Difference between revisions of "cpp/language/this"

Revision as of 12:14, 20 October 2015

Syntax

The keyword this is a prvalue expression whose value is the address of the object, on which the member function is being called. It can appear in the following contexts:

The type of this in a member function of class X is X* (pointer to X). If the member function is cv-qualified, the type of this is cv X* (pointer to identically cv-qualified X). Since constructors and destructors cannot be cv-qualified, the type of this in them is always X*, even when constructing or destroying a const object.

When a non-static class member is used in any of the contexts where the this keyword is allowed (non-static member function bodies, member initializer lists, default member initializers), the implicit this-> is automatically added before the name, resulting in a member access expression (which, if the member is a virtual member function, results in a virtual function call).

In class templates, this is a dependent expression, and explicit this-> may be used to force another expression to become dependent.

It is possible to execute delete this;, if the program can guarantee that the object was allocated by new, however, this renders every pointer to the deallocated object invalid, including the this pointer itself: after delete this; returns, such member function cannot refer to a member of a class (since this involves an implicit dereference of this) and no other member function may be called. This is used, for example, in the member function of the control block of std::shared_ptr responsible for decrementing the reference count, when the last reference to the managed object goes out of scope.

Example

class T
{
    int x;
 
    void foo()
    {
        x = 6;       // same as this->x = 6;
        this->x = 5; // explicit use of this->
    }
 
    void foo() const
    {
//        x = 7; // Error: *this is constant
    }
 
    void foo(int x) // parameter x shadows the member with the same name
    {
        this->x = x; // unqualified x refers to the parameter
                     // 'this->' required for disambiguation
    }
 
    int y;
    T(int x) : x(x), // uses parameter x to initialize member x
               y(this->x) // uses member x to initialize member y
    {}
 
    T& operator= ( const T& b )
    {
        x = b.x;
        return *this; // many overloaded operators return *this
    }
};
 
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
        };
    }
}
int main()
{
    T t;
}