58

Can you do something like this with a macro in C?

#define SUPERMACRO(X,Y) #define X Y then SUPERMACRO(A,B) expands to #define A B

I have a feeling not because the preprocessor only does one pass.

Official gcc only. No third-party tools please.

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6 Answers 6

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42

Macros can't expand into preprocessing directives. From C99 6.10.3.4/3 "Rescanning and further replacement":

The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one,

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20

You cannot define macros in other macros, but you can call a macro from your macro, which can get you essentially the same results.

#define B(x) do {printf("%d", (x)) }while(0) #define A(x) B(x)

so, A(y) is expanded to do {printf("%d", (y)) }while(0)

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4

No. The order of operations is such that all preprocessor directives are recognized before any macro expansion is done; thus, if a macro expands into something that looks like a preprocessor directive, it won't be recognized as such, but will rather be interpreted as (erroneous) C source text.

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3

Sorry, you cannot. You can call other macros in macros but not define new ones.

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2 Comments

how about if you did 2 passes of the pre processor
The syntax is not valid. # is kind of a reserved character in macros. # must be followed by a macro parameter to be valid. This will 'stringify' the parameter.
1

You could try running it through with only the preprocess option, then compiling with the preprocessed file.

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-2

You might do this though: #define SUPERMACRO(X,Y) define X Y

Then you can use your editors macro-expansion functionality and paste in the missing #.

Or even better: Use a different, more powerful string-processing language as your preprocessor.

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