qthreadstorage.3qt

Trolltech公司发布的基于C++图形开发环境

'\" t
.TH QThreadStorage 3qt "21 January 2005" "Trolltech AS" \" -*- nroff -*-
.\" Copyright 1992-2001 Trolltech AS.  All rights reserved.  See the
.\" license file included in the distribution for a complete license
.\" statement.
.\"
.ad l
.nh
.SH NAME
QThreadStorage \- Per-thread data storage
.SH SYNOPSIS
All the functions in this class are thread-safe when Qt is built with thread support.</p>
.PP
\fC#include <qthreadstorage.h>\fR
.PP
.SS "Public Members"
.in +1c
.ti -1c
.BI "\fBQThreadStorage\fR ()"
.br
.ti -1c
.BI "\fB~QThreadStorage\fR ()"
.br
.ti -1c
.BI "bool \fBhasLocalData\fR () const"
.br
.ti -1c
.BI "T & \fBlocalData\fR ()"
.br
.ti -1c
.BI "T \fBlocalData\fR () const"
.br
.ti -1c
.BI "void \fBsetLocalData\fR ( T data )"
.br
.in -1c
.SH DESCRIPTION
The QThreadStorage class provides per-thread data storage.
.PP
QThreadStorage is a template class that provides per-thread data storage.
.PP
\fINote that due to compiler limitations, QThreadStorage can only store pointers.\fR
.PP
The setLocalData() function stores a single thread-specific value for the calling thread. The data can be accessed later using the localData() functions. QThreadStorage takes ownership of the data (which must be created on the heap with \fInew\fR) and deletes it when the thread exits (either normally or via termination).
.PP
The hasLocalData() function allows the programmer to determine if data has previously been set using the setLocalData() function. This is useful for lazy initializiation.
.PP
For example, the following code uses QThreadStorage to store a single cache for each thread that calls the \fIcacheObject()\fR and \fIremoveFromCache()\fR functions. The cache is automatically deleted when the calling thread exits (either normally or via termination).
.PP
.nf
.br
    QThreadStorage<QCache<SomeClass> *> caches;
.br
.br
    void cacheObject( const QString &key, SomeClass *object )
.br
    {
.br
        if ( ! caches.hasLocalData() )
.br
            caches.setLocalData( new QCache<SomeClass> );
.br
.br
        caches.localData()->insert( key, object );
.br
    }
.br
.br
    void removeFromCache( const QString &key )
.br
    {
.br
        if ( ! caches.hasLocalData() )
.br
            return; // nothing to do
.br
.br
        caches.localData()->remove( key );
.br
    }
.br
.fi
.SH "Caveats"
.IP
.TP
As noted above, QThreadStorage can only store pointers due to compiler limitations. Support for value-based objects will be added when the majority of compilers are able to support partial template specialization.
.IP
.TP
The destructor does \fInot\fR delete per-thread data. QThreadStorage only deletes per-thread data when the thread exits or when setLocalData() is called multiple times.
.IP
.TP
QThreadStorage can only be used with threads started with QThread. It \fIcannot\fR be used with threads started with platform-specific APIs.
.IP
.TP
As a corollary to the above, platform-specific APIs cannot be used to exit or terminate a QThread using QThreadStorage. Doing so will cause all per-thread data to be leaked. See QThread::exit() and QThread::terminate().
.IP
.TP
QThreadStorage \fIcan\fR be used to store data for the \fImain()\fR thread \fIafter\fR QApplication has been constructed. QThreadStorage deletes all data set for the \fImain()\fR thread when QApplication is destroyed, regardless of whether or not the \fImain()\fR thread has actually finished.
.IP
.TP
The implementation of QThreadStorage limits the total number of QThreadStorage objects to 256. An unlimited number of threads can store per-thread data in each QThreadStorage object.
.IP
.PP
See also Environment Classes and Threading.
.SH MEMBER FUNCTION DOCUMENTATION
.SH "QThreadStorage::QThreadStorage ()"
Constructs a new per-thread data storage object.
.SH "QThreadStorage::~QThreadStorage ()"
Destroys the per-thread data storage object.
.PP
Note: The per-thread data stored is \fInot\fR deleted. Any data left in QThreadStorage is leaked. Make sure that all threads using QThreadStorage have exited before deleting the QThreadStorage.
.PP
See also hasLocalData().
.SH "bool QThreadStorage::hasLocalData () const"
Returns TRUE if the calling thread has non-zero data available; otherwise returns FALSE.
.PP
See also localData().
.SH "T & QThreadStorage::localData ()"
Returns a reference to the data that was set by the calling thread.
.PP
Note: QThreadStorage can only store pointers. This function returns a \fIreference\fR to the pointer that was set by the calling thread. The value of this reference is 0 if no data was set by the calling thread,
.PP
See also hasLocalData().
.SH "T QThreadStorage::localData () const"
This is an overloaded member function, provided for convenience. It behaves essentially like the above function.
.PP
Returns a copy of the data that was set by the calling thread.
.PP
Note: QThreadStorage can only store pointers. This function returns a pointer to the data that was set by the calling thread. If no data was set by the calling thread, this function returns 0.
.PP
See also hasLocalData().
.SH "void QThreadStorage::setLocalData ( T data )"
Sets the local data for the calling thread to \fIdata\fR. It can be accessed later using the localData() functions.
.PP
If \fIdata\fR is 0, this function deletes the previous data (if any) and returns immediately.
.PP
If \fIdata\fR is non-zero, QThreadStorage takes ownership of the \fIdata\fR and deletes it automatically either when the thread exits (either normally or via termination) or when setLocalData() is called again.
.PP
Note: QThreadStorage can only store pointers. The \fIdata\fR argument must be either a pointer to an object created on the heap (i.e. using \fInew\fR) or 0. You should not delete \fIdata\fR yourself; QThreadStorage takes ownership and will delete the \fIdata\fR itself.
.PP
See also localData() and hasLocalData().

.SH "SEE ALSO"
.BR http://doc.trolltech.com/qthreadstorage.html
.BR http://www.trolltech.com/faq/tech.html
.SH COPYRIGHT
Copyright 1992-2001 Trolltech AS, http://www.trolltech.com.  See the
license file included in the distribution for a complete license
statement.
.SH AUTHOR
Generated automatically from the source code.
.SH BUGS
If you find a bug in Qt, please report it as described in
.BR http://doc.trolltech.com/bughowto.html .
Good bug reports help us to help you. Thank you.
.P
The definitive Qt documentation is provided in HTML format; it is
located at $QTDIR/doc/html and can be read using Qt Assistant or with
a web browser. This man page is provided as a convenience for those
users who prefer man pages, although this format is not officially
supported by Trolltech. 
.P
If you find errors in this manual page, please report them to
.BR [e-mail] .
Please include the name of the manual page (qthreadstorage.3qt) and the Qt
version (3.3.4).