Difference between revisions of "cpp/language/this"

Revision as of 19:10, 25 April 2020

Syntax

The keyword this is a prvalue expression whose value is the address of the implied object argument. (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.

During construction of an object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified. In other words, the this pointer cannot be aliased in a constructor:

extern struct D d;
struct D {
    D(int a) : a(a), b(d.a) {} // b(a) or b(this->a) would be correct
    int a, b;
};
D d = D(1);   // because b(d.a) did not obtain a through this, d.b is now unspecified

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.

class ref
{
    // ...
    void incRef() { ++mnRef; }
    void decRef() { if (--mnRef == 0) delete this; }
};

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
        };
    }
}