The this pointer

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The this pointer

  • Article
  • 06/03/2022
  • 3 minutes to read

In this article

The this pointer is a pointer accessible only within the nonstatic member functions of a class, struct, or union type. It points to the object for which the member function is called. Static member functions don’t have a this pointer.

Syntax

this
this->member-identifier

Remarks

An object’s this pointer isn’t part of the object itself. It’s not reflected in the result of a sizeof statement on the object. When a nonstatic member function is called for an object, the compiler passes the object’s address to the function as a hidden argument. For example, the following function call:

myDate.setMonth( 3 );

can be interpreted as:

setMonth( &myDate, 3 );

The object’s address is available from within the member function as the this pointer. Most this pointer uses are implicit. It’s legal, though unnecessary, to use an explicit this when referring to members of the class. For example:

void Date::setMonth( int mn )
{
   month = mn;            // These three statements
   this->month = mn;      // are equivalent
   (*this).month = mn;
}

The expression *this is commonly used to return the current object from a member function:

return *this;

The this pointer is also used to guard against self-reference:

if (&Object != this) {
// do not execute in cases of self-reference

Note

Because the this pointer is nonmodifiable, assignments to the this pointer are not allowed. Earlier implementations of C++ allowed assignment to this.

Occasionally, the this pointer is used directly — for example, to manipulate self-referential data structures, where the address of the current object is required.

Example

// this_pointer.cpp
// compile with: /EHsc

#include <iostream>
#include <string.h>

using namespace std;

class Buf
{
public:
    Buf( char* szBuffer, size_t sizeOfBuffer );
    Buf& operator=( const Buf & );
    void Display() { cout << buffer << endl; }

private:
    char*   buffer;
    size_t  sizeOfBuffer;
};

Buf::Buf( char* szBuffer, size_t sizeOfBuffer )
{
    sizeOfBuffer++; // account for a NULL terminator

    buffer = new char[ sizeOfBuffer ];
    if (buffer)
    {
        strcpy_s( buffer, sizeOfBuffer, szBuffer );
        sizeOfBuffer = sizeOfBuffer;
    }
}

Buf& Buf::operator=( const Buf &otherbuf )
{
    if( &otherbuf != this )
    {
        if (buffer)
            delete [] buffer;

        sizeOfBuffer =  strlen( otherbuf.buffer ) + 1;
        buffer = new char[sizeOfBuffer];
        strcpy_s( buffer, sizeOfBuffer, otherbuf.buffer );
    }
    return *this;
}

int main()
{
    Buf myBuf( "my buffer", 10 );
    Buf yourBuf( "your buffer", 12 );

    // Display 'my buffer'
    myBuf.Display();

    // assignment operator
    myBuf = yourBuf;

    // Display 'your buffer'
    myBuf.Display();
}

my buffer
your buffer

Type of the this pointer

The this pointer’s type can be modified in the function declaration by the const and volatile keywords. To declare a function that has either of these attributes, add the keyword(s) after the function argument list.

Consider an example:

// type_of_this_pointer1.cpp
class Point
{
    unsigned X() const;
};
int main()
{
}

The preceding code declares a member function, X, in which the this pointer is treated as a const pointer to a const object. Combinations of cv-mod-list options can be used, but they always modify the object pointed to by the this pointer, not the pointer itself. The following declaration declares function X, where the this pointer is a const pointer to a const object:

// type_of_this_pointer2.cpp
class Point
{
    unsigned X() const;
};
int main()
{
}

The type of this in a member function is described by the following syntax. The cv-qualifier-list is determined from the member function’s declarator. It can be const or volatile (or both). class-type is the name of the class:

[cv-qualifier-list] class-type * const this

In other words, the this pointer is always a const pointer. It can’t be reassigned. The const or volatile qualifiers used in the member function declaration apply to the class instance the this pointer points at, in the scope of that function.

The following table explains more about how these modifiers work.

Semantics of this modifiers

Modifier
Meaning

const
Can’t change member data; can’t invoke member functions that aren’t const.

volatile
Member data is loaded from memory each time it’s accessed; disables certain optimizations.

It’s an error to pass a const object to a member function that isn’t const.

Similarly, it’s also an error to pass a volatile object to a member function that isn’t volatile.

Member functions declared as const can’t change member data — in such functions, the this pointer is a pointer to a const object.

Note

Constructors and destructors can’t be declared as const or volatile. They can, however, be invoked on const or volatile objects.

See also

Keywords

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