CODEWITHSUNDEEP
c++templatesc++11template-meta-programming
NmdMystery
NmdMystery

Reputation: 2868

Efficiently get the size of a parameter pack up to a certain index

I want to be able to determine the number of bytes that are in a subset of a parameter pack from 0 to a given index.

Right now I'm using a non-constexpr way of doing this. Below is my code:

template <size_t index, typename... args> struct pack_size_index;

template <size_t index, typename type_t, typename... args>
struct pack_size_index <index, type_t, args...> {
    static const size_t index_v = index;

    static const size_t value(void) {
        if (index_v > 0) {
            return sizeof(type_t) + pack_size_index<index - 1, args...>::value();
        }

        return 0;
    }
};

template <size_t index> struct pack_size_index <index> {
    static const size_t index_v = index;

    static const size_t value(void) { return 0; }
};

Usage:

//output: 5  (equal to 1 + 4)
std::cout << pack_size_index<2, bool, float, int, double>::value() << std::endl;

//output: 20 (equal to 8 + 8 + 4)
std::cout << pack_size_index<3, double, double, float, int>::value() << std::endl;

This gets the job done, but this uses runtime comparison and the resulting executable increases in size rapidly whenever this is used. What's a less expensive way of doing this?

Upvotes: 1

Views: 325

Answers (1)

NmdMystery
NmdMystery

Reputation: 2868

Solved, I think:

template <size_t index, typename... args> struct pack_size_index;

template <size_t index, typename type_t, typename... args>
struct pack_size_index <index, type_t, args...> {
    static const size_t value = (index > 0)?
        (sizeof(type_t) + pack_size_index<index - 1, args...>::value):0;
};

template <size_t index> struct pack_size_index <index> {
    static const size_t value = 0;
};

Upvotes: 3

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