C++ Classes/Structures Access specifiers
Example
There are three keywords that act as access specifiers. These limit the access to class members following the specifier, until another specifier changes the access level again:
| Keyword | Description |
|---|---|
public | Everyone has access |
protected | Only the class itself, derived classes and friends have access |
private | Only the class itself and friends have access |
When the type is defined using the class keyword, the default access specifier is private, but if the type is defined using the struct keyword, the default access specifier is public:
struct MyStruct { int x; };
class MyClass { int x; };
MyStruct s;
s.x = 9; // well formed, because x is public
MyClass c;
c.x = 9; // ill-formed, because x is private
Access specifiers are mostly used to limit access to internal fields and methods, and force the programmer to use a specific interface, for example to force use of getters and setters instead of referencing a variable directly:
class MyClass {
public: /* Methods: */
int x() const noexcept { return m_x; }
void setX(int const x) noexcept { m_x = x; }
private: /* Fields: */
int m_x;
};
Using protected is useful for allowing certain functionality of the type to be only accessible to the derived classes, for example, in the following code, the method calculateValue() is only accessible to classes deriving from the base class Plus2Base, such as FortyTwo:
struct Plus2Base {
int value() noexcept { return calculateValue() + 2; }
protected: /* Methods: */
virtual int calculateValue() noexcept = 0;
};
struct FortyTwo: Plus2Base {
protected: /* Methods: */
int calculateValue() noexcept final override { return 40; }
};
Note that the friend keyword can be used to add access exceptions to functions or types for accessing protected and private members.
The public, protected, and private keywords can also be used to grant or limit access to base class subobjects. See the Inheritance example.
