Member Access Control (C++)

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01/03/2018

The latest version of this topic can be found at Member Access Control (C++).

Access controls enable you to separate the public interface of a class from the private implementation details and the protected members that are only for use by derived classes. The access specifier applies to all members declared after it until the next access specifier is encountered.

class Point  
{  
public:                   
    Point( int, int ) // Declare public constructor.;  
    Point();// Declare public default constructor.  
    int &x( int ); // Declare public accessor.  
    int &y( int ); // Declare public accessor.  
  
private:                 // Declare private state variables.  
    int _x;  
    int _y;  
  
protected:      // Declare protected function for derived classes only.  
    Point ToWindowCoords();  
};

The default access is private in a class, and public in a struct or union. Access specifiers in a class can be used any number of times in any order. The allocation of storage for objects of class types is implementation dependent, but members are guaranteed to be assigned successively higher memory addresses between access specifiers.

Member-Access Control

Type of Access	Meaning
private	Class members declared as `private` can be used only by member functions and friends (classes or functions) of the class.
protected	Class members declared as `protected` can be used by member functions and friends (classes or functions) of the class. Additionally, they can be used by classes derived from the class.
public	Class members declared as public can be used by any function.

Access control helps prevent you from using objects in ways they were not intended to be used. This protection is lost when explicit type conversions (casts) are performed.

Note

Access control is equally applicable to all names: member functions, member data, nested classes, and enumerators.

Access Control in Derived Classes

Two factors control which members of a base class are accessible in a derived class; these same factors control access to the inherited members in the derived class:

Whether the derived class declares the base class using the public access specifier in the class-head (class-head is described in the Grammar section in Defining Class Types).
What the access to the member is in the base class.

The following table shows the interaction between these factors and how to determine base-class member access.

Member Access in Base Class

private	protected	Public
Always inaccessible regardless of derivation access	Private in derived class if you use private derivation	Private in derived class if you use private derivation
	Protected in derived class if you use protected derivation	Protected in derived class if you use protected derivation
	Protected in derived class if you use public derivation	Public in derived class if you use public derivation

The following example illustrates this:

// access_specifiers_for_base_classes.cpp  
class BaseClass  
{  
public:  
    int PublicFunc();    // Declare a public member.  
protected:  
    int ProtectedFunc(); // Declare a protected member.  
private:  
    int PrivateFunc();   // Declare a private member.  
};  
  
// Declare two classes derived from BaseClass.  
class DerivedClass1 : public BaseClass  
{  
};  
  
class DerivedClass2 : private BaseClass  
{  
};  
  
int main()  
{  
}

In DerivedClass1, the member function PublicFunc is a public member and ProtectedFunc is a protected member because BaseClass is a public base class. PrivateFunc is private to BaseClass, and it is inaccessible to any derived classes.

In DerivedClass2, the functions PublicFunc and ProtectedFunc are considered private members because BaseClass is a private base class. Again, PrivateFunc is private to BaseClass, and it is inaccessible to any derived classes.

You can declare a derived class without a base-class access specifier. In such a case, the derivation is considered private if the derived class declaration uses the class keyword. The derivation is considered public if the derived class declaration uses the struct keyword. For example, the following code:

class Derived : Base  
...

is equivalent to:

class Derived : private Base  
...

Similarly, the following code:

struct Derived : Base  
...

is equivalent to:

struct Derived : public Base  
...

Note that members declared as having private access are not accessible to functions or derived classes unless those functions or classes are declared using the friend declaration in the base class.

A union type cannot have a base class.

Note

When specifying a private base class, it is advisable to explicitly use the private keyword so users of the derived class understand the member access.

Access control and static members

When you specify a base class as private, it affects only nonstatic members. Public static members are still accessible in the derived classes. However, accessing members of the base class using pointers, references, or objects can require a conversion, at which time access control is again applied. Consider the following example:

// access_control.cpp  
class Base  
{  
public:  
    int Print();             // Nonstatic member.  
    static int CountOf();    // Static member.  
};  
  
// Derived1 declares Base as a private base class.  
class Derived1 : private Base  
{  
};  
// Derived2 declares Derived1 as a public base class.  
class Derived2 : public Derived1  
{  
    int ShowCount();    // Nonstatic member.  
};  
// Define ShowCount function for Derived2.  
int Derived2::ShowCount()  
{  
   // Call static member function CountOf explicitly.  
    int cCount = Base::CountOf();     // OK.  
  
   // Call static member function CountOf using pointer.  
    cCount = this->CountOf();  // C2247. Conversion of  
                               //  Derived2 * to Base * not  
                               //  permitted.  
    return cCount;  
}

In the preceding code, access control prohibits conversion from a pointer to Derived2 to a pointer to Base. The this pointer is implicitly of type Derived2 *. To select the CountOf function, this must be converted to type Base *. Such a conversion is not permitted because Base is a private indirect base class to Derived2. Conversion to a private base class type is acceptable only for pointers to immediate derived classes. Therefore, pointers of type Derived1 * can be converted to type Base *.

Note that calling the CountOf function explicitly, without using a pointer, reference, or object to select it, implies no conversion. Therefore, the call is allowed.

Members and friends of a derived class, T, can convert a pointer to T to a pointer to a private direct base class of T.

Access to virtual functions

The access control applied to virtual functions is determined by the type used to make the function call. Overriding declarations of the function do not affect the access control for a given type. For example:

// access_to_virtual_functions.cpp  
class VFuncBase  
{  
public:  
    virtual int GetState() { return _state; }  
protected:  
    int _state;  
};  
  
class VFuncDerived : public VFuncBase  
{  
private:  
    int GetState() { return _state; }  
};  
  
int main()  
{  
   VFuncDerived vfd;             // Object of derived type.  
   VFuncBase *pvfb = &vfd;       // Pointer to base type.  
   VFuncDerived *pvfd = &vfd;    // Pointer to derived type.  
   int State;  
  
   State = pvfb->GetState();     // GetState is public.  
   State = pvfd->GetState();     // C2248 error expected; GetState is private;  
}

In the preceding example, calling the virtual function GetState using a pointer to type VFuncBase calls VFuncDerived::GetState, and GetState is treated as public. However, calling GetState using a pointer to type VFuncDerived is an access-control violation because GetState is declared private in class VFuncDerived.

Warning

The virtual function GetState can be called using a pointer to the base class VFuncBase. This does not mean that the function called is the base-class version of that function.

Access control with multiple inheritance

In multiple-inheritance lattices involving virtual base classes, a given name can be reached through more than one path. Because different access control can be applied along these different paths, the compiler chooses the path that gives the most access. See the following figure.

Access Along Paths of an Inheritance Graph

In the figure, a name declared in class VBase is always reached through class RightPath. The right path is more accessible because RightPath declares VBase as a public base class, whereas LeftPath declares VBase as private.

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