C++ Constants

What is const

Const is keyword used do declare variable that cannot be changed.

Const with different contexts

Const can be used in different places and each of them mean something different. For example:

Const Variables
You declare that variable cannot be changed using const keyword
```
const int max_size = 5;
```

Const with pointers
If const keyword is in front of the * it means that you can't modify contents of pointer, the value at memory adress it points to but you can modify memory adress that it points to.

int max_size = 5;
int min_size = 0;
const int* size = &max_size;
 
*size = 3; // you cannot do that
size = &min_size; // you can do that

If const keyword is after the * it means that you can modify contents of pointer but not the adress that it points to.

    int max_size = 5;
    int min_size = 0;
    int* const size = &max_size;
 
    *size = 3; // you can do that
    size = &min_size; // you cannot do that

Const with classes
If you write const in methods you can't change any values in this method.

class Entity{
private:
   int m_X, m_Y;
 
public:
   int getX() const{
       m_X = 2; // you cant do that
       return m_X;
   }
};

But if you need to change something you can use mutable keyword

class Entity{
private:
  int m_X, m_Y;
  mutable int var;
 
public:
  int getX() const{
      m_X = 2; // you cant do that
      var = 2; // you can do that because of mutable keyword
      return m_X;
  }
};

C++ Constants

What is const

Const with different contexts