c++ Unpacking parameter pack from template arguments

prestokeys Published at Dev

prestokeys

How to achieve want I want below? The paramater pack I want to unpack is not in a function argument list but template argument list.

#include <iostream>
#include <array>

const std::size_t SIZE = 10;

template <int...ARGS>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b;
    // I want to set b[n] = true, where n takes on all values from ARGS...
    // what to put in here???
    return b;
}

// Example of what I want to achieve:
int main() {
    const std::array<bool, SIZE> b = func<1,3,7>();
    // I want b[1]==true, b[3]==true, b[7]==true, all others false
    for (int x: b) std::cout << x << std::endl;
}

I have to use this form for func (instead of func(1,3,7)) to get my bigger program working (I'm dealing with multiple inheritance issues).

Sam Cristall

Recursive template solution:

// recursive helper struct
template <int n, int First, int ...Rest>
struct helper {
  static void funcImpl(std::array<bool, SIZE>& temp) {
    temp[First] = true;
    helper<n - 1, Rest...>::funcImpl(temp);
  }
};

// partial specialization to catch base case
template <int First>
struct helper<0, First> {
  static void funcImpl(std::array<bool, SIZE>& temp) {
    temp[First] = true;
  }
};

template <int ...Args>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b = {}; // 0 inititalize array
    helper<sizeof...(Args) - 1, Args...>::funcImpl(b);
    return b;
}

EDIT: A super simplified version inspired by iavr's solution:

template <int... A>
std::array<bool, SIZE> func() {
    std::array<bool, SIZE> b = {};
    auto values = {A...};
    std::for_each(values.begin(), values.end(), [&](int n){b[n] = true;});
    return b;
}

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