使用gcc使用-D选项编译多个文件

Question

I have a project with multiple .cpp and .h files. I have a file called Globals.h which is included in all .cpp files. Now when compiling this file I use some -D options. These options affect all files not just Globals.h.

In my makefile I had to use the same -D options when compiling every file. Of course when removing those options some "undefined" errors arise.

This means every time I change one of these options I have to recompile the whole project. Is there a way that would let me compile ONLY Globals.h with these options?

This is a part from my makefile: lines that create Globals.o :

Globals.o : $(GlobalsLib) $(GlobalsInc)
       g++ -c -Wall $(CodeDefined) $(UserDefined) $(GlobalsLib) -std=c++0x -o Globals.o

$(GlobalsLib) and $(GlobalsInc) are paths to files Globals.h and Globals.cpp $(CodeDefined) and $(UserDefined) are -U and -D options this code is for creating another .o file:

NT_FFT_Decomp.o : $(GlobalsLib) $(GlobalsInc) $(NT_DecompLib) $(NT_DecompInc)
       g++ -c -Wall $(CodeDefined) $(UserDefined) $(NT_DecompLib) -std=c++0x -o NT_FFT_Decomp.o

(NT_DecompLib) $(NT_DecompInc) are also paths to files. Notice that I had to add the same options in the g++ command. Is there anyway around that?

Answer 1

No, that's not how headers work.

Headers are not separate compilation units. Perhaps this will make more sense if you precompile all of your files, via the -E flag (I think). This will leave you with a bunch of .i (I think) files that will actually be compiled. Note that the headers are now gone, but their contents are spread across the different source files. That is what you actually have to recompile each time.

There are ways to minimize source recompilation. For instance this situation:

foo.h

int foo = 1;

Can be turned into this one:

foo.h

extern int foo;

foo.cpp

int foo = 1;

So you can now change the value of foo without recompiling.

But unfortunately, it also sounds like your header is just too big. If you throw everything in one header you're going to be doing a lot of recompilation. Factor more specifically and this may be more avoidable.

Also (or alternatively?) use precompiled headers to reduce compilation time. I've never programmed with them but they solve this problem I understand.