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Global Qt Declarations

The <QtGlobal> header provides basic declarations and is included by all other Qt headers. The declarations include types, functions and macros.

The type definitions are partly convenience definitions for basic types (some of which guarantee certain bit-sizes on all platforms supported by Qt), partly types related to Qt message handling. The functions are related to generating messages, Qt version handling and comparing and adjusting object values. And finally, some of the declared macros enable programmers to add compiler or platform specific code to their applications, while others are convenience macros for larger operations.

Types

The header file declares several type definitions that guarantee a specified bit-size on all platforms supported by Qt for various basic types, for example qint8 which is a signed char guaranteed to be 8-bit on all platforms supported by Qt. The header file also declares the qlonglong type definition for long long int (__int64 on Windows).

Several convenience type definitions are declared: qreal for double, uchar for unsigned char, uint for unsigned int, ulong for unsigned long and ushort for unsigned short.

Finally, the QtMsgType definition identifies the various messages that can be generated and sent to a Qt message handler; QtMsgHandler is a type definition for a pointer to a function with the signature void myMsgHandler(QtMsgType, const char *).

Functions

The <QtGlobal> header file contains several functions comparing and adjusting an object's value. These functions take a template type as argument: You can retrieve the absolute value of an object using the qAbs() function, and you can bound a given object's value by given minimum and maximum values using the qBound() function. You can retrieve the minimum and maximum of two given objects using qMin() and qMax() respectively. All these functions return a corresponding template type; the template types can be replaced by any other type. For example:

The following code example is written in c++.
int myValue = 10;
int minValue = 2;
int maxValue = 6;

int boundedValue = qBound(minValue, myValue, maxValue);
// boundedValue == 6
<QtGlobal> also contains functions that generate messages from the given string argument: qCritical(), qDebug(), qFatal() and qWarning(). These functions call the message handler with the given message. For example:

The following code example is written in c++.
if (!driver()->isOpen() || driver()->isOpenError()) {
    qWarning("QSqlQuery::exec: database not open");
    return false;
}
The remaining functions are qRound() and qRound64(), which both accept a qreal value as their argument returning the value rounded up to the nearest integer and 64-bit integer respectively, the qInstallMsgHandler() function which installs the given QtMsgHandler, and the qVersion() function which returns the version number of Qt at run-time as a string.

Macros

The <QtGlobal> header file provides a range of macros (Q_CC_*) that are defined if the application is compiled using the specified platforms. For example, the Q_CC_SUN macro is defined if the application is compiled using Forte Developer, or Sun Studio C++. The header file also declares a range of macros (Q_OS_*) that are defined for the specified platforms. For example, Q_OS_X11 which is defined for the X Window System.

The purpose of these macros is to enable programmers to add compiler or platform specific code to their application.

The remaining macros are convenience macros for larger operations: The QT_TRANSLATE_NOOP() and QT_TR_NOOP() macros provide the possibility of marking text for dynamic translation, i.e. translation without changing the stored source text. The Q_ASSERT() and Q_ASSERT_X() enables warning messages of various level of refinement. The Q_FOREACH() and foreach() macros implement Qt's foreach loop.

The Q_INT64_C() and Q_UINT64_C() macros wrap signed and unsigned 64-bit integer literals in a platform-independent way. The Q_CHECK_PTR() macro prints a warning containing the source code's file name and line number, saying that the program ran out of memory, if the pointer is 0. The qPrintable() macro represent an easy way of printing text.

Finally, the QT_POINTER_SIZE macro expands to the size of a pointer in bytes, and the QT_VERSION and QT_VERSION_STR macros expand to a numeric value or a string, respectively, specifying Qt's version number, i.e the version the application is compiled against.

See also:
, and QSysInfo.


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