Token pasting and __LINE__

Question

I\'m writing a simple macro to show TRACE information.

This is what I\'m using ,

#ifdef __DEBUG__
#define TRACE  { PrintErrorMsg(\"Trace exception at         


        

Answer 1

You need this kind of silliness, unfortunately.

#include <stdio.h>

#define TRACE2(f,l) printf("I am at file: " f " and line: " #l "\n")
#define TRACE1(f,l) TRACE2(f,l)
#define TRACE() TRACE1(__FILE__, __LINE__)

int main(void)
{
    TRACE();
    TRACE();
}

I am at file: test.cpp and line: 9
I am at file: test.cpp and line: 10

Answer 2

When you try to stringize something with #x, that x must be a macro parameter:

#define FOO #__LINE__ /* this is not okay */
#define BAR(x) #x     /* this is okay */

But you cannot simply say BAR(__LINE__), because this will pass the token __LINE__ into BAR, where it is immediately turned into a string without expansion (this is by design), giving "__LINE__". The same thing happens with the token-pasting operator ##: expansion of their operands never happens.

The solution is to add indirection. You should always have these in your codebase somewhere:

#define STRINGIZE(x) STRINGIZE_SIMPLE(x)
#define STRINGIZE_SIMPLE(x) #x

#define CONCAT(first, second) CONCAT_SIMPLE(first, second)
#define CONCAT_SIMPLE(first, second) first ## second

Now STRINGIZE(__LINE__) turns to STRINGIZE_SIMPLE(__LINE__) which gets fully expanded to (for example) #123, which results in "123". Phew! I leave STRINGIZE_SIMPLE around on the off chance I want the original behavior. So your code would be something like:

#include <iostream>

#define STRINGIZE(x) STRINGIZE_SIMPLE(x)
#define STRINGIZE_SIMPLE(x) #x

#define TRACE()                                                 \
        PrintErrorMsg("Trace exception in " __FILE__            \
                      " at line number " STRINGIZE(__LINE__)    \
                      " in function " __FUNCTION__ ".")

void PrintErrorMsg(const char* str)
{
    std::cout << str << std::endl;
}

int main()
{
    TRACE();
}