CODEWITHSUNDEEP
c++templatesmetaprogrammingtemplate-meta-programmingtype-traits
phatgreen
phatgreen

Reputation: 13

C++ Template Type Traits Issue

I am new to making my own type traits and I want to make a type trait that allows me to identify if the passed type is the expected container. 

template<typename T, typename ... Types>
struct is_array
{
    static constexpr bool value = false;
};

template<typename ... Types>
struct is_array<std::array<Types...>>
{
    static constexpr bool value = true;
};

I've adopted the format above and it works for all container types except for array:

constexpr bool state = is_array<std::array<int, 5>>::value;

The above evaluates to false when it should be true. This only happens for the array class and I believe it is due to it having 2 template parameters that do not have default values. I am unable to figure out a fix for this.

Upvotes: 0

Views: 103

Answers (1)

O&#39;Neil
O&#39;Neil

Reputation: 3849

First, your trait should have one, and only one template parameter: the expected array type:

template<typename T/*, typename ... Types*/>
struct is_array {
    static constexpr bool value = false;
};

Second, std::array is defined as:

template<
    class T,
    std::size_t N
> struct array;

as first type parameter T and a non-type parameter N of type std::size_t.

Therefore, your true specialization should be:

template<typename T, std::size_t N>
struct is_array<std::array<T,N>>
{
    static constexpr bool value = true;
};

Note, that instead of defining a member value, we prefer to inherit from std::true_type and std::false_type which provide some other member alias + conversion operator. The code then becomes:

template<typename T>
struct is_array : std::false_type {};

template<typename T, std::size_t N>
struct is_array<std::array<T,N>> : std::true_type {};

Upvotes: 3

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