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237 lines
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<!-- /home/espenr/tmp/qt-3.3.8-espenr-2499/qt-x11-free-3.3.8/doc/shclass.doc:36 -->
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<font color="#004faf">Home</font></a>
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<font color="#004faf">All Classes</font></a>
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<font color="#004faf">Main Classes</font></a>
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<font color="#004faf">Annotated</font></a>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Shared Classes</h1>
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<p> <!-- index reference counting --><a name="reference-counting"></a><!-- index implicit sharing --><a name="implicit-sharing"></a><!-- index explicit sharing --><a name="explicit-sharing"></a><!-- index implicitly shared --><a name="implicitly-shared"></a><!-- index explicitly shared --><a name="explicitly-shared"></a><!-- index explicit sharing --><a name="explicit-sharing"></a><!-- index shared implicitly --><a name="shared-implicitly"></a><!-- index shared explicitly --><a name="shared-explicitly"></a>
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<p> Many C++ classes in TQt use <em>explicit</em> and <em>implicit</em> data sharing
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to maximize resource usage and minimize copying of data.
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<p> <!-- toc -->
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<ul>
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<li><a href="#1"> Overview
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</a>
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<li><a href="#2"> A TQByteArray Example
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</a>
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<li><a href="#3"> Explicit vs. Implicit Sharing
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</a>
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<li><a href="#4"> Explicitly Shared Classes
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</a>
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<li><a href="#5"> Implicitly Shared Classes
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</a>
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<li><a href="#6"> TQCString: implicit or explicit?
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</a>
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</ul>
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<!-- endtoc -->
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<p> <h2> Overview
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</h2>
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<a name="1"></a><p> A shared class consists of a pointer to a shared data block that
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contains a reference count and the data.
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<p> When a shared object is created, it sets the reference count to 1. The
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reference count is incremented whenever a new object references the
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shared data, and decremented when the object dereferences the shared
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data. The shared data is deleted when the reference count becomes
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zero.
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<p> <!-- index deep copy --><a name="deep-copy"></a><!-- index shallow copy --><a name="shallow-copy"></a>
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<p> When dealing with shared objects, there are two ways of copying an
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object. We usually speak about <em>deep</em> and <em>shallow</em> copies. A deep
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copy implies duplicating an object. A shallow copy is a reference
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copy, i.e. just a pointer to a shared data block. Making a deep copy
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can be expensive in terms of memory and CPU. Making a shallow copy is
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very fast, because it only involves setting a pointer and incrementing
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the reference count.
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<p> Object assignment (with operator=()) for implicitly and explicitly
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shared objects is implemented using shallow copies. A deep copy can be
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made by calling a copy() function or by using <a href="ntqdeepcopy.html">TQDeepCopy</a>.
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<p> The benefit of sharing is that a program does not need to duplicate
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data unnecessarily, which results in lower memory use and less copying
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of data. Objects can easily be assigned, sent as function arguments,
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and returned from functions.
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<p> Now comes the distinction between <em>explicit</em> and <em>implicit</em> sharing.
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Explicit sharing means that the programmer must be aware of the fact
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that objects share common data. Implicit sharing means that the
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sharing mechanism takes place behind the scenes and the programmer
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does not need to worry about it.
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<p> <h2> A <a href="qbytearray.html">TQByteArray</a> Example
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</h2>
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<a name="2"></a><p> TQByteArray is an example of a shared class that uses explicit sharing.
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Example:
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<pre>
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//Line a= b= c=
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<a href="qbytearray.html">TQByteArray</a> a(3),b(2) // 1: {?,?,?} {?,?}
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b[0] = 12; b[1] = 34; // 2: {?,?,?} {12,34}
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a = b; // 3: {12,34} {12,34}
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a[1] = 56; // 4: {12,56} {12,56}
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<a href="qbytearray.html">TQByteArray</a> c = a; // 5: {12,56} {12,56} {12,56}
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a.<a href="ntqmemarray.html#detach">detach</a>(); // 6: {12,56} {12,56} {12,56}
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a[1] = 78; // 7: {12,78} {12,56} {12,56}
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b = a.<a href="ntqmemarray.html#copy">copy</a>(); // 8: {12,78} {12,78} {12,56}
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a[1] = 90; // 9: {12,90} {12,78} {12,56}
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</pre>
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<p> The assignment <tt>a = b</tt> on line 3 throws away <tt>a</tt>'s original shared
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block (the reference count becomes zero), sets <tt>a</tt>'s shared block to
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point to <tt>b</tt>'s shared block and increments the reference count.
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<p> On line 4, the contents of <tt>a</tt> is modified. <tt>b</tt> is also modified,
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because <tt>a</tt> and <tt>b</tt> refer to the same data block. This is the
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difference between explicit and implicit sharing (explained below).
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<p> The <tt>a</tt> object detaches from the common data on line 6. Detaching
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means that the shared data is copied to make sure that an object has
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its own private data. Therefore, modifying <tt>a</tt> on line 7 does not
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affect <tt>b</tt> or <tt>c</tt>.
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<p> Finally, on line 8 we make a deep copy of <tt>a</tt> and assign it to <tt>b</tt>,
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so that when <tt>a</tt> is modified on line 9, <tt>b</tt> remains unchanged.
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<p> <h2> Explicit vs. Implicit Sharing
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</h2>
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<a name="3"></a><p> Implicit sharing automatically detaches the object from a shared block
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if the object is about to change and the reference count is greater
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than one. (This is often called "copy-on-write".) Explicit sharing
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leaves this job to the programmer. If an explicitly shared object is
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not detached, changing an object will change all other objects that
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refer to the same data.
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<p> Implicit sharing optimizes memory use and copying of data without
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this side effect. So why didn't we implement implicit sharing for all
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shared classes? The answer is that a class that allows direct access
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to its internal data (for efficiency reasons), like <a href="qbytearray.html">TQByteArray</a>, cannot
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be implicitly shared, because it can be changed without letting
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TQByteArray know.
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<p> An implicitly shared class has total control of its internal data. In
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any member functions that modify its data, it automatically detaches
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before modifying the data.
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<p> The <a href="ntqpen.html">TQPen</a> class, which uses implicit sharing, detaches from the shared
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data in all member functions that change the internal data.
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<p> Code fragment:
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<pre>
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void TQPen::setStyle( PenStyle s )
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{
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detach(); // detach from common data
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data->style = s; // set the style member
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}
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void TQPen::detach()
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{
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if ( data->count != 1 ) // only if >1 reference
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*this = copy();
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}
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</pre>
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<p> This is clearly not possible for TQByteArray, because the programmer
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can do the following:
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<p> <pre>
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<a href="qbytearray.html">TQByteArray</a> array( 10 );
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array.<a href="ntqmemarray.html#fill">fill</a>( 'a' );
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array[0] = 'f'; // will modify array
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array.<a href="ntqmemarray.html#data">data</a>()[1] = 'i'; // will modify array
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</pre>
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<p> If we monitor changes in a <a href="qbytearray.html">TQByteArray</a>, the TQByteArray class would
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become unacceptably slow.
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<p> <h2> Explicitly Shared Classes
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</h2>
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<a name="4"></a><p> All classes that are instances of the <a href="ntqmemarray.html">TQMemArray</a> template class are
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explicitly shared:
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<p> <ul>
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<li> <a href="ntqbitarray.html">TQBitArray</a>
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<li> <a href="ntqpointarray.html">TQPointArray</a>
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<li> <a href="qbytearray.html">TQByteArray</a>
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<li> Any other instantiation of <a href="ntqmemarray.html">TQMemArray<type></a>
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</ul>
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<p> These classes have a detach() function that can be called if you want
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your object to get a private copy of the shared data. They also have a
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copy() function that returns a deep copy with a reference count of 1.
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<p> The same is true for <a href="ntqimage.html">TQImage</a>, which does not inherit TQMemArray. <a href="ntqmovie.html">TQMovie</a> is also explicitly shared, but it does not support detach() or
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copy().
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<p> <h2> Implicitly Shared Classes
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</h2>
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<a name="5"></a><p> The TQt classes that are implicitly shared are:
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<ul>
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<li> <a href="ntqbitmap.html">TQBitmap</a>
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<li> <a href="ntqbrush.html">TQBrush</a>
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<li> <a href="ntqcursor.html">TQCursor</a>
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<li> <a href="ntqfont.html">TQFont</a>
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<li> <a href="ntqfontinfo.html">TQFontInfo</a>
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<li> <a href="ntqfontmetrics.html">TQFontMetrics</a>
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<li> <a href="ntqiconset.html">TQIconSet</a>
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<li> <a href="ntqmap.html">TQMap</a>
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<li> <a href="ntqpalette.html">TQPalette</a>
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<li> <a href="ntqpen.html">TQPen</a>
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<li> <a href="ntqpicture.html">TQPicture</a>
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<li> <a href="ntqpixmap.html">TQPixmap</a>
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<li> <a href="ntqregion.html">TQRegion</a>
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<li> <a href="ntqregexp.html">TQRegExp</a>
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<li> <a href="ntqstring.html">TQString</a>
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<li> <a href="ntqstringlist.html">TQStringList</a>
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<li> <a href="ntqvaluelist.html">TQValueList</a>
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<li> <a href="ntqvaluestack.html">TQValueStack</a>
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</ul>
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<p> These classes automatically detach from common data if an object is
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about to be changed. The programmer will not even notice that the
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objects are shared. Thus you should treat separate instances of them
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as separate objects. They will always behave as separate objects but
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with the added benefit of sharing data whenever possible. For this
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reason, you can pass instances of these classes as arguments to
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functions by value without concern for the copying overhead.
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<p> Example:
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<pre>
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<a href="ntqpixmap.html">TQPixmap</a> p1, p2;
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p1.<a href="ntqpixmap.html#load">load</a>( "image.bmp" );
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p2 = p1; // p1 and p2 share data
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<a href="ntqpainter.html">TQPainter</a> paint;
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paint.<a href="ntqpainter.html#begin">begin</a>( &p2 ); // cuts p2 loose from p1
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paint.<a href="ntqpainter.html#drawText">drawText</a>( 0,50, "Hi" );
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paint.<a href="ntqpainter.html#end">end</a>();
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</pre>
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<p> In this example, <tt>p1</tt> and <tt>p2</tt> share data until <a href="ntqpainter.html#begin">TQPainter::begin</a>() is
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called for <tt>p2</tt>, because painting a pixmap will modify it. The same
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also happens if anything is <a href="ntqimage.html#bitBlt">bitBlt()</a>'ed into
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<tt>p2</tt>.
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<p> <b>Warning:</b> Do not copy an implicitly shared container (<a href="ntqmap.html">TQMap</a>,
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<a href="ntqvaluevector.html">TQValueVector</a>, etc.) while you are iterating over it.
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<p> <h2> TQCString: implicit or explicit?
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</h2>
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<a name="6"></a><p> <a href="ntqcstring.html">TQCString</a> uses a mixture of implicit and explicit sharing. Functions
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inherited from <a href="qbytearray.html">TQByteArray</a>, such as data(), employ explicit sharing, while
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those only in <a href="ntqcstring.html">TQCString</a> detach automatically. Thus, TQCString is rather an
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"experts only" class, provided mainly to ease porting from TQt 1.x to TQt 2.0.
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We recommend that you use <a href="ntqstring.html">TQString</a>, a purely implicitly shared class.
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<p>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>TQt 3.3.8</div>
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</table></div></address></body>
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