you don’t want to change the value.

int main(){
	const int MAX_AGE = 90;
	// MAX_AGE =2 ; not work
	
	const int* a = new int; // you can't change the content of a
	int const* a = new int; // you can't change the content of a
	int* const a = new int; // you can't reassign the actual pointer itself
	

class Entity{
private: 
	int m_X, m_Y;
	int* p_X;
	mutable int var;

public:
	int GetX() const // it means read-only.
	{
		// m_X = 2; // dosn't work.
		var = 2; // it works, because it is mutable.
		return m_X;
	}
	int SetX(int x) const // it means read-only.
	{
		// m_X = x; // dosn't work.
	}
	const int* const GetX() const
	{
		return m_X;
	}
	// first const: returning a pointer which can't be modified.
	// second const: the content of pointer can't be modified.
	// third const: this function can't modify the actual Entity class.
};

void PrintEntity (const Entity & e)
{
	// we can't modify the Entity.
	std::cout << e.GetX() << std::endl;
	// If there is no const in GetX(), then can't use e.GetX().
	// Because the value could be changed.
}