Re: C++ equivalent of C FAMs and subsequent issues with offsetof

Carl Barron <>
Sun, 10 Jun 2007 03:13:45 CST
In article <>, Charles
Bailey <> wrote:

I appreciate that considering all corner cases and constructing
unambiguous and clear wording for a standard would be a lot of effort
so it would take considerable collective desire for this feature to be
considered but I would definitely be in favour of it. It would be a
shame for C and C++ to diverge more than necessary.

    Seems like a lot of work compared to merely providing in global
namespace when this problem occurs, as the most difficult of the
extern "C" functions are already [hopefully] written in the supporting
C code. The getters are one liners if they don't already exist and
need to be compiled in C, if missing.

    extern "C"
       struct foo;
       int get_len_from foo(foo *);
       double *get_data_froo(foo *);
       foo * create_foo(int);
       destroy_foo(foo *);

    std::tr1::shared_ptr<foo> CreateFoo(int len)
       return std:shared_ptr<foo>(create_foo(len),destroy_foo);
   or better yet contain the shared_ptr in a genuine C++ class
providing access to the C functions and opague struct.

class cpp_foo
    std::tr1::shared_ptr<foo> p_foo;
    explicit cpp_foo(int n):p_foo(create_foo(n),destroy_foo){}
    int len() const {return get_len_from_foo(p_foo.get());}
    double & operator [] (int n)
    {return *(get_data_from_foo(p_foo.get())+n);}
    double *data() { return get_data_from_foo(p_foo.get());}
    operator foo *() { return p_foo.get();}// for easy calling of C
    or something similiar.
  This handles exceptions well, and automatic correct deletions using
C++ idioms and is easy to impliment and will be accepted C++09 standard
as of now. [requires shared_ptr from boost or tr1 presently]

Given this kind of workaround for this problem and
other more pressing considerations I stiil doubt a change, but who
knows. I do know that no proposal means no change.

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