Variadic constructor fails to initialise an std::array - c++

Question

Here is a snippet from my code:
template <typename Type, unsigned int NumberOfRows, unsigned int NumberOfColumns>
class HexMatrix
{
private:
std::array<Type, NumberOfRows*NumberOfColumns> values;
// ...
public:
template <typename... OtherType>
HexMatrix(OtherType...);
// ...
};
template <typename Type, unsigned int NumberOfRows, unsigned int NumberOfColumns>
template <typename... OtherType>
HexMatrix<Type, NumberOfRows, NumberOfColumns>::HexMatrix(OtherType... args) : values({ args... })
{
static_assert(NumberOfRows != 0u);
static_assert(NumberOfColumns != 0u);
}
Until recently, I was able to use this code as is, and initialise the std::array with an std::vector using the constructor above. Then my PC got formatted. Now this doesn't compile anymore, even though I was using and am still using -std=c++2a. Here is the compilation report:
In file included from foo.cpp:3:
LinearAlgebra.hpp: In instantiation of ‘HexMatrix< <template-parameter-1-1>, <anonymous>, <anonymous> >::HexMatrix(OtherType ...) [with OtherType = {std::vector<long double, std::allocator<long double> >}; Type = long double; unsigned int NumberOfRows = 6; unsigned int NumberOfColumns = 6]’:
LinearAlgebra.hpp:230:9: required from ‘static HexMatrix<Type, NumberOfRows, NumberOfColumns> HexMatrix< <template-parameter-1-1>, <anonymous>, <anonymous> >::Make(OtherType ...) [with OtherType = {std::vector<long double, std::allocator<long double> >}; Type = long double; unsigned int NumberOfRows = 6; unsigned int NumberOfColumns = 6]’
foo.cpp:93:80: required from here
LinearAlgebra.hpp:299:98: error: no matching function for call to ‘std::array<long double, 36>::array(<brace-enclosed initializer list>)’
299 | HexMatrix<Type, NumberOfRows, NumberOfColumns>::HexMatrix(OtherType... args) : values({ args... })
| ^
In file included from /usr/include/c++/9/tuple:39,
from /usr/include/c++/9/bits/hashtable_policy.h:34,
from /usr/include/c++/9/bits/hashtable.h:35,
from /usr/include/c++/9/unordered_map:46,
from Using.hpp:5,
from LinearAlgebra.hpp:4,
from foo.cpp:3:
/usr/include/c++/9/array:94:12: note: candidate: ‘std::array<long double, 36>::array()’
94 | struct array
| ^~~~~
/usr/include/c++/9/array:94:12: note: candidate expects 0 arguments, 1 provided
/usr/include/c++/9/array:94:12: note: candidate: ‘constexpr std::array<long double, 36>::array(const std::array<long double, 36>&)’
/usr/include/c++/9/array:94:12: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘const std::array<long double, 36>&’
/usr/include/c++/9/array:94:12: note: candidate: ‘constexpr std::array<long double, 36>::array(std::array<long double, 36>&&)’
/usr/include/c++/9/array:94:12: note: no known conversion for argument 1 from ‘<brace-enclosed initializer list>’ to ‘std::array<long double, 36>&&’
make: *** [Makefile:16: foo.o] Error 1
What's wrong? I tried to remove the parenthesis, and/or add another pair of brackets around args... but that didn't change anything. Please help?
Reproductible example:
int main(void)
{
const std::vector<long double> foo = { 1.L, 2.L, -6.L, 0.L };
HexMatrix<long double, 2u, 2u> givenMatrix(foo);
return 0;
}

Answer 1

If you want to pass a vector in the constructor, add a new constructor to HexMatrix:
#include <vector>
#include <array>
#include <algorithm>
template <typename Type, unsigned int NumberOfRows, unsigned int NumberOfColumns>
class HexMatrix
{
private:
std::array<Type, NumberOfRows*NumberOfColumns> values;
// ...
public:
template <typename... OtherType>
HexMatrix(OtherType...);
HexMatrix(const std::vector<Type>&);
// ...
};
template <typename Type, unsigned int NumberOfRows, unsigned int NumberOfColumns>
template <typename... OtherType>
HexMatrix<Type, NumberOfRows, NumberOfColumns>::HexMatrix(OtherType... args) : values({ args... })
{
static_assert(NumberOfRows != 0u);
static_assert(NumberOfColumns != 0u);
}
template <typename Type, unsigned int NumberOfRows, unsigned int NumberOfColumns>
HexMatrix<Type, NumberOfRows, NumberOfColumns>::HexMatrix(const std::vector<Type>& vec)
{
static_assert(NumberOfRows != 0u);
static_assert(NumberOfColumns != 0u);
std::copy_n(vec.begin(), NumberOfRows*NumberOfColumns, values.begin());
}
int main(void)
{
const std::vector<long double> foo = { 1.L, 2.L, -6.L, 0.L };
HexMatrix<long double, 2u, 2u> givenMatrix(foo);
return 0;
}