Q3DeepCopy Class Reference
[Qt3Support module]

Detailed Description

The Q3DeepCopy class is a template class which ensures that implicitly shared and explicitly shared classes reference unique data.

Normally, shared copies reference the same data to optimize memory use and for maximum speed. In the example below, s1, s2, s3, s4 and s5 share data.

 // all 5 strings share the same data
 QString s1 = "abcd";
 QString s2 = s1;
 QString s3 = s2;
 QString s4 = s3;
 QString s5 = s2;

Q3DeepCopy can be used several ways to ensure that an object references unique, unshared data. In the example below, s1, s2 and s5 share data, while neither s3 nor s4 share data.

 // s1, s2 and s5 share the same data, neither s3 nor s4 are shared
 QString s1 = "abcd";
 QString s2 = s1;
 Q3DeepCopy<QString> s3 = s2;  // s3 is a deep copy of s2
 QString s4 = s3;             // s4 is a deep copy of s3
 QString s5 = s2;

In the example below, s1, s2 and s5 share data, and s3 and s4 share data.

 // s1, s2 and s5 share the same data, s3 and s4 share the same data
 QString s1 = "abcd";
 QString s2 = s1;
 QString s3 = Q3DeepCopy<QString>( s2 );  // s3 is a deep copy of s2
 QString s4 = s3;                        // s4 is a shallow copy of s3
 QString s5 = s2;

Q3DeepCopy can also provide safety in multithreaded applications that use shared classes. In the example below, the variable global_string is used safely since the data contained in global_string is always a deep copy. This ensures that all threads get a unique copy of the data, and that any assignments to global_string will result in a deep copy.

 Q3DeepCopy<QString> global_string;  // global string data
 QMutex global_mutex;               // mutex to protext global_string

 ...

 void setGlobalString( const QString &str )
 {
     global_mutex.lock();
     global_string = str;           // global_string is a deep copy of str
     global_mutex.unlock();
 }

 ...

 void MyThread::run()
 {
     global_mutex.lock();
     QString str = global_string;          // str is a deep copy of global_string
     global_mutex.unlock();

     // process the string data
     ...

     // update global_string
     setGlobalString( str );
 }

Warning: It is the application developer's responsibility to protect the object shared across multiple threads.

The examples above use QString, which is an implicitly shared class. The behavior of Q3DeepCopy is the same when using explicitly shared classes like QByteArray.

Currently, Q3DeepCopy works with the following classes:

• QMemArray (including subclasses like QByteArray and QCString)
• QMap
• QString
• QValueList (including subclasses like QStringList and QValueStack)
• QValueVector

See also Thread Support in Qt.

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