How to see what a file looks like after preprocessing?

Question

How to check the preprocessing results? For example, let's say I have the following code:

#define CONCATENATE(X, Y) X ## Y
#define STRING_1 First
#define STRING_2 Second
#define STRING_3 CONCATENATE(STRING_1, STRING_2)

Is there any way to make sure it STRING_3will be expanded to FirstSecondlater in the program?

Answer 1

Each compiler must provide a switch to save pre-processed code.

• gcc: -E
• MS Visual Studio: Keep preprocessed filesin settings or /Pswitch

For other compilers, I'm sure you will find a suitable switch in the documentation

Answer 2

I think it is best to run the C ++ file under g++ -E <file> -o <file>.out, and then check the result this way.

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