Is there a way to create an array of combined functions compile-time in c++?

Question

I'm working on an NES emulator in c++ and figured that the most efficient way to run opcodes would be to call a function pointer in an array of functions that do exactly what the opcode does.

The problem is that each opcode has a specific operation and memory address. While searching for a solution, I stumbled upon lambda expressions. This is definitely good enough for a NES emulator on modern hardware. However, I can't find a solution such that each function in the array contains the machine code for both the operation and the addressing without defining 256 separate functions.

This is along what I had in mind for a similar function that combines f and g:

int addone(int x) {
  return x + 1;
}

int multtwo(int x) {
  return 2 * x;
}

something combine(function<int(int)> f, function <int(int)> g) {
  /* code here */
}

/*
combine(addone, multtwo) creates a function h that has the same machine code as
int h(x) {
  return 2 * x + 1;
}
*/

Any ideas? If I had to take a guess, it would have something to do with templates. Thanks!

Answer 1

I'd say that when you want to write generics for functions that it's kind of a "design pattern" to switch to functors: Compilers are desigined to handle types easily, but handling function pointers for stuff you want to mis-match and keep optimised at compile-time gets ugly!

So we either write our functions as functors, or we wrap them as functors:

struct A
{
    static constexpr int Func (int x)
    {
        return -3*x + 1;
    }
};

struct B
{
    static constexpr int Func (int x)
    {
        return -2*x - 5;
    }
};

// etc...

If we have nice symmetry in how we'll use them, then we can manage them systematically. Eg. if we always want to combine them like f(g(h(...y(z())...))), then we can solve as follows:

template <class T, class ... Ts>
struct Combine
{
    static constexpr int Get ()
    {
        int x = Combine<Ts...>::Get();
        return T::Func(x);
    }
};

template <class T>
struct Combine <T> // The base case: the last function in the list
{
    static constexpr int Get ()
    {
        return T::Func();
    }
};

demo

Or if we're in no such luck, we'll have to resort to more old-fashioned inputs like you suggested:

template <class Funcs, class Data>
constexpr int Combine (const Data & d) 
{
    Funcs F;
    // Some use without much symmetry:
    return F.f(F.g(d)) - F.h(d);
}

int main ()
{
    struct FuncArgs
    {
        A f;
        B g;
        C h;
    };

    return Combine<FuncArgs>(5);
}

demo

Note that in the second example I've changed from static methods to non-static. This doesn't really matter - the compiler should optimise these fully regardless, but I think in this case it makes the syntax slightly nicer (and shows an alternative style).