Difference between revisions of "cpp/language/this"

Revision as of 22:55, 19 May 2023

Syntax

The expression this is a prvalue expression whose value is the address of the implicit object parameter (object on which the non-static member function(until C++23)implicit object member function(since C++23) is being called). It can appear in the following contexts:

Explanation

this can only associate with the innermost enclosing class of its appearance, even if the appearance is invalid in the context:

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
                                  // 'this' is not associated with Outer
                                  // even if it is inside a member function of Outer
        };
    }
};

The type of this in a member function of class X is X* (pointer to X). If the member function is declared with a cv-qualifier sequence cv, the type of this is cv X* (pointer to identically cv-qualified X). Since constructors and destructors cannot be declared with cv-qualifiers, 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.

template<typename T>
struct B
{
    int var;
};
 
template<typename T>
struct D : B<T>
{
    D()
    {
        // var = 1;    // error: 'var' was not declared in this scope
        this->var = 1; // OK
    }
};

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 can be used in the member function of the reference-counting pointer (for example, std::shared_ptr)(since C++11) 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; }
};

Keywords

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
    }
};

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.