How to create a wrapper on printf?

Question

I want to write an conditional printf, something like this

class ConditionalPrintf
{
public:
    ConditionalPrintf(bool print)
        : print_(print)
    {}

   void printf(int x, double y, char b, const char* format, ...) const
   {
        // use x, y and b

        va_list argptr;
        va_start(argptr, format);

        if (print_)
            printf(format, argptr);

        va_end(argptr);
   }

private:
   bool print_;
};

But it prints garbage. Is there anything wrong? May implicit this parameter change things?

Also, if this isn't good idea whatsoever, what other solutions are there? I just don't want to write if (print) printf(...) billion times.

have a look at vprintf: http://en.cppreference.com/w/cpp/io/c/vfprintf — Richard Hodges, Jul 06 '16 at 12:32
No I don't want to use it, I need to understand what's wrong with my program. — Mister Nobody, Jul 06 '16 at 12:35
what's wrong with your program is that you're not using it. It's designed for forwarding a va_list into printf! — Richard Hodges, Jul 06 '16 at 12:37
val_list is an argument for vprintf, notfor printf. Thus you see garbage. Use vprintf — dimba, Jul 06 '16 at 12:37
I feel sort of bad marking this C++ question a duplicate of a C question, but since you are using C functions and the answer is exactly the same... — Cody Gray - on strike, Jul 06 '16 at 12:38
You **cannot** wrap `printf`. You **must** wrap `vprintf` instead. There is a reason why every standard varargs function has a "v" counterpart. That's the way varargs work. (Now that we have C++11 you may be able to wrap printf with a C++11 variadic template, but there's no reason to). — n. m. could be an AI, Jul 06 '16 at 12:41
Well, and can I just call `printf_another(format, argptr)` from that code? (printf_another is another member function) — Mister Nobody, Jul 06 '16 at 12:46

score 3 · Answer 1 · answered Jul 06 '16 at 12:40

vprintf forwards the arg list to printf

#include <stdio.h>
#include <stdarg.h>

class ConditionalPrintf
{
public:
    ConditionalPrintf(bool print)
        : print_(print)
    {}

   void printf(int x, double y, char b, const char* format, ...) const
   {
        // use x, y and b

        va_list argptr;
        va_start(argptr, format);

        if (print_)
            vprintf(format, argptr);

        va_end(argptr);
   }

private:
   bool print_;
};

How to create a wrapper on printf?

1 Answers1