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Suppose I have #define foo in various header files. It may expand to some different things. I would like to know (when compiling a .cc file) when a #define is encountered, to what it will expand, it which file it is and where it got included from.

Is it possible? If not, are there any partial solutions that may help?

Feel free to add comments with clarification requests.

Edit: current answers seem to concentrate on the case when there is one #define and I just want to jump to definition or know what the definition is. That's the simple case and yes, your solutions work. But when I have the same #define in different files, and want to know which one kicks in first, none of these techniques is useful. Okay, I actually used #warning carefully to find the right place. But this requires much work.

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  • What exactly do you want it to do when it runs into your macros on compilation? Commented Nov 14, 2008 at 18:33
  • @Greg: print information: such and such just got defined. Would expand to (...). This is in file (...), line (...). Included from (a "backtrace of #includes going back to my .cc file). Commented Nov 14, 2008 at 18:42
  • 1
    I'd have a very bad feeling about any software where this made a difference. Commented Nov 14, 2008 at 22:17

6 Answers 6

Reset to default
19

Use -E :

# shows preprocessed source with cpp internals removed
g++ -E -P file.cc
# shows preprocessed source kept with macros and include directives 
g++ -E -dD -dI -P file.cc

The internals above are line-markers for gcc which are kinda confusing when you read the output. -P strips them

 -E  Stop after the preprocessing stage; do not run the compiler proper.  
     The output is in the form of preprocessed source code, which is sent to the 
     standard output.

     Input files which don't require preprocessing are ignored.

Note: comments correctly complain this is only a partial solution. It won't tell you when a macro will be replaced. It shows you the preprocessed source, which can be helpful anyway.

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3 Comments

This works on entire files, not individual #defines, and it doesn't provide the rest of the information @phjr was looking for.
Indeed, it's only a partial solution :)
If '-P' is not used (keep internals), in which file macro is defined can be seen easily.
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I would like to know (when compiling a .cc file) when a #define is encountered,

I know a solution to that. Compile the file with the symbol already defined as illegal C++ code (the article linked to uses '@'). So, for GCC you would write

gcc my_file.c -Dfoo=@

When that expands it's guaranteed to cause a syntax error, and the compiler should tell you which file that syntax error is in, which will be very helpful.

If you use the trick Raymond Chen suggests, the compiler may tell you where the "conflicting" definition came from, and may give you a list of how it got included. But there's no guarantee. Since I don't use macros (I prefer const and enum) I can't say if GCC is one of the smarter compilers in this regard. I don't believe the C or C++ standards say anything about this, other than once the preprocessor runs you lose all sorts of useful information.

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1 Comment

Nice idea, but that won't work if the definition is guarded by a #ifndef foo.
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It wont help you find where it was defined but you can see the definition for a particular file by using the -E -dM flags

g++ -E -dM file.cpp | grep MACRO
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And then look for your macro from bottom. and to know the file you can spot a corresponding _HEADER_FILE_H definition.
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for the "to what it will expand" I use the -E switch in gcc which gives the preprocessed output. But there is no backtrace which macro came where from (or if there was a macro at all).

Another option you might use is -g3, this adds debug information regarding the macros, i.e. you can later see in your debugger the definition of each macro.

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1

A Good IDE can do this for you on demand via some form of jump to definition.

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0

Use #warning. It's described here.

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