C++ General: What is the 'this' pointer?

[cilu]

Q: What is the 'this' pointer?

A: It is a misbelief that the 'this' pointer is a hidden member of a class or struct. It is a hidden parameter of non-static member functions. When you declare a function the compiler adds an extra parameter to function's prototype. The type of the parameter depends on how the function is declared. According to C++ standard, 9.3.2.1:

In the body of a nonstatic member function, the keyword this is a non-lvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*. If the member function is declared const, the type of this is const X*, if the member function is declared volatile, the type of this is volatile X*, and if the member function is declared const volatile, the type of this is const volatile X*.

For instance

class T
{
public:
  void foo(int a);
  int goo() const;
};

is actually:

class T
{
public:
  void foo(T* this , int a);
  int goo(const T* this) const;
};

Static member functions, which don’t have class scope, do not have this extra parameter. One consequence is that you cannot use a non-static member function as a thread function even if it has the correct prototype

UINT ThreadFunction(LPVOID param);

because that in fact the prototype (when non-static) is

UINT ThreadFunction(T* this, LPVOID param);