ERROR "nonstatic member reference must be relative to a specific object

Question

#ifndef WORLD_H_
#define WORLD_H_
using namespace std;

class World{
public:
    friend class DoodleBug;
    friend class Ant;
    friend class Organism;
    int GRID_SIZE;
    
    World();   
    ~World();   
    
    void Draw();  
    int global_get_ID(int x, int y);  
    Organism* get_Ptr(int x, int y);  
    void set_Ptr(int x, int y, Organism* newOrg);  
    void TimeStepForward();  
    
protected:
    Organism* grid[GRID_SIZE][GRID_SIZE];
};
#endif

In this .h file on line Organism* grid[GRID_SIZE][GRID_SIZE] I get this error : Error : a nonstatic member reference must be relative to a specific object.

What does it mean and how can I fix this error?

Answer 1

The problem is that in standard C++, the size of an array must be a compile time constant. But since GRID_SIZE is a non-static data member, the expression GRID_SIZE inside the class is actually equivalent to the expression this->GRID_SIZE. But the this pointer is more of a runtime construct and the expression this->Grid_SIZE is not a constant expression and cannot be used to specify the size of the array.

To solve this you can make the data member GRID_SIZE a constexpr static as shown below:

class World{
public:
//--vvvvvvvvvvvvvvvv--------------------------->constexpr static added here
    constexpr static int GRID_SIZE = 10;
       
protected:
    Organism* grid[GRID_SIZE][GRID_SIZE];
};

You can even use static const.

Working demo