错误：函数不是“类”的静态数据成员 - C++

Question

I'm very sorry if this has already been answered somewhere, but after hours of searching, I couldn't find or understand anything.

Being quite new to OOP, I'm exercising myself with classes by trying to create a class where I pre-defined a 2 dimensions character matrix. I keep getting the following error:

error: 'char Matrix2d::keyss [4][4]' is not a static data member of 'class Matrix2d' char Matrix2d::keyss [ROWS][COLS] =

My header is the following:

const int ROWS = 4;
const int COLS = 4;

class Matrix2d
{
  public:

   char keys [ROWS][COLS];

  private:

};

And My .cpp being this:

char Matrix2d::keys [ROWS][COLS] =

{
{'1','2','3','A'},
{'4','5','6','B'},
{'7','8','9','C'},
{'*','0','#','D'}
};

This is a simple code made to run with an arduino keypad.

Thank you in advance for any help and hope I gave enough information as it's my first time posting here.

Answer 1

If the class is defined like that :

const int ROWS = 4;
const int COLS = 4;

class Matrix2d
{
  public:

   char keys [ROWS][COLS];

  private:

};

that means keys is an attributes of the instance of Matrix2d , but the form

char Matrix2d::keys [ROWS][COLS] =

{
{'1','2','3','A'},
{'4','5','6','B'},
{'7','8','9','C'},
{'*','0','#','D'}
};

defines and initializes an attribute of the class Matrix2d , this is incompatible

---

If you want an attribute of the class (a 'static' one) do

const int ROWS = 4;
const int COLS = 4;

class Matrix2d
{
  public:
     static char keys [ROWS][COLS];    
};

char Matrix2d::keys [ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

If you want an attribute of instances having that default value do

class Matrix2d
{
  public:
     char keys [ROWS][COLS] = {
       {'1','2','3','A'},
       {'4','5','6','B'},
       {'7','8','9','C'},
       {'*','0','#','D'}
     };
};

In both case I encourage you to change the visibility