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Getting around the need for a vararg #define just to automatically use __FILE__ and __LINE__ in a TRACE macro

, 9 Jan 2003
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Ever wrote #define TRACE, #define TRACE1, #define TRACE2, etc., just so you could use the __FILE__ macro and printf style formatting in a macro at the same time?

Introduction

Someone else has probably done this before, but I haven't seen it, so I had to make this up.

If you want to create a preprocessor macro that outputs something to a log, you generally want one that works something like this:

MYTRACE( "something something: %s (%d)", str, i );

It's a little tricky, because you want it to accept variable arguments, but I often see something like:

#define MYTRACE MyTrace
static inline MyTrace( const char* msg, ... )
{
    // ...
}

__FILE__ and __LINE__

But, if you wanted MYTRACE to automatically put __FILE__ and __LINE__ into the trace message, you're stuck. You can't use them inside the MyTrace() function because __FILE__ and __LINE__ will always point to the header that defines it instead of the place you call MYTRACE -- and it isn't possible to have a vararg preprocessor macro.

If you're stubborn about such things, you do something like this:

#define MYTRACE1( m, a ) MyTrace( __FILE__, __LINE__, m, a )
#define MYTRACE2( m, a, b ) MyTrace( __FILE__, __LINE__, m, a, b )
#define MYTRACE3( m, a, b, c ) MyTrace( __FILE__, __LINE__, m, a, c )

(And so on). It's horrible to use, though, because you have to keep changing the macro you use when you add an argument.

A solution!

With a little imagination it's possible to work around this, though (at least in C++):

class tracing_output_debug_string
{
private:
    const char* m_file;
    int m_line;

    enum { MY_BUFFER_SIZE = 1024 };

public:
    tracing_output_debug_string( const char* file, int line ) :
        m_file( file ),
        m_line( line )
    {
    }

    void operator()( const char* Format, ... )
    {
        //  vararg stuff
    }
};

#define MYTRACE (tracing_output_debug_string( __FILE__, __LINE__ ))

The downloadable project has an implementation (which writes to ::OutputDebugString()) in a .h file and a little test that shows it, you know, actually works.

I've only tried it on VS.NET (that's what I have), but I'm sure it would work on ... well, just about anything, really.

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About the Author

Paul Mclachlan
Web Developer
United States United States
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Comments and Discussions

 
GeneralExcellent, great idea! [modified] PinmemberXia Xiongjun28-Oct-09 8:29 
GeneralWorks as advertised Pinmemberrcc20-May-08 20:37 
GeneralThanks for this--like others noticed, it is useful and widely open to customizations PinmemberCuratica31-Dec-06 13:22 
GeneralVar Args in C - Another approach PinmemberSDX-{5A853460-2944-42f8-84B8-2432DE3657EF}12-Dec-06 20:46 
GeneralVisual Studio 2005 (VC8) supports varags in Macros PinmemberJeremyRemington2-Mar-06 14:16 
Generalvararg preprocessor possible. PinsussAnonymous19-Aug-05 11:10 
GeneralEven simpler solution PinsussAnonymous31-Jan-05 10:43 
GeneralSome MACRO crimes against humanity ... PinmemberAmanjit Gill23-Oct-04 13:57 
GeneralRe: Some MACRO crimes against humanity ... PinmemberRick York31-Jan-05 11:30 
GeneralI didn't see the MYTRACE message. PinmemberWREY5-Sep-03 11:44 
Generalchaining operator() Pinmemberilmcuts12-Jun-03 22:26 
GeneralA C solution PinsussAnonymous28-Mar-03 15:29 
GeneralRe: A C solution - This is not thread-safe PinmemberAndrew Schetinin30-Mar-03 6:34 
GeneralRe: A C solution - This is not thread-safe Pinmemberroel_13-Feb-04 6:28 
GeneralRe: A C solution - This is not thread-safe PinmemberAndrew Schetinin15-Feb-04 6:15 
GeneralNice One. Check also boost PinmemberRamon Casellas11-Jan-03 8:48 
GeneralGood one! PinmemberDalle11-Jan-03 7:36 
GeneralVery nice! PinsitebuilderMichael Dunn10-Jan-03 21:16 

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