An example of a std::variant-like type implementation.
The implementation creates dispatch table at compile time. Therefore, the invocation of the callable object in visit function is guaranteed to be is in constant time, independently of the number of types in variant.
This implementation has no constexpr support, because reinterpret_cast and placement new (::new (ptr) T (val)) are currently not allowed in constexpr context. An implementation with constexpr support is discussed e.g here