Can't use inheritance in a std::visit functor?

Question

I have the following code:

class multitype {
public:
 int get_number() const;
 .....
private:
std::variant<int, std::string, ..... > field;
};

.....

struct zero_return {
    template<typename X> int operator()(const X&) const { return 0; }
};

inline int multitype::get_number() const
{
    struct visitor : public zero_return {
        int operator()(int n) const { return n; }
    };
    return std::visit(visitor(), field);
}

While the actual code has many more types in the std::variant and is much more complex, the above code fragment is sufficient to reproduce the issue by itself.

The idea behind the zero_return struct in this example was supposed to provide a particular default return value for any type not specifically handled by a subclass functor.

However, the line with the call to std::visit fails with the following message:

error: no matching function for call to '__invoke(multitype::get_number() const::visitor, const std::__cxx11::basic_string<char>&)'
  972 |    return std::__invoke(std::forward<_Visitor>(__visitor),
      |           ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  973 |        __element_by_index_or_cookie<__indices>(
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  974 |   std::forward<_Variants>(__vars))...);

Can anyone please tell me what am I doing wrong?

A TL;DR of the dupe: you need a `using zero_return::operator();` — StoryTeller - Unslander Monica, Jan 29 '20 at 15:00
@markt1964: That's irrelevant; you have to do the `using` trick regardless of how many base classes you use. — Nicol Bolas, Jan 29 '20 at 15:49
I thought you would only need using to disambiguate if multiple base classes had a match — markt1964, Jan 29 '20 at 15:51

Can't use inheritance in a std::visit functor?

0 Answers0