Question

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Are there gotchas using varargs with reference parameters

Hi, I have a problem with varargs.
Look at my code(Microsoft Visual Studio 2005 or 2008).

#include <stdarg.h>

struct Test { int a; };

void T1(int n, ...) {
 va_list args;
 va_start(args, n);
 char* p = va_arg(args, char*);
 va_end(args);
}

void T2(Test n, ...) {
 va_list args;
 va_start(args, n);
 char* p = va_arg(args, char*);
 va_end(args);
}

void T3(const Test& n, ...) {
 va_list args;
 va_start(args, n);
 char* p = va_arg(args, char*);  // p corrupt!!
 va_end(args);
}

int _tmain(int argc, _TCHAR* argv[]) {
 const Test t;
 T1(1, "Test1");
 T2(t, "Test2");
 T3(t, "Test3");
 return 0;
}

function T1, T2 work well. But T3 function have a problem. The pointer p doesn’t point “Test3”. Can’t I use va_start with pass-by-reference?
Thanks in advance.

Answer 1

You cannot use references with va_start according to C++ Standard 18.7/3:

The restrictions that ISO C places on the second parameter to the va_start() macro in header
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.