c++ macro to import all names of base template class [duplicate]

问题

When deriving a class from a template base class, one is forced to use the syntax this->member, or using Base::member to get access to a member of the base class. The same would happen if the base class is a generic template parameter and one is sure that some members exists.

Would it be possible to write a macro that "import" all members of the base class that I know (or assume) to be existent?

Simple example:

struct A {
   int x = 0;
};

struct B {
  int x = 0;
  int y = 1;
};

template <class T>
struct C {
   int x = 0;
   T z = 0;
};

template <class BaseClass>
struct Derived : BaseClass {
  using BaseClass::x;
  void setx1() { x = 1; }
};

template <class T>
struct Derived_from_C : C<T> {
  using C<T>::x;
  void setx1() { x = 1; }
};

int main()
{
    Derived<A> a;
    a.setx1();

    Derived<B> b;
    b.setx1();

    Derived_from_C<double> c;
    c.setx1();

    return 0;
}

In defining struct Derived I assume that the template parameter BaseClass contains a member x, but to use it in the member setx1 I have to manually declare a using. The same happens for Derived_from _C where the base class is a template class. But if the base class contains many members that I would like to use, it becomes tedious and error-prone to add a using for each member.

It it possible to write a macro that does this semi-automatically? Something like

#define USING(class, __VA_ARGS__)

such that

USING(BaseClass, x, y, z)

expands to

using BaseClass::x;
using BaseClass::y;
using BaseClass::z;

回答1:

Based on Marvin's solution to the question Is it possible to iterate over arguments in variadic macros here is a solution

#define FE_1(CLASS, X) using CLASS::X
#define FE_2(CLASS, X, ...) using CLASS::X;FE_1(CLASS, __VA_ARGS__)
#define FE_3(CLASS, X, ...) using CLASS::X;FE_2(CLASS, __VA_ARGS__)
#define FE_4(CLASS, X, ...) using CLASS::X;FE_3(CLASS, __VA_ARGS__)
#define FE_5(CLASS, X, ...) using CLASS::X;FE_4(CLASS, __VA_ARGS__)
#define FE_6(CLASS, X, ...) using CLASS::X;FE_5(CLASS, __VA_ARGS__)
#define FE_7(CLASS, X, ...) using CLASS::X;FE_6(CLASS, __VA_ARGS__)
#define FE_8(CLASS, X, ...) using CLASS::X;FE_7(CLASS, __VA_ARGS__)
#define FE_9(CLASS, X, ...) using CLASS::X;FE_8(CLASS, __VA_ARGS__)
#define FE_10(CLASS, X, ...) using CLASS::X;FE_9(CLASS, __VA_ARGS__)
#define FE_11(CLASS, X, ...) using CLASS::X;FE_10(CLASS, __VA_ARGS__)
#define FE_12(CLASS, X, ...) using CLASS::X;FE_11(CLASS, __VA_ARGS__)
//... repeat as needed

#define GET_MACRO(_1,_2,_3,_4,_5,_6,_7,_8,_9,_10,_11,_12,NAME,...) NAME 
#define FOR_EACH_USING(class,...) \
  GET_MACRO(__VA_ARGS__,FE_12,FE_11,FE_10,FE_9,FE_8,FE_7,FE_6,FE_5,FE_4,FE_3,FE_2,FE_1,)(class,__VA_ARGS__)

来源：https://stackoverflow.com/questions/56323134/c-macro-to-import-all-names-of-base-template-class

标签

c++

inheritance

preprocessor

variadic-macros