Macro Tricks

Stringification

The # operator allows to create a string out of a macro parameter. With the ‘stringize’ trick any defined constant can be converted into a string literal.

#define stringize(s) _stringize(s)
#define _stringize(s) #s
#define IMPORTANT_CONST 4

If you’d use the ‘stringize’ operator # directly in a macro you won’t get the intended string:

_stringize(IMPORTANT_CONST)

would lead to:

"IMPORTANT_CONST"

That’s why there is another macro calling the first one:

stringize(IMPORTANT_CONST)

is replaced to:

stringize(4)  

which leads to:

_stringize(4)  

and finally:

"4"  

Concatenation

With the ## operator in a preprocessor macro it’s possible to combine two tokens.

#define CREATE_ID(name) ID_##name

CREATE_ID(IMPORTANT_THING)

ID_IMPORTANT_THING

Further Reading

There is a good explanation in the GCC online docs: Macros