Re: variable arg list issue with reference types
Cyril Comparon wrote:
Hi,
I encountered a problem with variable-arg-list functions and MSVC.NET and
MSVC2005 compilers. It seems like having the starting variable (in va_start)
as a reference type messes up the values returned by va_arg().
See the enclosed code and output.
NB: this works fine with GCC 4.x.
It's undefined behaviour to use a reference parameter with va_start:
18.7/3
The restrictions that ISO C places on the second parameter to the
va_start() macro in header <stdarg.h> are different in this
International Standard. The parameter parmN is the identifier of the
rightmost parameter in the variable parameter list of the function
definition (the one just before the ...). If the parameter parmN is
declared with a function, array, or reference type, or with a type that
is not compatible with the type that results when passing an argument
for which there is no parameter, the behavior is undefined.
SEE ALSO: ISO C subclause 4.8.1.1.
So the behaviours of GCC and VC2005 are both correct.
In C++ you can simulate variable arguments using templates and
overloading, up to some pre-defined limited number of args. The
generation of the overloads can be automated using a preprocessor
meta-programming library. See www.boost.org for examples, though it is
quite complicated. On the bright side, you do get full type safety with
that kind of approach.
Consider also approaches like operator overloading, as cout does (e.g.
printf_s(s) << arg1 << arg2 << arg3;
or whatever. Finally, take a look at boost.format.
Tom