GNU如何理解是否设置了标志？

Question

I am working on a C++ project with a makefile. I have to make several modifications to it but before that, I have a question about the way flags are interpreted in GNU make. To elaborate, in the following snippet, I have two options to enable or disable a feature during the compilation of my project,

    # Two options for a feature:
      FEATURE=on off

    # other lines in the make file

    # By default, the feature is turned off
      ifndef FEATURE
       FEATURE = off
      endif

   # other lines in the make file

   # A number of other flags are defined here

   # I have defined a flag to indicate that my feature is disabled
     CXXFLAGS_off = -DFEATURE_DISABLED

   # Adding all the flags
     CXXFLAGS += $(CXXFLAGS_$(FEATURE)) #Other flags also added

Now, somewhere in my code, I have this line:

    #ifdef FEATURE_DISABLED
       //Don't invoke the functions for the feature
    #else
      //Invoke the functions for the feature
    #endif

Now during compilation, when I say make FEATURE = on , I see that the program works fine, with the feature enabled. When I say make FEATURE = off, it is disabled.

My problem however is that I don't exactly understand how the compiler is interpreting my choice. For example, I am just saying, "make FEATURE = off", how is this line mapped to the fact that the flag for off is enabled and how does the code get compiled with the feature turned off? As I wrote above, I do add the flags for my feature to the CXXFLAGS, but how does make understand that FEATURE = off means that the FEATURE_DISABLED flag is set?

Thank you very much for any explanation.

Answer 1

Because you wrote

CXXFLAGS += $(CXXFLAGS_$(FEATURE))

which, when you supply FEATURE = off on the make command line, will expand to

CXXFLAGS += $(CXXFLAGS_off)

which, because you have also defined

CXXFLAGS_off = -DFEATURE_DISABLED

in turn expands to

CXXFLAGS += -DFEATURE_DISABLED

which means that the compiler will be run with -DFEATURE_DISABLED as an extra argument.