Is there a way to declare cyclic type aliases in C++

Question

I have a piece of code that looks something like

class Something {};
using MyUnionType = std::variant<A, B>;
using A = Something;
using B = std::vector<MyUnionType>;

but this does not compile, and I get

error: use of undeclared identifier 'A' in the declaration of MyUnionType

and error: use of undeclared identifier 'MyUnionType' in the declaration of B.

Is there a way to declare these types (aliases) or is C++ not advanced enough to do this?

I have tried to forward-declare A and B as follows

class Something {};
class A;
class B;
using MyUnionType = std::variant<A, B>;
using A = Something;
using B = std::vector<MyUnionType>;

but this also does not compile.

error: typedef redefinition with different types ('Something' vs 'A')

error: typedef redefinition with different types ('std::vector<MyUnionType>' (aka 'vector<variant<A, B>>') vs 'B')

When I try to move the declaration of MyUnionType to the bottom and forward-declare MyUnionType instead, I get a similar error.

Answer 1

If you want to reference the class itself, you have to define a new type.

#include <variant>
#include <vector>
struct Something {};
struct MyUnionType;
using A = Something;
using B = std::vector<MyUnionType>;
struct MyUnionType {
    std::variant<A, B> v;
};

You may be interested in some references: https://en.cppreference.com/w/cpp/language/type_alias , https://en.cppreference.com/w/cpp/language/declarations .