Shared Classes

Implicit sharing automatically detaches the object from a shared block if the object is about to change and the reference count is greater than one. (This is often called "copy-on-write".) Explicit sharing leaves this job to the programmer. If an explicitly shared object is not detached, changing an object will change all other objects that refer to the same data.

Implicit sharing optimizes memory use and copying of data without this side effect. So why didn't we implement implicit sharing for all shared classes? The answer is that a class that allows direct access to its internal data (for efficiency reasons), like QByteArray, cannot be implicitly shared, because it can be changed without letting QByteArray know.

An implicitly shared class has total control of its internal data. In any member functions that modify its data, it automatically detaches before modifying the data.

The QPen class, which uses implicit sharing, detaches from the shared data in all member functions that change the internal data.

Code fragment:

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

    void QPen::detach()
    {
        if ( data->count != 1 ) // only if >1 reference
            *this = copy();
    }

This is clearly not possible for QByteArray, because the programmer can do the following:

    QByteArray array( 10 );
    array.fill( 'a' );
    array[0] = 'f';        // will modify array
    array.data()[1] = 'i'; // will modify array

If we monitor changes in a QByteArray, the QByteArray class would become unacceptably slow.

Explicitly Shared Classes

    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();