gcc error with Python C API code - “ISO C++ forbids casting between pointer-to-function and pointer-to-object”

Question

The following code fragment does nothing, but illustrates the problem. It was extracted from some Boost Python code, which uses the Numpy C API. This was tested with the backport of a gcc 4.7 snapshot from Debian unstable to squeeze.

#include <boost/python/object.hpp>
#include <numpy/arrayobject.h>

int main(void)
{
  PyObject* obj=0;
  npy_int64 val;
  PyArray_ScalarAsCtype(obj, &val);
  return 0;
}

I'm compiling like this.

g++-4.7 -o warn.o -c -isystem /usr/include/python2.6 -fdiagnostics-show-option -ftemplate-depth-100 -fno-strict-aliasing -ansi -pedantic -Wextra -Wall -Werror -Wno-unused-function -Wc++0x-compat -g -O3 -std=c++11 -I/usr/include/python2.6 warn.cc
warn.cc: In function 'int main()':
warn.cc:8:3: error: ISO C++ forbids casting between pointer-to-function and pointer-to-object [-Werror]
cc1plus: all warnings being treated as errors

The problem is the -pedantic and the PyArray_ScalarAsCtype line of code. Without -pedantic the following compiles without error

g++-4.7 -o warn.o -c -isystem /usr/include/python2.6 -fdiagnostics-show-option -ftemplate-depth-100 -fno-strict-aliasing -ansi -Wextra -Wall -Werror -Wno-unused-function -Wc++0x-compat -g -O3 -std=c++11 -I/usr/include/python2.6 warn.cc
g++-4.7 -o warn warn.o -L/usr/lib/python2.6/config -lpython2.6 -lboost_python

Note: I added the =0 to suppress an uninitialized warning. Like I said, the code doesn't do anything.

I'd like to either suppress or remove the warning and keep the -pedantic flag. From what I've read, there is no error as such here, but this falls within some disputed section of the standard. I don't really understand the issue, or how it pertains to this line of code. The new gcc diagnostics allow one to selectively suppress warnings in a section of code, but they require you to know what specific flag is triggering the warning, and I don't know. Without the -Werror flag I get

warn.cc:8:3: warning: ISO C++ forbids casting between pointer-to-function and pointer-to-object [enabled by default]

Answer 1

In Standard C++, you cannot convert between, say, an int* and int(*)(). Likely, this is what's happening under the hood in your implementation. Most platforms allow it, but not all.

Of course, there is nothing illegal about any library only executing on platforms where it is legal.