Discussion about implicit sharing or copy on write. More...

Detailed Description

There are several classes in CopperSpice which still use "implicit data sharing" which in C++ is more commonly referred to as "copy on write". This was done to minimize copying and was implemented before C++ had move semantics. Implicitly shared classes were more efficient before the addition of move semantics in C++. The basic idea is to defer a copy of the data as long as possible.

Refer to references for a detailed description of rvalues and move semantics.

Overview

An implicitly shared class consists of a pointer to a shared area of memory, which contains a reference count and the actual data. When an implicitly shared object is created the reference count is 1. The reference count is incremented when a new object references the shared data and decremented when the object no longer references the shared data. The shared data is deleted when the reference count becomes zero.

Implicit sharing allows a program to reduce memory usage when the same data is referenced by many instances. If you want to implement an implicitly shared class, use the QSharedData and QSharedDataPointer classes. For information on how implicit sharing works with threads, refer to Threads and Implicitly Shared Classes.

List of Classes

The classes listed above are implicitly shared. In this example p1 and p2 share data until QPainter::begin() is called for p2, because painting a pixmap will modify it.

Warning

Do not copy an implicitly shared container while you are iterating over it using a non-const STL-style iterator.