CODEWITHSUNDEEP
c++templatesshared-ptrenable-ifweak-ptr
Денис Кулаков
Денис Кулаков

Reputation: 11

C++ std::enable_if member template function overloading for template class

I'm trying to 'overload' template function in class, depending on specialisation argument's parent template class. However, I get the following error.

error C2938: 'std::enable_if_t<false,bool>' : Failed to specialize alias template

What is wrong here?

#include <memory>
#include <type_traits>

template <template <typename...> class BaseTemplate, typename Derived, typename TCheck = void>
struct test_base_template;

template <template <typename...> class BaseTemplate, typename Derived>
using is_base_template_of = typename test_base_template<BaseTemplate, Derived>::is_base;

//Derive - is a class. Let inherit from Derive, so it can cast to its protected parents
template <template <typename...> class BaseTemplate, typename Derived>
struct test_base_template<BaseTemplate, Derived, std::enable_if_t<std::is_class_v<Derived>>> : Derived
{
    template<typename...T>
    static constexpr std::true_type test(BaseTemplate<T...>*);
    static constexpr std::false_type test(...);
    using is_base = decltype(test((test_base_template*) nullptr));
};

//Derive - is not a class, so it always false
template <template <typename...> class BaseTemplate, typename Derived>
struct test_base_template<BaseTemplate, Derived, std::enable_if_t<!std::is_class_v<Derived>>>
{
    using is_base = std::false_type;
};

template<typename type>
class MyClass
{
public:
    template <typename Dummy = type, typename std::enable_if_t<is_base_template_of<std::weak_ptr, type>::value, bool> = bool()>
    std::shared_ptr<typename type::element_type> get()
    {
        return m_data.lock();
    }

    template <typename Dummy = type, typename std::enable_if_t<is_base_template_of<std::shared_ptr, type>::value, bool> = bool()>
    std::shared_ptr<typename type::element_type> get()
    {
        return m_data;
    }

private:
    type m_data;
};

int main()
{
    MyClass<std::weak_ptr<int>> t;
    auto a1 = t.get();

    MyClass<std::shared_ptr<int>> t2;
    auto a2 = t2.get();
}

Readed this thread std::is_base_of for template classes

Upvotes: 1

Views: 121

Answers (1)

user12002570
user12002570

Reputation: 1

Does that mean it is impossible to have one of theese functions at a time depending on m_data type?

You can use constexpr if as shown below:

template<typename type>
class MyClass
{
public:
       std::shared_ptr<typename type::element_type> get()
    {
        if constexpr(is_base_template_of<std::weak_ptr, type>::value)
        {
               return m_data.lock();
        }
        else if constexpr(is_base_template_of<std::shared_ptr, type>::value)
        {
               return m_data;  
        } 
        //make sure that this non-void returning function returns something if control reaches here
       
    }

};

Working demo

Upvotes: 0

Related Questions