Implicitly Shared Classes

A hybrid kind of managed container is the implicitly shared container. Container classes in the Qt library implement "lazy copy on write," or implicit sharing. This means they have reasonably fast[3] copy constructors and assignment operators. Only when the copy is actually modified are the collected objects cloned from the original container. That is when there will be a time/memory penalty.

[3] Operations should be "not much slower" than the time it takes to do one pointer-copy and one integer increment.

QString and QStringList are both implemented this way, meaning that it is fast to pass and return these objects by value. If you need to change values stored in the container from inside a function, you can pass the container by reference. It is still faster to pass by const reference, which allows C++ to optimize out the copy operation entirely. With const reference, the function cannot make changes to the container at all.

Implicitly shared classes work by reference-counting, to prevent the accidental deletion of shared managed objects. Since implicitly shared memory is encapsulated, the user of the class does not need to be concerned with reference counts or direct memory pointers.

We look in more detail at the implementation of a reference counted class in Section 24.2.


Part I: Introduction to C++ and Qt 4

C++ Introduction

Classes

Introduction to Qt

Lists

Functions

Inheritance and Polymorphism

Part II: Higher-Level Programming

Libraries

Introduction to Design Patterns

QObject

Generics and Containers

Qt GUI Widgets

Concurrency

Validation and Regular Expressions

Parsing XML

Meta Objects, Properties, and Reflective Programming

More Design Patterns

Models and Views

Qt SQL Classes

Part III: C++ Language Reference

Types and Expressions

Scope and Storage Class

Statements and Control Structures

Memory Access

Chapter Summary

Inheritance in Detail

Miscellaneous Topics

Part IV: Programming Assignments

MP3 Jukebox Assignments

Part V: Appendices

MP3 Jukebox Assignments

Bibliography

MP3 Jukebox Assignments



An Introduction to Design Patterns in C++ with Qt 4
An Introduction to Design Patterns in C++ with Qt 4
ISBN: 0131879057
EAN: 2147483647
Year: 2004
Pages: 268

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