linkage? |
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Ian <noone (AT) nowhere (DOT) com> wrote in news:1122500290.491907@drone2-svc-
skyt.qsi.net.nz:
| Quote: | The title says it all.
I can see the case where a function is to be called directly from C, the
name mangling will stuff this up.
But I can't see a reason why a template function can't be given extern
"C" linkage where it is to be assigned to a C function pointer.
Ian
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Um... how would your C program (which doesn't know anything about
templates) include the header file where the template function is declared?
Keep in mind that a template function doesn't actually exist until it is
instantiated.... (not sure that "instantiated" is the right word....)
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Victor Bazarov
Guest
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| Posted: Wed Jul 27, 2005 10:23 pm Post subject: Re: why can't a template function have extern "C" linkage? |
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Andre Kostur wrote:
| Quote: | [..]
Keep in mind that a template function doesn't actually exist until it is
instantiated.... (not sure that "instantiated" is the right word....)
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It is the right word.
V
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Ian
Guest
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| Posted: Wed Jul 27, 2005 11:08 pm Post subject: Re: why can't a template function have extern "C" linkage? |
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Andre Kostur wrote:
| Quote: | Ian <noone (AT) nowhere (DOT) com> wrote in news:1122500290.491907@drone2-svc-
skyt.qsi.net.nz:
The title says it all.
I can see the case where a function is to be called directly from C, the
name mangling will stuff this up.
But I can't see a reason why a template function can't be given extern
"C" linkage where it is to be assigned to a C function pointer.
Ian
Um... how would your C program (which doesn't know anything about
templates) include the header file where the template function is declared?
Keep in mind that a template function doesn't actually exist until it is
instantiated.... (not sure that "instantiated" is the right word....)
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As I said, I understand why this case can't be, but the problem I had
involved setting up some callbacks for a C driver. The driver API has a
number of structs with function pointers for various callbacks.
So in this case, the C function pointers will be assigned in C++ code,
after the templates have been instantiated. the names are irrelevant to
the C code.
Ian
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Jay Nabonne
Guest
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| Posted: Wed Jul 27, 2005 11:21 pm Post subject: Re: why can't a template function have extern "C" linkage? |
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On Thu, 28 Jul 2005 11:08:17 +1200, Ian wrote:
| Quote: | As I said, I understand why this case can't be, but the problem I had
involved setting up some callbacks for a C driver. The driver API has a
number of structs with function pointers for various callbacks.
So in this case, the C function pointers will be assigned in C++ code,
after the templates have been instantiated. the names are irrelevant to
the C code.
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What problem are you running into? I'm not sure of your distinction of "C"
vs. "C++" functions, unless you mean stand-alone vs. class member
functions. Is your template creating a template function or a templated
class? If it's a class, then you can't use the address of the member
functions whether it's a template or not.
As far as I know, 'extern "C"' is irrelevant as far as function
pointers go.
If all else fails, create a standalone (non-template) function that calls
the template one, and set a pointer to it instead.
- Jay
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Ian
Guest
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| Posted: Wed Jul 27, 2005 11:47 pm Post subject: Re: why can't a template function have extern "C" linkage? |
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Jay Nabonne wrote:
| Quote: | On Thu, 28 Jul 2005 11:08:17 +1200, Ian wrote:
As I said, I understand why this case can't be, but the problem I had
involved setting up some callbacks for a C driver. The driver API has a
number of structs with function pointers for various callbacks.
So in this case, the C function pointers will be assigned in C++ code,
after the templates have been instantiated. the names are irrelevant to
the C code.
What problem are you running into? I'm not sure of your distinction of "C"
vs. "C++" functions, unless you mean stand-alone vs. class member
functions. Is your template creating a template function or a templated
class? If it's a class, then you can't use the address of the member
functions whether it's a template or not.
In an ideal world, I wanted something like: |
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T>
struct Fred : FredFromC
{
Fred() { fnPointer = bla<T>; }
};
Where fnPointer is a member of FredFromC.
| Quote: | As far as I know, 'extern "C"' is irrelevant as far as function
pointers go.
That's just it, there is a difference and it is relevant. |
Ian
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Jay Nabonne
Guest
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| Posted: Thu Jul 28, 2005 12:24 am Post subject: Re: why can't a template function have extern "C" linkage? |
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On Thu, 28 Jul 2005 11:47:09 +1200, Ian wrote:
| Quote: | In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
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This compiles for me under Visual C++ 7.0 and gcc 3.2.3:
// Begin code
struct FredFromC
{
int (*fnPointer)();
};
template <typename T> int bla() { /*return T().bla();*/ return 0; }
template <typename T>
struct Fred : FredFromC
{
Fred() { fnPointer = bla<T>; }
};
Fred<int> myStruct;
// End code
It even works if I put 'extern "C"' around the "FredFromC" structure
definition.
Again, what is the problem that you're having (e.g. what is the error
message).
- Jay
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Victor Bazarov
Guest
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| Posted: Thu Jul 28, 2005 12:26 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Ian wrote:
| Quote: | In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
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And how is it going to be used?
V
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Ian
Guest
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| Posted: Thu Jul 28, 2005 12:31 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Victor Bazarov wrote:
| Quote: | Ian wrote:
In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
And how is it going to be used?
By the operating system as a callback into the driver. FredFromC is a |
structure defined by the OS to be populated with callbacks for various
driver functions.
Ian
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Ian
Guest
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| Posted: Thu Jul 28, 2005 12:38 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Jay Nabonne wrote:
| Quote: | On Thu, 28 Jul 2005 11:47:09 +1200, Ian wrote:
In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
Again, what is the problem that you're having (e.g. what is the error
message).
If I compile the following (based on your example): |
extern "C" {
struct FredFromC
{
int (*fnPointer)();
};
}
template <typename T> int bla() { return 0; }
template <typename T>
struct Fred : FredFromC
{
Fred() { fnPointer = bla<T>; }
};
int
main()
{
Fred<int> myStruct;
}
The compiler correctly warns:
"/tmp/x.cc", line 13: Warning (Anachronism): Assigning int(*)() to
extern "C" int(*)().
In this example, I can see no practical reason why bla() couldn't have
been declared as extern "C".
Ian
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Jay Nabonne
Guest
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| Posted: Thu Jul 28, 2005 12:49 am Post subject: Re: why can't a template function have extern "C" linkage? |
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On Thu, 28 Jul 2005 12:38:08 +1200, Ian wrote:
| Quote: |
The compiler correctly warns:
"/tmp/x.cc", line 13: Warning (Anachronism): Assigning int(*)() to
extern "C" int(*)().
In this example, I can see no practical reason why bla() couldn't have
been declared as extern "C".
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Ah, I see. A warning. Unfortunately, I can't coax any of the compilers I
have at my disposal to warn me about that, so I can't even begin to try to
figure out how to solve it (as I have no way to test).
I suppose this does no better:
template <typename T>
struct Fred : FredFromC
{
static int bla() { /*return T().bla();*/ return 0; }
Fred() { fnPointer = bla<T>; }
};
Good luck...
- Jay
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Jay Nabonne
Guest
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| Posted: Thu Jul 28, 2005 12:53 am Post subject: Re: why can't a template function have extern "C" linkage? |
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On Thu, 28 Jul 2005 00:49:45 +0000, Jay Nabonne wrote:
| Quote: | On Thu, 28 Jul 2005 12:38:08 +1200, Ian wrote:
template <typename T
struct Fred : FredFromC
{
static int bla() { /*return T().bla();*/ return 0; }
Fred() { fnPointer = bla
|
// Oops. I meant:
Fred() { fnPointer = bla; }
| Quote: | };
Good luck...
- Jay
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Victor Bazarov
Guest
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| Posted: Thu Jul 28, 2005 1:12 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Ian wrote:
| Quote: | Victor Bazarov wrote:
Ian wrote:
In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
And how is it going to be used?
By the operating system as a callback into the driver. FredFromC is a
structure defined by the OS to be populated with callbacks for various
driver functions.
|
Sorry, that basically means nothing here. You should consider posting
the C code that calls whatever callbacks you are trying to define.
V
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Ian
Guest
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| Posted: Thu Jul 28, 2005 2:07 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Victor Bazarov wrote:
| Quote: | Ian wrote:
Victor Bazarov wrote:
Ian wrote:
In an ideal world, I wanted something like:
extern "C" {
template <typename T> int bla() { return T().bla(); }
}
so I could set up the C struct thus:
template <typename T
struct Fred : FredFromC
{
Fred() { fnPointer = bla
};
Where fnPointer is a member of FredFromC.
And how is it going to be used?
By the operating system as a callback into the driver. FredFromC is a
structure defined by the OS to be populated with callbacks for various
driver functions.
Sorry, that basically means nothing here. You should consider posting
the C code that calls whatever callbacks you are trying to define.
It's part of an operating system and I don't have it. |
The calling code is irrelevant to the original question. I'm still none
the wiser as to why template functions can't be given extern "C" linkage.
Apart form the mangled name, a template function is (well OK, can be) no
different than any other with the same signature:
#include <iostream>
template <typename T> void tem( int n ) { std::cout << n << std::endl; }
void normal( int n ) { std::cout << n*2 << std::endl; }
typedef void (*Fn)( int );
int
main()
{
Fn fn1 = &tem
Fn fn2 = &normal;
fn1( 2 );
fn2( 2 );
}
but we can't do
extern "C"{
template <typename T> void tem( int n ) {}
void normal( int n ) {}
typedef void (*Fn)( int );
}
Ian
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Victor Bazarov
Guest
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| Posted: Thu Jul 28, 2005 3:28 am Post subject: Re: why can't a template function have extern "C" linkage? |
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Ian wrote:
| Quote: | [..]
The calling code is irrelevant to the original question. I'm still
none the wiser as to why template functions can't be given extern "C"
linkage.
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(a) It doesn't make sense since there is no way to use templates
from C -- it just doesn't have that mechanism. To learn more on
"why" certain things the way they are, post to comp.std.c++, that
is the "why" newsgroup. We here talk mostly "how".
| Quote: | Apart form the mangled name, a template function is (well OK, can be)
no different than any other with the same signature:
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(b) I think you're a bit confused. There are no "template functions".
There are only "function templates". What you have here is
a template, not a function.
(c) Templates can have linkage, but they cannot have "C" linkage because
the Standard prohibits it in 14/4.
| Quote: | #include <iostream
template
std::endl; } void normal( int n ) { std::cout << n*2 << std::endl; }
typedef void (*Fn)( int );
int
main()
{
Fn fn1 = &tem
Fn fn2 = &normal;
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There is no need for '&' here, BTW
| Quote: |
fn1( 2 );
fn2( 2 );
}
but we can't do
extern "C"{
template <typename T> void tem( int n ) {}
void normal( int n ) {}
typedef void (*Fn)( int );
}
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Correct, we can't.
V
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