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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?


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
    const char* m_file;
    int m_line;

    enum { MY_BUFFER_SIZE = 1024 };

    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
No Biography provided

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Comments and Discussions

GeneralExcellent, great idea! [modified] Pin
Xia Xiongjun28-Oct-09 7:29
memberXia Xiongjun28-Oct-09 7:29 
GeneralWorks as advertised Pin
rcc20-May-08 19:37
memberrcc20-May-08 19:37 
GeneralThanks for this--like others noticed, it is useful and widely open to customizations Pin
Curatica31-Dec-06 12:22
memberCuratica31-Dec-06 12:22 
GeneralVar Args in C - Another approach Pin
SDX-{5A853460-2944-42f8-84B8-2432DE3657EF}12-Dec-06 19:46
memberSDX-{5A853460-2944-42f8-84B8-2432DE3657EF}12-Dec-06 19:46 
GeneralVisual Studio 2005 (VC8) supports varags in Macros Pin
JeremyRemington2-Mar-06 13:16
memberJeremyRemington2-Mar-06 13:16 
Generalvararg preprocessor possible. Pin
Anonymous19-Aug-05 10:10
sussAnonymous19-Aug-05 10:10 
GeneralEven simpler solution Pin
Anonymous31-Jan-05 9:43
sussAnonymous31-Jan-05 9:43 
GeneralSome MACRO crimes against humanity ... Pin
Amanjit Gill23-Oct-04 12:57
memberAmanjit Gill23-Oct-04 12:57 
GeneralRe: Some MACRO crimes against humanity ... Pin
Rick York31-Jan-05 10:30
memberRick York31-Jan-05 10:30 
GeneralI didn't see the MYTRACE message. Pin
WREY5-Sep-03 10:44
memberWREY5-Sep-03 10:44 

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