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

From:

"Alf P. Steinbach" <alfps@start.no>

Newsgroups:

comp.lang.c++

Date:

Sat, 15 May 2010 14:38:38 +0200

Message-ID:

<hsm4o7$2s1$1@news.eternal-september.org>

On 15.05.2010 13:24, * Vaclav Haisman:

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

//-----8<--------
#include<cstdarg>
#include<cstdio>

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");
}
//-----8<--------

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_list_tag*'
va_test.cxx:7: error: initializing argument 2 of 'int vprintf(const char*,
__va_list_tag*)'

The following code reproduces your error message:

<code>
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 );
}
</code>

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

--
blog at <url: http://alfps.wordpress.com>