How do I create const arrays and calculated const values in C++ class?

Question

I'm running into some compiler errors I don't understand. I'm pretty sure I'm doing something very wrong here but I don't know what. I would like all the world constants to be defined as belonging to the class.

Notes:

I'm only using classes as structs with attached members. I'm not following strict Object-Orriented Design on purpose. Please do not comment the public variables.

I'm not concerned very much about the compiler inlining stuff. I'm using this structure because it's easy for me to use. (If it worked)

class Board{
public:
    enum PhysicsResult{ BOUNCE, OUT_OF_BOUNDS_TOP, OUT_OF_BOUNDS_BOTTOM, CONTINUE };
    //World constants
    const static float Height = 500;
    const static float Width = 300;
    //ERROR: 'Board::Width' cannot appear in a constant-expression.
    const static float PaddleWidth = Width/15; 
    const static float BallRadius = 5;
    const static float BounceDistance = 1.5;
    //World Objects
    Ball ball;
    Paddle paddle1;
    Paddle paddle2;
    /*
     1---2
     |   |
     0---3
     */
    //ERROR: a brace-enclosed initalizer is not allowed here before '{' token
    //ERROR: invalid in-class initalization of static data member of nonintegral type 'const Pair[4]'
    const static Pair corners[4] = {Pair(0, 0), Pair(0, Height), Pair(Width, Height), Pair(Width, 0)};

    //ERROR: a brace-enclosed initalizer is not allowed here before '{' token
    //ERROR: invalid in-class initalization of static data member of nonintegral type 'const Pair[2]'
    const static Pair left_wall[2]   = {corners[0], corners[1]};

    //ERROR: a brace-enclosed initalizer is not allowed here before '{' token
    //ERROR: invalid in-class initalization of static data member of nonintegral type 'const Pair[2]'
    const static Pair right_wall[2]  = {corners[3], corners[2]};

    //ERROR: a brace-enclosed initalizer is not allowed here before '{' token
    //ERROR: invalid in-class initalization of static data member of nonintegral type 'const Pair[2]'
    const static Pair top_wall[2]    = {corners[1], corners[2]};

    //ERROR: a brace-enclosed initalizer is not allowed here before '{' token
    //ERROR: invalid in-class initalization of static data member of nonintegral type 'const Pair[2]'
    const static Pair bottom_wall[2] = {corners[0], corners[3]};

If it is possible to do this, what is the proper syntax for doing this? If this is not possible, what alternative should I use?

Answer 1

You must initialize const members in the constructor initialization list.

class blah
{
    public:
            blah() : _constant( 1 ) { }

    private:
            const int _constant;
};

Answer 2

Defining the static consts outside of the body of the class compiles and executes with gcc.

#include <iostream>
using namespace std;

struct Pair { int a; int b; Pair(int x, int y) : a(x),b(y) {}};
struct F {
  static const float blah = 200.0;
  static const Pair corners[4];
};

// square boards are so ordinary
const Pair F::corners[4] = { Pair(0,0), Pair(0,1), Pair(2,0), Pair(2,2) };

const float F::blah ;

int main(int, char **) {
  cout << F::corners[0].a << endl ;
  cout << F::blah << endl;
  return 0;
}

I cannot overemphasize the importance of ebo's comment about order of initialization.

Answer 3

Static members of C++ objects need to be defined outside of the declaration. This is because the compiler doesn't know which translation unit (.o file) to put the members in.

Usually I will define them in the .cpp file of the implementation. You don't generally want to put them into the header file because they will end up in multiple .o files, and it will generate compiler errors because the same thing is defined multiple times.