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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit - Collection Classes</title><style type="text/css"><!--h3.fn,span.fn { margin-left: 1cm; text-indent: -1cm; }a:link { color: #004faf; text-decoration: none }a:visited { color: #672967; text-decoration: none }body { background: white; color: black; }--></style></head><body bgcolor="#ffffff"><p><table width="100%"><tr><td><a href="index.html"><img width="100" height="100" src="qtlogo.png"alt="Home" border="0"><img width="100"height="100" src="face.png" alt="Home" border="0"></a><td valign="top"><div align="right"><img src="dochead.png" width="472" height="27"><br><a href="classes.html"><b>Classes</b></a>- <a href="annotated.html">Annotated</a>- <a href="hierarchy.html">Tree</a>- <a href="functions.html">Functions</a>- <a href="index.html">Home</a>- <a href="topicals.html"><b>Structure</b> <font face="Arial,Helvetica,Geneva,Swiss,SunSans-Regular" align="center" size=32>Qte</font></a></div></table><h1 align="center"> Collection Classes</h1><br clear="all">A collection class is a class that can contain a number of items in acertain data structure and perform operations on the contained items;insert, remove, find etc.<p>Qt has many reference based collection classes and some value basedcollections.<p>The reference based collections are:<ul><li> <a href="qcache.html">QCache</a> QCache and <a href="qintcache.html">QIntCache</a> LRU (least recently used) cache structures.<li> <a href="qdict.html">QDict</a>, <a href="qintdict.html">QIntDict</a> and <a href="qptrdict.html">QPtrDict</a> dictionary structures.<li> <a href="qlist.html">QList</a> a double linked list structure.<li> <a href="qqueue.html">QQueue</a> a FIFO (first in, first out) queue structure.<li> <a href="qstack.html">QStack</a> a LIFO (last in, first out) stack structure.<li> <a href="qvector.html">QVector</a> a vector structure.</ul><p>The value based collections are:<ul><li> <a href="qvaluelist.html">QValueList</a> a value based list<li> <a href="qvaluestack.html">QValueStack</a> a value based stack structure<li> <a href="qmap.html">QMap</a> a value based dictionary structure</ul><p>The reference based collection classes work with pointers to items,while the value based classes store copies of their items. Anexception is <a href="qarray.html">QArray</a>. It is neither reference nor value based, butmemory based. For maximum efficiency with the simple data types usuallyused in arrays, it uses bitwise operations to copy and compare arrayelements.<p>Some of these classes have corresponding iterators. An iteratoris a class for traversing the items in a collection:<ul><li> <a href="qcacheiterator.html">QCacheIterator</a> and <a href="qintcacheiterator.html">QIntCacheIterator</a><li> <a href="qdictiterator.html">QDictIterator</a>, <a href="qintdictiterator.html">QIntDictIterator</a>, and <a href="qptrdictiterator.html">QPtrDictIterator</a><li> <a href="qlistiterator.html">QListIterator</a><li> <a href="qvaluelistiterator.html">QValueListIterator</a>, and <a href="qvaluelistconstiterator.html">QValueListConstIterator</a><li> <a href="qmapiterator.html">QMapIterator</a>, and <a href="qmapconstiterator.html">QMapConstIterator</a></ul><p>The value based collections plus algorithms operating on them aregrouped together in <a href="qtl.html">the Qt TemplateLibrary</a>. See the respective documentation for details. The rest ofthis page talks about the reference based containers only.<p><h2>Architecture of the reference based containers</h2><p>There are three internal base classes for the reference basedcontainers; <a href="qgcache.html">QGCache</a>, <a href="qgdict.html">QGDict</a> and <a href="qglist.html">QGList</a> that operate on <code>void*</code> pointers. A thin template layer implements the actualcollections by casting item pointers to and from <code>void*.</code><p>This strategy allows Qt's templates to be very economical on space(instantiating one of these templates adds only inline-able calls tothe base classes), while it does not hurt performance too much.<p><h2>A QList Example</h2><p>This example shows how to store Employee items in a list and printsthem out in the reverse order:<p><pre> #include <qlist.h> #include <qstring.h> #include <stdio.h> class Employee { public: Employee( const char *name, int salary ) { n=name; s=salary; } const char *name() const { return n; } int salary() const { return s; } private: <a href="qstring.html">QString</a> n; int s; }; void main() { <a href="qlist.html">QList</a><Employee> list; // list of pointers to Employee list.<a href="qcollection.html#a8ef9f">setAutoDelete</a>( TRUE ); // delete items when they are removed list.<a href="qlist.html#c5746a">append</a>( new Employee("Bill", 50000) ); list.<a href="qlist.html#c5746a">append</a>( new Employee("Steve",80000) ); list.<a href="qlist.html#c5746a">append</a>( new Employee("Ron", 60000) ); <a href="qlistiterator.html">QListIterator</a><Employee> it(list); // iterator for employee list for ( it.<a href="qlistiterator.html#70faa8">toLast</a>(); it.<a href="qlistiterator.html#e58f06">current</a>(); --it) ) { Employee *emp = it.<a href="qlistiterator.html#e58f06">current</a>(); printf( "%s earns %d\n", emp->name(), emp->salary() ); } }</pre><p>Program output:<pre> Ron earns 60000 Steve earns 80000 Bill earns 50000</pre><p><h2>Managing Collection Items</h2><p>All reference based collections inherit the <a href="qcollection.html">QCollection</a> base class.This class knows only the number of items in the collection and thedelete strategy.<p>Items in a collection are by default not deleted when they are removedfrom the collection. The <a href="qcollection.html#a8ef9f">QCollection::setAutoDelete()</a> functionspecifies the delete strategy. In the list example, we enableauto-deletion to make the list delete the items when they are removedfrom the list.<p>When inserting an item into a collection, only the pointer is copied,not the item itself. This is called a shallow copy. It is possible tomake the collection copy all of the item's data (known as a deep copy)when an item is inserted. All collection functions that insert anitem call the virtual function <a href="qcollection.html#55065e">QCollection::newItem()</a> for the itemto be inserted. Inherit a collection and reimplement it if you wantto have deep copies in your collection.<p>When removing an item from a list, the virtual function <a href="qcollection.html#8d78e7">QCollection::deleteItem()</a> is called.The default implementation in all collection classes deletes the itemif auto-deletion is enabled.<p><h2>Usage</h2><p>A reference based collection class, such as QList<type>, defines acollection of <em>pointers</em> to <em>type</em> objects. The pointer (*) isimplicit.<p>We discuss <a href="qlist.html">QList</a> here, but the same techniques apply for allreference based collection classes and all collection class iterators.<p>Template instantiation:<pre> <a href="qlist.html">QList</a><Employee> list; // wherever the list is used</pre><p>The item's class or type, Employee in our example, must be defined priorto the list definition.<p><pre> // Does not work: Employee is not defined class Employee; <a href="qlist.html">QList</a><Employee> list; // This works: Employee is defined before it is used class Employee { ... }; <a href="qlist.html">QList</a><Employee> list;</pre><p><h2>Iterators</h2><p>Although <a href="qlist.html">QList</a> has member functions to traverse the list, it canoften be better to make use of an iterator. <a href="qlistiterator.html">QListIterator</a> is verysafe and can traverse lists that are being modified at the same time.Multiple iterators can work independently on the same collection.<p>A QList has an internal list of all iterators that are currently operatingon the list. When a list entry is removed, the list updates all iteratorsto point to this entry.<p>The <a href="qdict.html">QDict</a> and <a href="qcache.html">QCache</a> collections have no traversal functions. Totraverse these collections, you must use <a href="qdictiterator.html">QDictIterator</a> or <a href="qcacheiterator.html">QCacheIterator</a>.<p><h2>Predefined Collections</h2><p>Qt has the following predefined collection classes:<ul><li> String lists; <a href="qstrlist.html">QStrList</a>, <a href="qstrilist.html">QStrIList</a> (<a href="qstrlist-h.html">qstrlist.h</a> ) and <a href="qstringlist.html">QStringList</a> (<a href="qstringlist-h.html">qstringlist.h</a> )<li> String vectors; QStrVec and QStrIVec (qstrvec.h )</ul><p>In almost all cases you would choose QStringList, which is a valuelist of implicitly shared QString unicode strings. QStrList andQStrIList only store char* pointers.<p><h2>Comparison with the STL</h2><p>We often get questions about why Qt does not use the STL, and why Qt'scontainer templates are provided at all. Here are the major factorswhy we use and provide these templates: <ul><li>Qt's reference based container templates add less space wheninstantiated than the STL ones do. Size is important for a library,and Qt contains many instantiations of QDict, QList etc.<li>Qt's reference based containers are often not as fast as theSTL's, for several reasons such as Qt's safe iterators.This is however not very important forQt, as they are used in code that doesn't need to be very fast. (Thespeed-critical data structures in Qt are mostly caches - either QCacheinstantiations or custom-written, custom-optimized ones.)<li>Qt's containers are much more portable than the STL. When westarted writing Qt, STL was far away in the future, and when wereleased Qt 1.0, no widely-used compilers could compile the STL. Fora library such as Qt, it is of course very important to compile on thewidest possible variety of compilers. Even today, the STL isnot portable across all platforms supported by Qt.<li> Though primarily written for use internally in Qt, the containersare well documented and so we saw no reason to hide them.</ul><p>Value based containers in the style of the STL do have one majoradvantage, though: They are extremely efficient when being used withvalue objects that implement a fast copy constructor. This is the casewith all implicit shared Qt classes such as QString or any of the C++standard data types like int, bool or double. Since we needed thatfunctionality for Qt itself, we added a few <a href="qtl.html">valuebased containers</a> in Qt-2.0. As with the other containers, the newones have a well documented interface and a naming scheme that matchesthe usual Qt conventions - another advantage over the STL classes.<p>There are other differences, but the ones above are the importantreasons behind our decision to write, use and provide these classes.<p>Classes:<ul plain><li><a href="qasciicache.html">QAsciiCache</a> (Template class that provides a cache based on <code>char*</code> keys)<li><a href="qasciicacheiterator.html">QAsciiCacheIterator</a> (Iterator for <a href="qasciicache.html">QAsciiCache</a> collections)<li><a href="qasciidict.html">QAsciiDict</a> (Template class that provides a dictionary based on <code>char*</code> keys)<li><a href="qasciidictiterator.html">QAsciiDictIterator</a> (Iterator for <a href="qasciidict.html">QAsciiDict</a> collections)<li><a href="qcache.html">QCache</a> (Template class that provides a cache based on <code><a href="qstring.html">QString</a></code> keys)<li><a href="qcacheiterator.html">QCacheIterator</a> (Iterator for <a href="qcache.html">QCache</a> collections)<li><a href="qcollection.html">QCollection</a> (The base class of all Qt collections)<li><a href="qdict.html">QDict</a> (Template class that provides a dictionary based on <code><a href="qstring.html">QString</a></code> keys)<li><a href="qdictiterator.html">QDictIterator</a> (Iterator for <a href="qdict.html">QDict</a> collections)<li><a href="qintcache.html">QIntCache</a> (Template class that provides a cache based on <code>long</code> keys)<li><a href="qintcacheiterator.html">QIntCacheIterator</a> (Iterator for <a href="qintcache.html">QIntCache</a> collections)<li><a href="qintdict.html">QIntDict</a> (Template class that provides a dictionary based on <code>long</code> keys)<li><a href="qintdictiterator.html">QIntDictIterator</a> (Iterator for <a href="qintdict.html">QIntDict</a> collections)<li><a href="qlist.html">QList</a> (Template class that provides doubly linked lists)<li><a href="qlistiterator.html">QListIterator</a> (Iterator for <a href="qlist.html">QList</a> collections)<li><a href="qptrdict.html">QPtrDict</a> (Template class that provides a dictionary based on <code>void*</code> keys)<li><a href="qptrdictiterator.html">QPtrDictIterator</a> (Iterator for <a href="qptrdict.html">QPtrDict</a> collections)<li><a href="qqueue.html">QQueue</a> (Template class that provides a queue)<li><a href="qsortedlist.html">QSortedList</a> (List sorted by operator< and operator==)<li><a href="qstack.html">QStack</a> (Template class that provides a stack)<li><a href="qstrilist.html">QStrIList</a> (Doubly linked list of <code>char*</code> with case insensitive compare)<li><a href="qstrlist.html">QStrList</a> (Doubly linked list of <code>char*.</code>)</ul><p><address><hr><div align="center"><table width="100%" cellspacing="0" border="0"><tr><td>Copyright ⌨️ 快捷键说明
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