Re: Passing std::va_list by reference to const.

"Alf P. Steinbach" <>
Sat, 15 May 2010 14:38:38 +0200
On 15.05.2010 13:24, * Vaclav Haisman:

I have the following code. I wonder if it is valid to pass std::va_list
around by reference to const like this:


void foo (char const * fmt, std::va_list const& l)
   std::vprintf(fmt, l);

void bar (char const * fmt, ...)
   std::va_list l;
   va_start (l, fmt);
   foo (fmt, l);
   va_end (l);

int main ()
   bar ("%s", "test\n");

This code produces the following error with GCC on FreeBSD/AMD64, but
compiles and works well on 32bit host:

va_test.cxx: In function 'void foo(const char*, const __va_list_tag (&)[1])':
va_test.cxx:7: error: invalid conversion from 'const __va_list_tag*' to
va_test.cxx:7: error: initializing argument 2 of 'int vprintf(const char*,

The following code reproduces your error message:

struct VaListTag {};
typedef VaListTag VaList[1];

void vPrintf( char const fmt[], VaList ) {}

void foo (char const * fmt, VaList const& arglist )
     vPrintf( fmt, arglist );

int main ()
   VaList argList = {};
   foo( "%s", argList );

A practical solution is to pass by value.

I can't find any place where the C++ the standard says anything about what kind
of type a std::va_list can be.

Cheers & hth.,

- Alf

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