How can SFINAE be used to create a type trait for identifying STL containers in C ?

Type Traits for Identifying STL Containers

In C , a type trait can be used to determine the properties of a given type. One common need is to check whether a type represents a container, such as a vector, set, or map.

Custom Implementation for Vector

To start with, let's consider creating a type trait specifically for vectors. However, the following attempt fails to compile:

<code class="cpp">template<class T, typename Enable = void>
struct is_vector {
  static bool const value = false;
};

template<class T, class U>
struct is_vector<T, typename boost::enable_if<boost::is_same<T, std::vector<U>> >::type> {
  static bool const value = true;
};</code>

This code generates the error "template parameters not used in partial specialization: U." This is because the template parameter U is not used within the partial specialization.

SFINAE-Based Solution for Containers

A more general approach is to use Substitution Failure Is Not An Error (SFINAE) to create a type trait that works for a wide range of STL containers. Here's an example:

<code class="cpp">template<typename T, typename _ = void>
struct is_container : std::false_type {};

template<typename... Ts>
struct is_container_helper {};

template<typename T>
struct is_container<
        T,
        std::conditional_t<
            false,
            is_container_helper<
                typename T::value_type,
                typename T::size_type,
                typename T::iterator,
                typename T::const_iterator,
                decltype(std::declval<T>().size()),
                decltype(std::declval<T>().begin()),
                decltype(std::declval<T>().end()),
                decltype(std::declval<T>().cbegin()),
                decltype(std::declval<T>().cend())
                >,
            void
            >
        > : public std::true_type {};</code>

This type trait checks for the presence of essential members and methods that are common to most STL containers. If all these members are present, is_container evaluates to true; otherwise, it evaluates to false.

Note: To ensure that the type trait only identifies STL containers, you may need to adjust the checks to verify specific requirements common to STL containers.

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