Implicit Sharing
Many C++ classes in Qt use implicit data sharing to maximize resource usage and minimize copying. Implicitly shared classes are both safe and efficient when passed as arguments, because only a pointer to the data is passed around, and the data is copied only if and when a function writes to it, i.e., copy-on-write.
Overview
A shared class consists of a pointer to a shared data block that contains a reference count and the data.
When a shared object is created, it sets the reference count to 1. The reference count is incremented whenever a new object references the shared data, and decremented when the object dereferences the shared data. The shared data is deleted when the reference count becomes zero.
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 mostly takes place behind the scenes; the programmer rarely needs to worry about it. However, Qt's container iterators have different behavior than those from the STL. Read Implicit sharing iterator problem.
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.
Implicit Sharing in Detail
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 or value semantics.)
An implicitly shared class has control of its internal data. In any member functions that modify its data, it automatically detaches before modifying the data. Notice, however, the special case with container iterators; see Implicit sharing iterator problem.
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(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 } }
List of Classes
The classes listed below automatically detach from common data if an object is about to be changed. The programmer will not even notice that the objects are shared. Thus you should treat separate instances of them as separate objects. They will always behave as separate objects but with the added benefit of sharing data whenever possible. For this reason, you can pass instances of these classes as arguments to functions by value without concern for the copying overhead.
Example:
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();
In this example, p1
and p2
share data until QPainter::begin() is called for p2
, because painting a pixmap will modify it.
Warning: Be careful with copying an implicitly shared container (QMap, QList, etc.) while you use STL-style iterator. See Implicit sharing iterator problem.
Monochrome (1-bit depth) pixmaps | |
List of byte arrays | |
View on an array of bytes with a read-only subset of the QByteArray API | |
Template class that provides a cache | |
Defines a possible command-line option | |
Template class that provides a contiguous cache | |
Output stream for debugging information | |
Describes the parameters of an area or region to be monitored for proximity | |
Contains information gathered on a global position, direction and velocity at a particular point in time | |
Contains basic information about a satellite | |
Direct access to the internal glyphs in a font | |
Template class that provides a dynamic array | |
Converts between numbers and their string representations in various languages | |
Represents the data to be broadcast during Bluetooth Low Energy advertising | |
Represents the parameters used for Bluetooth Low Energy advertising | |
Used to set up GATT service data | |
Used when requesting or reporting an update of the parameters of a Bluetooth LE connection | |
Used to create GATT service data | |
Used to set up GATT service data | |
Template class that provides an associative array | |
Represents a MQTT topic filter | |
Represents a MQTT topic name | |
Template class that provides an associative array with multiple equivalent keys | |
Represents a collection of page ranges | |
Defines how a QPainter should draw lines and outlines of shapes | |
Off-screen image representation that can be used as a paint device | |
Generic container that provides a queue | |
Pattern matching using regular expressions | |
The results of a matching a QRegularExpression against a string | |
Iterator on the results of a global match of a QRegularExpression object against a string | |
Template class that provides a hash-table-based set | |
Manipulates the fields in SQL database tables and views | |
Means of executing and manipulating SQL statements | |
Encapsulates a database record | |
API for accessing the extensions of an X509 certificate | |
Represents an SSL cryptographic cipher | |
Authentication data for pre shared keys (PSK) ciphersuites | |
Template class that provides a stack | |
Provides information about currently mounted storage and drives | |
Way to manipulate a key-value pairs in a URL's query |