CODEWITHSUNDEEP
c++templates
camino
camino

Reputation: 10594

Why class specialization from template<typename T1, typename T2> to template<typename T1, int i> not supported

I am wondering why class specialization from template<typename T1, typename T2> to template<typename T1, int i> is not supported.

For example:

template<typename T1, typename T2>
struct B{};

template<typename T1>
struct B<T1, T1>{};  //ok

template<typename T1>
struct B<T1, int>{}; //ok

template<typename T1,int i>
struct B<T1,i>{}; //error:   expected a type, got 'i'

template<typename T1,constexpr int i>
struct B<T1,i>{}; //error:   expected a type, got 'i'

Upvotes: 2

Views: 987

Answers (1)

skypjack
skypjack

Reputation: 50538

Your primary template expects a type parameter, but i is not a type, it is a non-type argument. int is a type.

You can use std::integral_constant to bind them somehow:

template<typename T1, int i>
struct B<T1, std::integral_constant<int, i>>{};

It is not exactly the same thing, but a using declaration can help to work around it:

template<typename T1, int i>
using BT = struct B<T1, std::integral_constant<int, i>>;

Upvotes: 6

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