fill std::array in the member initialization list

Question

The following code works but I would like to avoid the warning:

warning: 'fitness::vect_' should be initialized in the member initialization list [-Weffc++]

when it is compiled with the g++ -Weffc++ switch:

#include <array>

template<class T, unsigned N>
class fitness
{
public:
  explicit fitness(T v)
  {
    static_assert(N, "fitness zero length");

    vect_.fill(v);
  }

private:
  std::array<T, N> vect_;
};

int main()
{
  fitness<double, 4> f(-1000.0);

  return 0;
}

Should I ignore the warning? Is there a way to fill vect_ in the constructor initialization list (without changing its type)?

Answer 1

A function that generates a filled_array should have its return value be elided:

template<unsigned N, typename T>
std::array<T, N> filled_array_sized( T const& t ) {
  std::array<T, N> retval;
  retval.fill( t );
  return retval;
}

but that requires passing in at least the size N, if not the type T.

template<typename T>
struct array_filler {
  T && t;
  template<typename U, unsigned N>
  operator std::array<U, N>()&& {
    return filled_array_sized<N, U>( std::forward<T>(t) );
  }
  array_filler( T&& in ):t(std::forward<T>(in)) {}
};
template<typename T>
array_filler< T >
filled_array( T&& t ) {
  return array_filler<T>( t );
}

note that storing the return value of filled_array in an auto is not advised.

Use:

#include <array>

template<class T, unsigned N>
class fitness
{
public:
  explicit fitness(T v): vect_( filled_array( std::move(v) ) ) {
    //...
  }
//...

I do not know if the above code will generate a warning in the implementation of filled_array_size, but if it does, disable the warning locally.