30 Visibility in C++

It is a concept in OOP that refers to whether the members or methods of a class are visible. Visibility means who can access them, who can call them, and who can use them.

Visibility does not affect the actual running state of your program, nor does it affect aspects like program performance. It is purely a language-level concept that helps you write better code and organize your code more effectively.

1. Three Basic Visibility Modifiers

1. private

The difference between struct and public: struct defaults to public, while class defaults to private. See Lesson 19

class Entity
{
private:    // Only *this Entity class can access these variables
    int X, Y;
};

Only : Actually, in C++, there is also a keyword called friend that can mark other classes or functions as friends of the current class, allowing you to access the private* members of this class.

class Entity
{
private:
    int X, Y;
    void Print() {}
public:
    Entity()
    {
        X = 0;
        Print();  
    }
};

class Player : public Entity
{
public:
    Player()
    {
        X = 2;
        Print();  // Cannot access
    }
};

int main()
{
    Entity e;
    e.Print();    // Cannot access
    e.X = 2;      // Cannot access

    std::cin.get();
}

2. protected

The visibility of protected is higher than private but lower than public. It means that this class and all its derived classes can access these members.

protected:  // Derived classes can access
    int X, Y;

    void Print() {}
......

class Player : public Entity
{
public:
    Player()
    {
        X = 2;
        Print();   // Can access
    }
};

int main()   
{
    Entity e;
    e.Print();    // Not a subclass, cannot access
    e.X = 2;      // Cannot access

    std::cin.get();
}

3. public

class Entity
{
public:
    int X, Y;

    void Print() {}
public:
    Entity()
    {
        X = 0;
        Print();
    }
};

class Player : public Entity
{
public:
    Player()
    {
        X = 2;
        Print();   // Can access
    }
};

int main()
{
    Entity e;
    e.Print();     // Can access
    e.X = 2;       // Can access

    std::cin.get();
}

2. Why Set Visibility

First, setting everything to public is definitely a bad idea for developers.

Specifying visibility ensures that others do not call code they should not touch, causing some damage.

It also provides better readability, so you don't forget.