Generate initializer list of varying size depending on template parameter

Question

I have a class that contains an array of objects T without a default constructor. Here is an example:

#include <iostream>

struct Param {
    int x;
};

struct A {
    A(const Param &p) {
        std::cout << p.x << std::endl;
    }
};

template<class T, int n>
struct Array {
    T data[n];
                                 /*     n times     */
    Array(const Param &p) : data{/* T(p), ..., T(p) */} { }
};

int main() {
    Param p{42};
    Array<A, 3> arr(p);
    return 0;
}

I am looking for a way to initialize the data field using a varying size initializer list of elements T(p) containing exactly n elements. I have a feeling that it can be achieved by some construction like std::make_index_sequence, but I did not find anything appropriate.

If you're looking for a dynamically sized array, take a look at vectors — emsimpson92, Jun 11 '18 at 20:10
@emsimpson92 No, I do not want to allocate data on heap. It's a fixed-size array with size known at compile time — uranix, Jun 11 '18 at 20:11
Partial dupe: https://stackoverflow.com/questions/46899731/initializing-an-stdarray-of-non-default-constructible-elements I'm trying to find a more exact one — NathanOliver, Jun 11 '18 at 20:21

max66 · Accepted Answer · 2018-06-11T20:57:46.370

7

I suppose you can use a delegating contructor, std::index_sequence and std::make_index_sequence and rewrite Array as follows (Edit: answer improved by SergeyA and Rakete1111; thanks!)

template <typename T, std::size_t N>
struct Array
 {
    T data[N];

    template <std::size_t ... Is>
    Array (Param const & p, std::index_sequence<Is...> const &)
       : data { ((void)Is, T{p}) ... }
     { }

    Array(const Param &p) : Array{p, std::make_index_sequence<N>{}}
     { }
 };

Maybe you can make private the new constructor.

edited Jun 11 '18 at 20:57

answered Jun 11 '18 at 20:34

max66

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5

There is no need for `get_val` in the example, instead just use `data { ((void)Is, T{P})...}` – SergeyA Jun 11 '18 at 20:41
1

@SergeyA Wow! Didn't think of that. `data{((void)Is, T{P})...}`, it's safer ;) – Rakete1111 Jun 11 '18 at 20:42
@SergeyA - I was looking a way to avoid that ugly function; it was so simple and elegant! Thanks. – max66 Jun 11 '18 at 20:47
1

@Rakete1111 - maybe not safer (for a `std::size_t` isn't defined in a dangerous way the comma operator) but avoid an annoying warning. Thanks. – max66 Jun 11 '18 at 20:51
@max66 what stops me from defining a `bool operator,(std::size_t, T)` ;) – Rakete1111 Jun 11 '18 at 20:52
@HolyBlackCat - well... that `(void)` avoid an annoying warning. – max66 Jun 11 '18 at 20:52
@max66 templates + comma = asking for trouble :) – Rakete1111 Jun 11 '18 at 20:54

Generate initializer list of varying size depending on template parameter

1 Answers1