CODEWITHSUNDEEP
c++macros
Mohamad-Jaafar NEHME
Mohamad-Jaafar NEHME

Reputation: 1215

Is there anyway to define/run a macro on specific condition in C++?

Is there anyway to define/run a macro on specific condition in C++?

My application takes some parameters on running, say ./test -l

I want to define a macro if -l is provided as a parameter, so I tried:

if  (strcmp(argv [2],"-l")==0)
        #define LOOPBACK

but it's wrong. My application is always defining LOOPBACK!!

Upvotes: 0

Views: 84

Answers (2)

Rob
Rob

Reputation: 1974

Preprocessor macros perform text substitution, and the output of the preprocessor is then compiled as code.

That means, by definition, that macros can only be defined, redefined, undefined or expanded at (strictly speaking, before) compile time. There is no way for a macro to have a different expansion based on runtime data.

It also means that macros don't honour any rules of scope.

Which is why your LOOPBACK macro is always defined - the expansion is unrelated to the if statement.

Upvotes: 2

Maged Elghareib
Maged Elghareib

Reputation: 119

All preprocessor directives (e.g: #include, #define, ...) are performed/ evaluated by the preprocessor, which runs before the compiler. So the macro you're defining is defined without knowing about the if(...) statement.

Upvotes: 1

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