How to know (in GCC) when given macro/preprocessor symbol gets declared?

Question

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, t

Answer 1

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.