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c++constexprstdarray

Zero-initializing a `std::array` of arithmetic `value_type` at compile-time results in a missing constructor note

To initialize an std::array of an arithmetic type AT at compile-time, I did this: 

#include <array> 
#include <iostream>

template<typename AT, auto DIM, auto N = 0> 
constexpr void assign(std::array<AT, DIM>& arr, AT value)
{
    arr[N] = value; 
    if constexpr (N < std::size(arr) - 1)
        assign<AT, DIM, N + 1>(arr, value);
}

template<typename AT, auto DIM>
constexpr std::array<AT, DIM> zero_array() 
{
    std::array<AT, DIM> result;  
    assign(result, AT{0});
    return result;
}

template<typename Container> 
void print(Container&& cont)
{
    for (const auto& el : cont)
        std::cout << el << " "; 
    std::cout << std::endl;
}

int main()
{
    auto zero10 = zero_array<double, 10>(); 
    print(zero10); 
}

I godbolted it, and it seems to me it worked:

enter image description here

However, when I compile it with

g++  -O3 -std=c++2a -Wall -Wpedantic -Wunused-parameter -I /usr/include main.cpp -o main

using g++ (GCC) 8.2.1 20181127, I get a "note" 

In file included from main.cpp:1:
main.cpp: In instantiation of ‘constexpr std::array<AT, DIM> zero_array() [with AT = double; auto DIM = 10]’:
main.cpp:30:42:   required from here
/usr/include/c++/8.2.1/array:94:12: note: ‘struct std::array<double, 10>’ has no user-provided default constructor
     struct array
            ^~~~~
/usr/include/c++/8.2.1/array:110:56: note: and the implicitly-defined constructor does not initialize ‘double std::array<double, 10>::_M_elems [10]’
       typename _AT_Type::_Type                         _M_elems;

Why is this note there? Can I ignore it? If so, how to get rid of it?

Solution

  • std::array does not have a default constructor. You need to use:

    std::array<AT, DIM> result = {};

    What's puzzling to me is why you think you need zero_array at all. If you use

    std::array<double, 10> a = {};

    you get a zero-initialized object.