C++11 initialize array with uniform value in constexpr function

Question

I have a class template which builds a simple array based on the template parameters as one of its members. I need to be able to initialize every element in the array to a single value in one of the constructors. Unfortunately this constructor must be constexpr.

The relevant part boils down to:

template <typename T, size_t N>
class foo
{
  T data[N];

  constexpr foo(T val)
  {
    // initialize data with N copies of val
  }
};

Using std::fill or a loop is incompatible with the constexpr requirement. Initializing with : data{val} only sets the first element of the array and zero-initializes the remainder.

How can this be achieved?

I feel like there should be a solution with variadic templates and tuples etc...

@BryanChen: Not so much, that question isn't about constexpr — Ben Voigt
– Ben Voigt, Commented May 28, 2014 at 1:32
There's a straightforward way to achieve what you want if constructor argument of "foo(T val)" could be turned into a template argument "template <T val> foo()". — oakad
– oakad, Commented May 28, 2014 at 2:37

oakad · Accepted Answer · 2014-05-28 03:35:53Z

8

Rather curiously, a solution for the problem exists in c++14 (compile the example with -std=c++1y on gcc; also see comments for a bit more verbose c++11 solution by Praetorian):

template <size_t N>
struct bar {
    template <typename T, typename ...Tn>
    static constexpr auto apply(T v, Tn ...vs)
    {
        return bar<N - 1>::apply(v, v, vs...);
    }
};

template <>
struct bar<1> {
    template <typename T, typename ...Tn>
    static constexpr auto apply(T v, Tn ...vs)
    {
        return std::array<T, sizeof...(vs) + 1>{v, vs...};
    }

};

template <typename T, size_t N>
struct foo {
    std::array<T, N> data;

    constexpr foo(T val)
    : data(bar<N>::apply(val))
    {}
};

(I replaced the POD array with std::array - suppose it should not make any problems for your use case).

Live example is here: http://coliru.stacked-crooked.com/a/4731a10ee54563b9

edited May 28, 2014 at 3:35

answered May 28, 2014 at 3:08

oakad

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2 Comments

Praetorian Over a year ago

+1 Works with -std=c++11 also if you specify the return types for the functions. coliru.stacked-crooked.com/a/668bd7f546654971

oakad Over a year ago

Of course - somehow decltype() had slipped my mind.

Jarod42 · Accepted Answer · 2014-05-28 07:39:06Z

6

You may use the following: (https://ideone.com/xTacMP)

namespace detail
{
    template <typename T, std::size_t...Is>
    constexpr std::array<T, sizeof...(Is)> make_array(T val, index_sequence<Is...>)
    {
        return {(static_cast<void>(Is), val)...};
    }
}

template <std::size_t N, typename T>
constexpr std::array<T, N> make_array(T val)
{
    return detail::make_array(val, make_index_sequence<N>());
}

And then call it:

constexpr foo(T val) : data(make_array<N>(val)) {}

answered May 28, 2014 at 7:39

Jarod42

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