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