why MACROS are used between class name and class keyword

[babu198649]

Hi
What is the use of using MACRO in between the class name and class keyword such as the following declaration.

class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict
{
...
}

class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict
{
...
}

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[spirit]

it's used for correct compilation of dll on windows.

[babu198649]

Thanks,
Is it legal to define macro between the class keyword and class name or it is compiler specific.
Does defining macro between class name and class keyword makes any difference on the linux.
Does anyone knows a good link which explains about this(googling had not helped).

[spirit]

actually import/export specifiers are used only on windows on Linux they have no effect.
take a look at QTDIR/src/corelib/global/qglobal.h where you can find Q_DECL_EXPORT/Q_DECL_IMPORT macros

#ifndef Q_DECL_EXPORT
#  ifdef Q_OS_WIN
#    define Q_DECL_EXPORT __declspec(dllexport)
#  elif defined(QT_VISIBILITY_AVAILABLE)
#    define Q_DECL_EXPORT __attribute__((visibility("default")))
#  endif
#  ifndef Q_DECL_EXPORT
#    define Q_DECL_EXPORT
#  endif
#endif
#ifndef Q_DECL_IMPORT
#  if defined(Q_OS_WIN)
#    define Q_DECL_IMPORT __declspec(dllimport)
#  else
#    define Q_DECL_IMPORT
#  endif
#endif

#ifndef Q_DECL_EXPORT
#  ifdef Q_OS_WIN
#    define Q_DECL_EXPORT __declspec(dllexport)
#  elif defined(QT_VISIBILITY_AVAILABLE)
#    define Q_DECL_EXPORT __attribute__((visibility("default")))
#  endif
#  ifndef Q_DECL_EXPORT
#    define Q_DECL_EXPORT
#  endif
#endif
#ifndef Q_DECL_IMPORT
#  if defined(Q_OS_WIN)
#    define Q_DECL_IMPORT __declspec(dllimport)
#  else
#    define Q_DECL_IMPORT
#  endif
#endif

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so, if you need to create your own dll you need to specify these macros, see detailes here dll & Qt

[babu198649]

Thanks again,
As per the c++ language standard can a macro be present between class-name and class keyword or it is an extension of the compiler.

[Ginsengelf]

Hi, you should be able to place a macro nearly everywhere as it will just be replaced by the preprocessor.

Some of the replacements are compiler specific (e.g. __declspec), some define calling conventions like the order of parameters on the stack for a function call.

Ginsengelf

[babu198649]

you should be able to place a macro nearly everywhere as it will just be replaced by the preprocessor

Right,Unless the macro doesnot contains any value

Some of the replacements are compiler specific (e.g. __declspec)

Thanks ,That cleared my doubt.