Question

The C++11 standard specifies that vwscanf is available via header <cwchar> (and therefore <wchar.h>). However it seems to be lacking in Visual C++. With that function I could write …

inline int scanf( CodingValue const* format, ... )
{
    va_list args;
    va_start( args, format );
    return ::vwscanf( format->ptr(), args );
}

but without it, i.e. with Visual C++ 10.0, which also appears to lack support for C++11 variadic templates, I’m reduced to writing …

inline int scanf(
    CodingValue const* format,
    void* a01 = 0, void* a02 = 0, void* a03 = 0, void* a04 = 0, void* a05 = 0,
    void* a06 = 0, void* a07 = 0, void* a08 = 0, void* a09 = 0, void* a10 = 0,
    void* a11 = 0, void* a12 = 0
    )
{
    int const   nArgs = !!a01 + !!a02 + !!a03 + !!a04 + !!a05 + !!a06 +
                        !!a07 + !!a08 + !!a09 + !!a10 + !!a11 + !!a12;
    BasicCodingValue const* const   f   = format->ptr();

    switch( nArgs )
    {
    case  0:    return ::wscanf( f );
    case  1:    return ::wscanf( f,a01 );
    case  2:    return ::wscanf( f,a01,a02 );
    case  3:    return ::wscanf( f,a01,a02,a03 );
    case  4:    return ::wscanf( f,a01,a02,a03,a04 );
    case  5:    return ::wscanf( f,a01,a02,a03,a04,a05 );
    case  6:    return ::wscanf( f,a01,a02,a03,a04,a05,a06 );
    case  7:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07 );
    case  8:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07,a08 );
    case  9:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07,a08,a09 );
    case 10:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07,a08,a09,a10 );
    case 11:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07,a08,a09,a10,a11 );
    case 12:    return ::wscanf( f,a01,a02,a03,a04,a05,a06,a07,a08,a09,a10,a11,a12 );
    }
}

Or, I could do some assembly, as I did like 15 years ago when I encountered similar (or perhaps the same) problem, but that does not feel quite right.

Can you suggest some better way?

Answer 1

As I understand it’s not an error to call a variadic function with extra (i.e. more than asked for) arguments in C (and possibly in C++, although I’ve yet to locate a definitive statement either way on that).

The best I’ve got on the C++ side so far is 5.2.2 [expr.call]:

A function can be declared to accept fewer arguments (by declaring
default arguments (8.3.6)) or more arguments (by using the ellipsis,
…, or a function parameter pack (8.3.5)) than the number of
parameters in the function definition (8.4)

18.10 [support.runtime] defers to the ISO C 4.8.1.1 on the subject of restrictions for va_start().

I can’t see anywhere that says “you must not pass extra arguments”, so it seems that it wouldn’t be unreasonable to assume that it’s not prohibited.

If it’s not legal in C++ to give extra arguments then you can use C++ to handle the default arguments still and call a C function with the defaults having been “filled out” so that you only ever call the one C function, regardless of how many arguments were given.

So in C++ you’d do:

extern "C" {
  int real_scanf(
        const char* format,
        void* a01, void* a02, void* a03, void* a04, void* a05,
        void* a06, void* a07, void* a08, void* a09, void* a10,
        void* a11, void* a12
        );
}

inline int scanf(
    CodingValue const* format,
    void* a01 = 0, void* a02 = 0, void* a03 = 0, void* a04 = 0, void* a05 = 0,
    void* a06 = 0, void* a07 = 0, void* a08 = 0, void* a09 = 0, void* a10 = 0,
    void* a11 = 0, void* a12 = 0
    )
{
    BasicCodingValue const* const   f   = format->ptr();
    return real_scanf( f,a01,a02,a03,a04,a05,a06,a07,a08,a09,a10,a11,a12 );
}

and then in C you could do:

int real_scanf(
    const char* format,
    void* a01, void* a02, void* a03, void* a04, void* a05,
    void* a06, void* a07, void* a08, void* a09, void* a10,
    void* a11, void* a12
    )
{
    return wscanf( format,a01,a02,a03,a04,a05,a06,a07,a08,a09,a10,a11,a12 );
}