CODEWITHSUNDEEP
c++c++11templatessfinaecrtp
themagicalyang
themagicalyang

Reputation: 2553

Incomplete type while accessing templated derived class (CRTP)' static function while doing SFINAE

Demo

template <typename T>
struct A {
    void mem_func() {
        // This works!!! Can access static_func<int>();
        std::cout << T::template static_func<int>() << '\n';
    }
    template <typename K>
    static constexpr bool static_func() { return true; }

    template <typename T1>
    typename std::enable_if<T::template static_func<T1>(), void> sfinae_func() { 
        // This breaks!!! T is an incomplete type??
        std::cout << "This fails" <<'\n';
    }
};

struct B : A<B> {
};

Here, I don't get why T::static_func is working in the member function foo() but when I go to instantiate the sfinae_func the same T::static_func is inaccessible as T is incomplete! How can this be??

Follow Up is this: If the standards restricts me to access T::static_func in enable_if, is there a workaround to hide A::static_funct with T::static_func when required? 

template <typename T1>
    typename std::enable_if<T::template static_func<T1>(), void> sfinae_func()

Here what would need to be done to have enable if call T's static_func such that T's static_func hides A's static_func?

Upvotes: 2

Views: 220

Answers (1)

W.F.
W.F.

Reputation: 13988

To workaround the problem you could delay type instantiation by making T of sfinae_func additional template parameter's default value e.g. as follows (oh and don't forget to use inner type of std::enable_if to actually perform sfinae):

#include <iostream>
template <typename T>
struct A {
    void mem_func() {
        std::cout << T::template static_func<int>() << '\n';
    }
    template <typename K>
    static constexpr bool static_func() { return true; }

    template <typename T1, typename TT = T>
    typename std::enable_if<TT::template static_func<T1>(), void>::type sfinae_func() {
        std::cout << "This fails" <<'\n';
    }
};

struct B : A<B> {
};
int main() {
    B a;
    a.mem_func();
    a.sfinae_func<int>();
}

[live demo]

Upvotes: 3

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