7 Expressions [expr]

7.5 Primary expressions [expr.prim]

7.5.3 This [expr.prim.this]

The keyword this names a pointer to the object for which an implicit object member function ([class.mfct.non.static]) is invoked or a non-static data member's initializer ([class.mem]) is evaluated.
The current class at a program point is the class associated with the innermost class scope containing that point.
[Note 1: 
A lambda-expression does not introduce a class scope.
— end note]
If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” wherever X is the current class between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator.
It shall not appear within the declaration of a static or explicit object member function of the current class (although its type and value category are defined within such member functions as they are within an implicit object member function).
[Note 2: 
This is because declaration matching does not occur until the complete declarator is known.
— end note]
[Note 3: 
In a trailing-return-type, the class being defined is not required to be complete for purposes of class member access.
Class members declared later are not visible.
[Example 1: struct A { char g(); template<class T> auto f(T t) -> decltype(t + g()) { return t + g(); } }; template auto A::f(int t) -> decltype(t + g()); — end example]
— end note]
Otherwise, if a member-declarator declares a non-static data member ([class.mem]) of a class X, the expression this is a prvalue of type “pointer to X” wherever X is the current class within the optional default member initializer ([class.mem]).
The expression this shall not appear in any other context.
[Example 2: class Outer { int a[sizeof(*this)]; // error: not inside a member function unsigned int sz = sizeof(*this); // OK, in default member initializer void f() { int b[sizeof(*this)]; // OK struct Inner { int c[sizeof(*this)]; // error: not inside a member function of Inner }; } }; — end example]