CODEWITHSUNDEEP
c++templatesmultiple-inheritancec++17
got
got

Reputation: 9

Derived class in multiple inheritance behaves like an aggregate

I could get the following code (also found in cppreference http://en.cppreference.com/w/cpp/utility/variant/visit) to compile (https://wandbox.org/permlink/stCFKi0VQlF49Bxr)

#include <type_traits>
#include <utility>

template <typename... Types>
struct Overload : public Types... {
    using Types::operator()...;
};
template <typename... Types>
auto make_overload(Types&&... instances) {
    return Overload<std::decay_t<Types>...>{std::forward<Types>(instances)...};
}

int main() {
    auto overloaded = make_overload([](int) {}, [](double) {});
    static_cast<void>(overloaded);
}

How does the code above compile in C++17? It does not compile in C++14. What is happening behind the scenes? Also why does the variadic using declaration not work in C++14? Which new feature is this?

Upvotes: 0

Views: 66

Answers (1)

Jarod42
Jarod42

Reputation: 218323

As you can read in parameter_pack,

Introduced in C++17

The following is the list of all allowed contexts: [...]

Using-declarations
In using declaration, ellipsis may appear in the list of declarators, this is useful when deriving from a parameter pack:

template <typename... bases>
struct X : bases... {
    using bases::g...;
};
X<B, D> x; // OK: B::g and D::g introduced

Upvotes: 1

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