Fix integer overflow warning in python.cxx

[vigsterkr]

When compiling SWIG generated cxx file in c++11 mode ('-std=c++11') with '-Wc++11-narrowing' flag one will get an error (see the example below) since the 'tp_dictoffset' is basically 'unsigned long' to 'long', which could lead to integer overflow. Using 'static_cast<Py_ssize_t>(...)' can prevent this.

error example (with clang -std=c++11):
non-constant-expression cannot be narrowed from type 'size_t'
(aka 'unsigned long') to 'Py_ssize_t' (aka 'long') in initializer list
-Wc++11-narrowing(((char_)&((SwigPyObject ) 64L)->dict) - (char_) 64L), /...
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[ojwb]

It won't ever actually overflow, as it's computing the offset of a struct member, which will always be a fairly small number. It would be good to fix the warning though.

Does this code only get generated in C++ mode? If not, then static_cast<> isn't safe to use and we should just use a C-style cast to Py_ssize_t. What options are you passing to swig? I couldn't figure out the option combination to actually get this code generated...

The whole casting of 64L is rather nasty, and using offsetof() would be saner (and possibly more portable), but that's really a separate issue.

[vigsterkr]

@ojwb i dont know if this code is generated only in c++ mode.
i'm using the following flags for swig: -w473 -w454 -w312 -w325 -fvirtual builtin -modern -modernargs -c++

and i'm trying to compile the c++ generated code with clang and C++11 extension, i.e.:
clang++ -std=c++11

[ojwb]

OK, thanks for the flags. This code can be generated in C mode too - e.g. by this interface file:

%module foo
struct A { int x; };
typedef struct A A;

I've committed these two changes, which should fix this warning, and make this just use offsetof:

commit 628b4710e5309379475f55c95a42e1cf21c9be87
Author: Olly Betts <olly@survex.com>
Date:   Sat Aug 24 08:40:08 2013 +1200

    [Python] Fix clang++ warning in generated wrapper code.

commit b477cb66be5739a0acc2c2de15e988187971ce40
Author: Olly Betts <olly@survex.com>
Date:   Sat Aug 24 08:34:50 2013 +1200

    Use offsetof() rather than icky homemade equivalent

Thanks for reporting this issue.