Table of Contents
C Macro Expansion Rules
Introduce macro expansion rules in C.
#convert macro parameter to string.##combine multiple tokens into one token.- Nested macro expansion rules
- The preprocessor will first process macro parameters until they are expanded to non-macro parameters or blocked by
#or##. - The expansion of macro parameters will be blocked by
#or##. That is, the expansion of macro parameters will stop and then the pre-processor will use the expanded macro parameters to expand the macro.
There are some examples to help understanding.
Example 1
#define str(a) #a #define foo(a, b) a ## b
str(x)
> "x"
foo(x, y)
> xy
Example 2
#define str(_a) #_a
#define A a
#define B b
#define foo(_a, _b) _a##_b
#define foo2(_a, _b) foo(_a, _b)
foo2(A, B)
> foo2(a, b)
> ab
foo(A, B)
# Because A and B are arguments to foo, the extension of A and B are blocked by ##.
> AB
str(foo2(A, B))
# Because foo2 is an argument to foo, the extension of foo2 is blocked by #.
> "foo2(A, B)"
Example 3
#define A a
#define B b
#define foo(_a, _b) _a##_b _a
foo(A, B)
> AB A
> AB a
