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How can I create const arrays and const computed values ​​in a C ++ class?

I am facing some compiler errors that I do not understand. I'm pretty sure I'm doing something really bad here, but I don't know what. I would like all world constants to be defined as belonging to a class.

Notes:

I only use classes as structs with attached members. I am not strictly OO design. Please do not comment out public variables.

I'm not really worried about the compiler the information is embedded in. I use this framework because it is 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 possible, what is the correct syntax for this? If this is not possible, what alternative should I use?

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3 answers


Defining static constants outside of the class body is compiled and executed 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 overstate the importance of ebo's comment about initialization order.

+5


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You must initialize constant members in the constructor initialization list.



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

    private:
            const int _constant;
};
+2


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The static members of C ++ objects must be defined outside of the declaration. This is because the compiler does not know which translation unit (.o file) to put the members in.

I usually define them in the .cpp implementation file. Usually you don't want to put them in a header file because they will end up in multiple .o files and it will generate compiler errors because the same value is defined multiple times.

+1


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