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# Q3DeepCopy Class Reference
[Qt3Support module]

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

     #include <Q3DeepCopy>

This class is part of the Qt 3 support library. It is provided to keep old source code working. We strongly advise against using it in new code. See Porting to Qt 4 for more information.

Note: All the functions in this class are reentrant.

- List of all members, including inherited members

### Public Functions

- Q3DeepCopy ()
- Q3DeepCopy ( const T & t )
- operator T ()
- Q3DeepCopy<T> & operator= ( const T & t )

## 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.

## Member Function Documentation

### Q3DeepCopy::Q3DeepCopy ()

Constructs an empty instance of type T.

### Q3DeepCopy::Q3DeepCopy ( const T & t )

Constructs a deep copy of t.

### Q3DeepCopy::operator T ()

Returns a deep copy of the encapsulated data.

### Q3DeepCopy<T> & Q3DeepCopy::operator= ( const T & t )

Assigns a deep copy of t.