Classes

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General

• classes and structs are basically the same in C++, the only difference is the default access modifier:
  • public for structs
  • private for classes

Static data members

• static data members can be declared inline and initialized in the class definition since C++17

Non-static member functions

Ref-qualified member functions

A non-static member function can be declared with:

• no ref-qualifier - void f() {...}
  • implicit object parameter has type lvalue reference to cv-qualified X and is additionally allowed to bind rvalue implied object argument
• lvalue ref-qualifier - void f() & {...}
  • the implicit object parameter has type lvalue reference to cv-qualified X
• the rvalue ref-qualifier - void f() && {...}
  • the implicit object parameter has type rvalue reference to cv-qualified X

No ref-qualifed overload cannot be mixed with ref-qualified overloads.

Unlike cv-qualification, ref-qualification does not change the properties of the this pointer: within a rvalue ref-qualified function, *this remains an lvalue expression.

Nested classes

Class declared inside other class is nested class:

• the name of the nested class exists only in the scope of the enclosing class
• nested class can use any members of the enclosing class
  • it has access to all names to which enclosing class has access (private, protected, …)
  • it has no special access to this pointer of the enclosing class
• name lookup from a member function of a nested class visits the scope of the enclosing class after examining the scope of the nested class
• friend functions of nested class have no special access to the members of the enclosing class
• nested classes can be forward-declared and later defined, either within the same enclosing class body, or outside of it:

  class enclose {
      class nested1; // forward declaration
      class nested2; // forward declaration
      class nested1 {}; // definition of nested class
  };
  class enclose::nested2 { }; // definition of nested class
  

Local classes

Class declared inside body of a function is local class:

• the name of local class exists only in the function scope
• cannot have static data members
• member functions of a local class have no linkage and they need to be defined entirely inside the class body
• local class inside a function (including member function) can access the same names that the enclosing function can access