Define macro within another macro

Question

I want to define a macro within another macro, in order to save me some repetetive writing.

Currently I have to write this:

MyClass : public Base<MyClass> {
  Value<std::string> testString = value<std::string>(&MyClass::testString, "something");
  // many more values like this
}

And I want to write something like this, in order to reduce the cluttery of the above:

BASE(MyClass) {
  VALUE(std::string, testString, "something");
}

My solution would have been to define two function style macros and BASE and VALUE and within BASE define another constant named __CURRENT_CLASS that will be used for all calls of VALUE. But how do I define this constant within the BASE macro?

Answer 1

Not exactly the same syntax, but you can define a type alias to give the class name something that's always the same.

It's not possible to define a macro inside a macro. An alternative, used when the macros are very complex, is to #define some parameters that will be used and then #include a header file containing macro definitions. Then the parameters can be redefined and header included again.

#include <string>
template <typename T> class Base {};
template <typename T> struct Value {};
template <typename T, typename Container>
Value<T> value(Value<T> Container::*, const T&);

#define BASE(name) \
  class name: public Base<name> { \
    using ThisClass = name;
#define ENDBASE(name) }

#define VALUE(name, type, init) \
  Value<type> name = value<type>(&ThisClass::name, init)
 
BASE(MyClass)
  VALUE(testString, std::string, "something");
ENDBASE(MyClass);