How do you create a debug only function that takes a variable argument list? Like printf()

I still do it the old way, by defining a macro (XTRACE, below) which correlates to either a no-op or a function call with a variable argument list. Internally, call vsnprintf so you can keep the printf syntax:

#include <stdio.h>

void XTrace0(LPCTSTR lpszText)
{
   ::OutputDebugString(lpszText);
}

void XTrace(LPCTSTR lpszFormat, ...)
{
    va_list args;
    va_start(args, lpszFormat);
    int nBuf;
    TCHAR szBuffer[512]; // get rid of this hard-coded buffer
    nBuf = _vsnprintf(szBuffer, 511, lpszFormat, args);
    ::OutputDebugString(szBuffer);
    va_end(args);
}

Then a typical #ifdef switch:

#ifdef _DEBUG
#define XTRACE XTrace
#else
#define XTRACE
#endif

Well that can be cleaned up quite a bit but it's the basic idea.

This is how I do debug print outs in C++. Define 'dout' (debug out) like this:

#ifdef DEBUG
#define dout cout
#else
#define dout 0 && cout
#endif

In the code I use 'dout' just like 'cout'.

dout << "in foobar with x= " << x << " and y= " << y << '\n';

If the preprocessor replaces 'dout' with '0 && cout' note that << has higher precedence than && and short-circuit evaluation of && makes the whole line evaluate to 0. Since the 0 is not used the compiler generates no code at all for that line.

Here's something that I do in C/C++. First off, you write a function that uses the varargs stuff (see the link in Stu's posting). Then do something like this:

 int debug_printf( const char *fmt, ... );
 #if defined( DEBUG )
  #define DEBUG_PRINTF(x) debug_printf x
 #else
   #define DEBUG_PRINTF(x)
 #endif

 DEBUG_PRINTF(( "Format string that takes %s %s\n", "any number", "of args" ));

All you have to remember is to use double-parens when calling the debug function, and the whole line will get removed in non-DEBUG code.

Ah, vsprintf() was the thing I was missing. I can use this to pass the variable argument list directly to printf():

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

void DBG_PrintImpl(char * format, ...)
{
    char buffer[256];
    va_list args;
    va_start(args, format);
    vsprintf(buffer, format, args);
    printf("%s", buffer);
    va_end(args);
}

Then wrap the whole thing in a macro.

Another fun way to stub out variadic functions is:

#define function sizeof

@CodingTheWheel:

There is one slight problem with your approach. Consider a call such as

XTRACE("x=%d", x);

This works fine in the debug build, but in the release build it will expand to:

("x=%d", x);

Which is perfectly legitimate C and will compile and usually run without side-effects but generates unnecessary code. The approach I usually use to eliminate that problem is:

1. Make the XTrace function return an int (just return 0, the return value doesn't matter)

2. Change the #define in the #else clause to:

   0 && XTrace
   

Now the release version will expand to:

0 && XTrace("x=%d", x);

and any decent optimizer will throw away the whole thing since short-circuit evaluation would have prevented anything after the && from ever being executed.

Of course, just as I wrote that last sentence, I realized that perhaps the original form might be optimized away too and in the case of side effects, such as function calls passed as parameters to XTrace, it might be a better solution since it will make sure that debug and release versions will behave the same.

In C++ you can use the streaming operator to simplify things:

#if defined _DEBUG

class Trace
{
public:
   static Trace &GetTrace () { static Trace trace; return trace; }
   Trace &operator << (int value) { /* output int */ return *this; }
   Trace &operator << (short value) { /* output short */ return *this; }
   Trace &operator << (Trace &(*function)(Trace &trace)) { return function (*this); }
   static Trace &Endl (Trace &trace) { /* write newline and flush output */ return trace; }
   // and so on
};

#define TRACE(message) Trace::GetTrace () << message << Trace::Endl

#else
#define TRACE(message)
#endif

and use it like:

void Function (int param1, short param2)
{
   TRACE ("param1 = " << param1 << ", param2 = " << param2);
}

You can then implement customised trace output for classes in much the same way you would do it for outputting to std::cout.

What platforms are they not available on? stdarg is part of the standard library:

http://www.opengroup.org/onlinepubs/009695399/basedefs/stdarg.h.html

Any platform not providing it is not a standard C implementation (or very, very old). For those, you will have to use varargs:

http://opengroup.org/onlinepubs/007908775/xsh/varargs.h.html

Part of the problem with this kind of functionality is that often it requires variadic macros. These were standardized fairly recently(C99), and lots of old C compilers do not support the standard, or have their own special work around.

Below is a debug header I wrote that has several cool features:

• Supports C99 and C89 syntax for debug macros
• Enable/Disable output based on function argument
• Output to file descriptor(file io)

Note: For some reason I had some slight code formatting problems.

#ifndef _DEBUG_H_
#define _DEBUG_H_
#if HAVE_CONFIG_H
#include "config.h"
#endif

#include "stdarg.h"
#include "stdio.h"

#define ENABLE 1
#define DISABLE 0

extern FILE* debug_fd;

int debug_file_init(char *file);
int debug_file_close(void);

#if HAVE_C99
#define PRINT(x, format, ...) \
if ( x ) { \
if ( debug_fd != NULL ) { \
fprintf(debug_fd, format, ##__VA_ARGS__); \
} \
else { \
fprintf(stdout, format, ##__VA_ARGS__); \
} \
}
#else
void PRINT(int enable, char *fmt, ...);
#endif

#if _DEBUG
#if HAVE_C99
#define DEBUG(x, format, ...) \
if ( x ) { \
if ( debug_fd != NULL ) { \
fprintf(debug_fd, "%s : %d " format, __FILE__, __LINE__, ##__VA_ARGS__); \
} \
else { \
fprintf(stderr, "%s : %d " format, __FILE__, __LINE__, ##__VA_ARGS__); \
} \
}

#define DEBUGPRINT(x, format, ...) \
if ( x ) { \
if ( debug_fd != NULL ) { \
fprintf(debug_fd, format, ##__VA_ARGS__); \
} \
else { \
fprintf(stderr, format, ##__VA_ARGS__); \
} \
}
#else /* HAVE_C99 */

void DEBUG(int enable, char *fmt, ...);
void DEBUGPRINT(int enable, char *fmt, ...);

#endif /* HAVE_C99 */
#else /* _DEBUG */
#define DEBUG(x, format, ...)
#define DEBUGPRINT(x, format, ...)
#endif /* _DEBUG */

#endif /* _DEBUG_H_ */

Have a look at this thread:

• How to make a variadic macro (variable number of arguments)

It should answer your question.

Not exactly what's asked in the question . But this code will be helpful for debugging purposes , it will print each variable's value along with it's name . This is completely type independent and supports variable number of arguments. And can even display values of STL's nicely , given that you overload output operator for them

#define show(args...) describe(#args,args);
template<typename T>
void describe(string var_name,T value)
{
    clog<<var_name<<" = "<<value<<" ";
}

template<typename T,typename... Args>
void describe(string var_names,T value,Args... args)
{
    string::size_type pos = var_names.find(',');
    string name = var_names.substr(0,pos);
    var_names = var_names.substr(pos+1);
    clog<<name<<" = "<<value<<" | ";
    describe(var_names,args...);
}

Sample Use :

int main()
{
    string a;
    int b;
    double c;
    a="string here";
    b = 7;
    c= 3.14;
    show(a,b,c);
}

Output :

a = string here | b = 7 | c = 3.14