How to determine if va_list is empty
c++, variadic-functions, visual-c++, visual-studio-2010
Solution
There is no way to tell how many arguments are passed through `...`, nor what type they are. Variadic function parameters can only be used if you have some other way (e.g. a `printf`-style format string) to tell the function what to expect; and even then there is no way to validate the arguments.
C++11 provides type-safe variadic templates. I don't know whether your compiler supports these, or whether they would be appropriate for your problem.
Problem
I have been reading that some compilers support va_list with macros and users were able to overload the functionality with other macros in order to count the va_list. With visual studio, is there a way to determine if the va_list is empty (aka count==0)? Basically I would like to know this condition: ``` extern void Foo(const char* psz, ...); void Test() { Foo("My String"); // No params were passed } ``` My initial thought was to do something like this: ``` va_list vaStart; va_list vaEnd; va_start(vaStart, psz); va_end(vaEnd); if (vaStart == vaEnd) ... ``` The problem is that va_end only sets the param to null. ``` #define _crt_va_start(ap,v) ( ap = (va_list)_ADDRESSOF(v) + _INTSIZEOF(v) ) #define _crt_va_arg(ap,t) ( *(t *)((ap += _INTSIZEOF(t)) - _INTSIZEOF(t)) ) #define _crt_va_end(ap) ( ap = (va_list)0 ) ``` I was thinking of maybe incorporating a terminator but I would want it to be hidden from the caller so that existing code doesnt need to be changed.