CODEWITHSUNDEEP
c++c++17variadic-templates
PiotrK
PiotrK

Reputation: 4453

How to find out if integer_sequence contains given number in compile-time?

Given:

typedef std::integer_sequence<int, 0,4,7> allowed_args_t;

and:

template<int arg> void foo()
{
    static_assert( /*fire if arg not in allowed_args_t!*/ )
}

How should I write that static_assert to be as cheap as possible in compile-time?

I'm using C++17.

Upvotes: 4

Views: 412

Answers (2)

Jarod42
Jarod42

Reputation: 217398

You might want to use:

template <int ... Is>
constexpr bool is_in(int i, std::integer_sequence<int, Is...>)
{
    return ((i == Is) || ...);
}


typedef std::integer_sequence<int, 0, 4, 7> allowed_args_t;


template<int arg> void foo()
{
    static_assert(is_in(arg, allowed_args_t{}));
}

Upvotes: 14

Justin
Justin

Reputation: 25317

Unpack the integers and use a fold expression:

template <typename AllowedIntegers>
struct contains
{};

template <typename Int, Int... Is>
struct contains<std::integer_sequence<Int, Is...>>
{
    template <Int value>
    static constexpr bool contains = ((value == Is) || ...);
};

// ...

template <int arg>
void foo()
{
    static_assert(contains<allowed_args_t>::contains<arg>);
}

Godbolt link

Upvotes: 8

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