Implicit Sharing

When dealing with shared objects, there are two ways of copying an object. We usually speak about deep and shallow copies. A deep copy implies duplicating an object. A shallow copy is a reference copy, i.e. just a pointer to a shared data block. Making a deep copy can be expensive in terms of memory and CPU. Making a shallow copy is very fast, because it only involves setting a pointer and incrementing the reference count.

Object assignment (with operator=()) for implicitly shared objects is implemented using shallow copies.

The benefit of sharing is that a program does not need to duplicate data unnecessarily, which results in lower memory use and less copying of data. Objects can easily be assigned, sent as function arguments, and returned from functions.

Implicit sharing takes place behind the scenes; the programmer does not need to worry about it. Even in multithreaded applications, implicit sharing takes place, as explained in Threads and Implicitly Shared Classes.

When implementing your own implicitly shared classes, use the QSharedData and QSharedDataPointer classes.

 void QPen::setStyle(Qt::PenStyle style)
 {
     detach();           // detach from common data
     d->style = style;   // set the style member
 }

 void QPen::detach()
 {
     if (d->ref != 1) {
         ...             // perform a deep copy
     }
 }

 QPixmap p1, p2;
 p1.load("image.bmp");
 p2 = p1;                        // p1 and p2 share data

 QPainter paint;
 paint.begin(&p2);               // cuts p2 loose from p1
 paint.drawText(0,50, "Hi");
 paint.end();