Virtual array sizes-CodePudding

Is it possible for a class to declare an array which can be overridden with an array of a different length in a derived class? I know I could just use std::vector but it is for a performance critical stage of a game engine and since the lengths are known at compile time it seems like it should be possible to do it statically.

I am aiming for something like this, but without the impossible requirement of a member variable which is both virtual and static:

struct F {
  virtual static const size_t n;
  Signal[n] inputs;
  Signal getInput(size_t i)
  {
    if(i<n)
      return inputs[i];
  }
};
struct Binary : F {
  static const size_t n=2;
};

CodePudding user response：

When you say compile time think, template or constexpr. Like this :

#include <cassert>
#include <array>
#include <iostream>

//-----------------------------------------------------------------------------

struct Signal
{
    std::size_t id =   s_id;
    static std::size_t s_id;
};

std::size_t Signal::s_id{ 0 };

//-----------------------------------------------------------------------------

struct FBase
{
    virtual std::size_t size() const = 0;
    virtual ~FBase() = default;

protected:
    FBase() = default;

};

//-----------------------------------------------------------------------------


template<std::size_t N>
struct F : 
    public FBase
{
    Signal inputs[N]{};

    Signal getInput(size_t i)
    {
        assert(i < N);
        return inputs[i];
    }

    std::size_t size() const override
    {
        return N;
    }
};

//-----------------------------------------------------------------------------

struct Binary : 
    public F<2ul>
{
};

struct Trinary :
    public F<3ul>
{
};

//-----------------------------------------------------------------------------

int main()
{
    Binary bin;
    Trinary tri;

    auto signal = bin.getInput(1ul);
    std::cout << signal.id << "\n";

    std::cout << tri.size();
    
    return 0;
}

CodePudding user response：

Maybe you can use a pointer to Signal in the base class and then each inherited class will initialize the pointer to an array of different size. This will overcome the problem on the class dimension. Just remember to declar virtual de destructor and delete the memory of the array manaully.