Clause 11: handle self-assignment in operator =

Self-assignment occurs when an object is assigned to itself:

Class widget {

...

}

Widget W;

...

W = W // assign a value to yourself

There is also an implicit example of self-assignment:

A [I] = A [J]; // when I = J

The following is a class:

Class bitmap {};

Class widget {

...

PRIVATE:

Bitmap * pb;

};

L version 1

Widget & Widget: Operator = (const widget & RHs ){

Delete Pb;

PB = new Bitmap (* RHS. Pb );

Return * this;

}

Note: There is a problem here, that is, if RHS refers to this object. After deleting Pb, the Pb of this is deleted.

 

To prevent such an error, you can add a certificate to the test:

L Version 2

Widget & Widget: Operator = (const widget & RHs ){

If (this = RHs) return * This; // same as test

Delete Pb;

PB = new Bitmap (* RHS. Pb );

Return * this;

}

 

This is acceptable, but another problem is that if an exception occurs during the new bitmap operation, the widget object will eventually hold a pointer to a deleted bitmap. Such pointers are harmful. You cannot safely delete them or read them.

 

However, this error can be avoided after careful arrangement:

L Version 3:

Widget & Widget: Operator = (const widget & RHs ){

Bitmap * OBJ = RHS. Pb;

PB = new Bitmap (* RHS. Pb );

Delete OBJ;

Return * this;

}

If a new bitmap exception occurs, petabytes remain unchanged. Note: although we have not added the same certification test, the aboveCodeYou can also handle the phenomenon of Self-assignment.

 

Version 4:

There is also a replacement version for Version 3:

Class widget {

Public:

...

Swap (widget & RHs );

PRIVATE:

Bitmap * pb;

};

Widget & Widget: Operator = (const widget & RHs ){

Widget temp (RHs );

Swap (temp );

Return * this;

}

This technology is called copy and swap technology.

• Remember:
• Make sure that operator = has good behavior when the object is assigned a value. Technologies include:

1. Compare the addresses of "source object" and "target object"
2. Careful and thoughtful statement order
3. Copy and swap Technology

• Determine if any function operates on more than one object, and multiple objects are the same object, the behavior is correct.