C++ Preprocessor X-macros
Example
An idiomatic technique for generating repeating code structures at compile time.
An X-macro consists of two parts: the list, and the execution of the list.
Example:
#define LIST \
X(dog) \
X(cat) \
X(racoon)
// class Animal {
// public:
// void say();
// };
#define X(name) Animal name;
LIST
#undef X
int main() {
#define X(name) name.say();
LIST
#undef X
return 0;
}
which is expanded by the preprocessor into the following:
Animal dog;
Animal cat;
Animal racoon;
int main() {
dog.say();
cat.say();
racoon.say();
return 0;
}
As lists become bigger (let's say, more than 100 elements), this technique helps to avoid excessive copy-pasting.
Source: https://en.wikipedia.org/wiki/X_Macro
See also: X-macros
If defining a seamingly irrelevant X before using LIST is not to your liking, you can pass a macro name as an argument as well:
#define LIST(MACRO) \
MACRO(dog) \
MACRO(cat) \
MACRO(racoon)
Now, you explicitly specify which macro should be used when expanding the list, e.g.
#define FORWARD_DECLARE_ANIMAL(name) Animal name;
LIST(FORWARD_DECLARE_ANIMAL)
If each invocation of the MACRO should take additional parameters - constant with respect to the list, variadic macros can be used
//a walkaround for Visual studio
#define EXPAND(x) x
#define LIST(MACRO, ...) \
EXPAND(MACRO(dog, __VA_ARGS__)) \
EXPAND(MACRO(cat, __VA_ARGS__)) \
EXPAND(MACRO(racoon, __VA_ARGS__))
The first argument is supplied by the LIST, while the rest is provided by the user in the LIST invocation. For example:
#define FORWARD_DECLARE(name, type, prefix) type prefix##name;
LIST(FORWARD_DECLARE,Animal,anim_)
LIST(FORWARD_DECLARE,Object,obj_)
will expand to
Animal anim_dog;
Animal anim_cat;
Animal anim_racoon;
Object obj_dog;
Object obj_cat;
Object obj_racoon;
