VC++ error when using a pointer to a template function

Question

I'm trying to write a template callback function for libcurl. However, when using a pointer to an instance of the template function, VC++ 2008 and 2010 keep giving me this error:

template-callback.cpp(27) : error C2664: 'curl_easy_setopt' : cannot convert parameter 3 from 'size_t (__cdecl *)(void *,size_t,size_t,void *)' to '...' Context does not allow for disambiguation of overloaded function

But GCC (4.5.1) compiles the code without a problem. This is a trimmed version of the code:

#include <string>

template<typename String>
size_t callback(
    void*       ptr
  , size_t  size
  , size_t  nmemb
  , void*       userdata
)
{
  //String* str = static_cast<String*>(userdata);
  size_t    len = size*nmemb;
  //str->append(static_cast<char const*>(ptr), len);
  return len;
}

typedef size_t (*write_callback)(void*, size_t, size_t, void*);
struct CURL;
enum CURLOption { none };
void curl_easy_setopt(CURL*, CURLOption, ...);

void f()
{
  CURL* curl = NULL;
  CURLOption option = none;

  // This gives an error
  curl_easy_setopt(curl, option, &callback<std::string>);

  // This doesn't
  write_callback cb = &callback<std::string>;
  curl_easy_setopt(curl, option, cb);
}

Is this a bug in VC++ or am I doing something wrong?

@Pablo: is this the exact signature of curl_easy_setopt ? I am wondering if the issue does not come from the variadic argument. — Matthieu M.
– Matthieu M., Commented May 27, 2011 at 9:18
@Matthieu: The problem does indeed come from the variadic argument. — Xeo
– Xeo, Commented May 27, 2011 at 9:21
@Xeo: do you have any idea why, gcc-4.3.4 rejects the code too, but gcc-4.5.1 accepts it in C++0x mode. I don't use variadic arguments in general so I am a bit at a loss as to the origin of the issue :/ — Matthieu M.
– Matthieu M., Commented May 27, 2011 at 9:30
@Matthieu M. It is almost the exact signature, except trimmed down to make it compile without dependencies. — Pablo
– Pablo, Commented May 27, 2011 at 17:26

Matthieu M. · Accepted Answer · 2011-05-27 09:45:19Z

1

I have reproduced the issue on ideone (C++03 with gcc-4.3.4):

#include <iostream>

typedef void (*FuncType)(int);

void foo(FuncType);
void bar(...);

template <typename T>
void callback(T t) { std::cout << t << "\n"; }

int main() {
  foo(&callback<int>); // OK
  bar(static_cast<FuncType>(&callback<int>)); // OK
  bar(&callback<int>); // error: address of overloaded function
                       // with no contextual type information
}

The issue seems to come from the interaction of the variadic argument and the function pointer.

Note: in C++0x mode with gcc-4.5.1 it works fine

I surmise that the issue comes from the overload resolution of bar (or curl_easy_setopt in your case).

The problem is that in order to use the ellipsis, the compiler as to decide how to pass the argument: int, double, pointer, ... It seems that it is unable to decide, by itself, what the type of &callback<int> is.

When we use foo, or perform a cast, it is unambiguous because there is no choice.

I suspect a conversion issue, but I don't have a version of the C++03 standard to dig into.

edited May 27, 2011 at 9:45

answered May 27, 2011 at 9:28

Matthieu M.

303k58 gold badges495 silver badges754 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Johannes Schaub - litb Over a year ago

Clang is the only compiler that accepts bool b = &callback<int>; correctly, from what I can see :)

Matthieu M. Over a year ago

@Johannes: ah! great that you're here! Could you explain exactly what's going on ?

Johannes Schaub - litb Over a year ago

@Matthieu It shall accept that according to the FDIS specs, like v4.5 does. See llvm.org/bugs/show_bug.cgi?id=7505 and llvm.org/bugs/show_bug.cgi?id=9208 . IIRC there may be an issue in C++03 that didn't allow this (i.e identifying the correct function lvalue solely based on template arguments, without a correct target type context). Please see the referred-to PRs.

Pablo Over a year ago

@Johannes what are the FDIS specs? Googling doesn't help

Matthieu M. Over a year ago

@Pablo: FDIS stands for Final Draft ISO Standard. It's the last refinement of the drafts before the C++ ISO Standard is actually published (after the FDIS gets vetted by all National ISO bodies). It used to be available for download, but they seem to have removed it.

Collectives™ on Stack Overflow

VC++ error when using a pointer to a template function

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related