C++ Preprocessor Variables

May 17, 2007

I went several years programming in C/C++ without really knowing about these preprocessor variables. They can be very useful for debugging and logging of custom run time errors/warnings. I have not found a definitive list of them, but here is a list of some useful ones that I know of. (The following variables can vary by compiler, but generally work)

__LINE__ Contains the current line number being processed.
__FILE__ Contains the current file name (full path) being processed.
__DATE__ Contains the compile date, This is the date that the file was compiled, not necessarily the current date.
__TIME__ Contains the compile time. This is the time that the file was compiled, not necessarily the current time.
__TIMESTAMP__ Contains the current date and time. This is the date and time that the file was compiled, not necessarily the current date and time.
__FUNCTION__ Contains the function name. (this is part of C99, the new C standard. Not all C++ compilers support it)
__cplusplus Defined when compiling a C++ program. In some older compilers, this is also called c_plusplus.
__STDC__ Defined when compiling a C program, and may also be defined when compiling C++

Instead of explaining these variables, here is an example program that demonstrates how you may use some of them:

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#include <iostream>
 
#define PUKE_TRACE(X,Y) puke_trace(__FUNCTION__, __FILE__, __LINE__, __threadid(), X, Y)
void puke_trace(const char* func, const char* file, int line,
				unsigned long thread, const char* msg, std::ostream& os)
{
	os << msg << " : Line " << line << ", file " << file
		<< ", function " << func << ", thread " << thread << ".\n";
}
 
int main(int argc, char* argv[])
{
	PUKE_TRACE("Test 1.",std::cout);
	PUKE_TRACE("Test 2.",std::cerr);
	PUKE_TRACE("Test 3.",std::clog);
	return 0;
}

The above program will output something like this:

Test 1. : Line 13, file /path/to/test.cpp, function main, thread 3712.
Test 2. : Line 14, file /path/to/test.cpp, function main, thread 3712.
Test 3. : Line 15, file /path/to/test.cpp, function main, thread 3712.

__threadid() is not a preprocessor variable obviously, but it can be useful when tracing/debugging/logging multi-threaded applications, usually used in conjunction with preprocessor variables.

References:
http://www.codeguru.com/forum/showthread.php?t=231043
http://www.cppreference.com/preprocessor/preprocessor_vars.html

Filed under:
C++ by admin

One Response to “C++ Preprocessor Variables”

  1. Larry
    October 13th, 2007 at 8:16 am

    Did you sanitize your data input?

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