Const.md

Const in C++

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const is a keyword in cpp that 'promises' to not change something. this thing will be 'constant' =) You can break those promises too.

#include <iostream>
#include <string>
#include <stdlib.h>  // some C functions

int main()
{
    const int MAX_AGE = 5;

    // the compiler will warn you if you try to change it
    //MAX_AGE = 90;


    // const int*
    const int* a = new int; 	//the same as int const* a = new int;
    //*a = 2; // Cannot do this, we cannot change the value, to where it is pointing
    a = &MAX_AGE;

    // int* const
    int* const b = new int;
    *b = 2;
    // Cannot do this, we cannot change the pointer itself
    //b = &MAX_AGE;

    // Cannot change anything
    const int* const c = new int(8);
    // *c = &MAX_AGE;
    // *c = 7;

    std::cout << *a << std::endl;
    std::cout << MAX_AGE << std::endl;
    std::cout << *b << std::endl;
    std::cout << *c << std::endl;
    std::cin.get();
}

// const in class
class Entity
{
private:
    int m_X, m_Y;
    int* ptr;
    int* a, b;	// b is int, not int*
    int* c, *d;
    mutable int var;
public:
    // this method will not modify any of this private values in class
    int GetX() const
    {
        // This lines gives us compiler errors
        //m_X = 3;
        //m_Y = 5;
        var = 2; // we CAN modify mutable variables in const methods
        return m_X;
    }

    // 1st const: Returning a pointer that can't be modified
    // 2nd const: The content of the pointer returned cannot be modified
    // 3rd const: this method promises to not modified the entity class
    const int* const GetPrt() const
    {
        return ptr;
    }

    // Not using const in Setters
    int SetX(int x)
    {
        m_X = x;
    }

    // passing by reference, can only use const methods
    void PrintEntity(const Entity& e)
    {		
        std::cout << e.GetX() << std::endl;
    }
};