source: trunk/doc/src/howtos/sharedlibrary.qdoc@ 980

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/*!
    \page sharedlibrary.html
    \title Creating Shared Libraries
    \brief How to create shared libraries.
    \ingroup best-practices

    The following sections list certain things that should be taken into
    account when creating shared libraries.

    \section1 Using Symbols from Shared Libraries

    Symbols - functions, variables or classes - contained in shared libraries
    intended to be used by \e{clients}, such as applications or other
    libraries, must be marked in a special way. These symbols are called
    \e{public symbols} that are \e{exported} or made publicly visible.

    The remaining symbols should not be visible from the outside. On most
    platforms, compilers will hide them by default. On some platforms, a
    special compiler option is required to hide these symbols.

    When compiling a shared library, it must be marked for \e{export}. To use
    the shared library from a client, some platforms may require a special
    \e{import} declaration as well.

    Depending on your target platform, Qt provides special macros that contain
    the necessary definitions:
    \list
        \o  \c{Q_DECL_EXPORT} must be added to the declarations of symbols used
            when compiling a shared library.
        \o  \c{Q_DECL_IMPORT} must be added to the declarations of symbols used
            when compiling a client that uses the shared library.
    \endlist

    Now, we need to ensure that the right macro is invoked -- whether we
    compile a share library itself, or just the client using the shared
    library.
    Typically, this can be solved by adding a special header.

    Let us assume we want to create a shared library called \e{mysharedlib}.
    A special header for this library, \c{mysharedlib_global.h}, looks like
    this:

    \code
        #include <QtCore/QtGlobal>

        #if defined(MYSHAREDLIB_LIBRARY)
        #  define MYSHAREDLIB_EXPORT Q_DECL_EXPORT
        #else
        #  define MYSHAREDLIB_EXPORT Q_DECL_IMPORT
        #endif
    \endcode

    In the \c{.pro} file of the shared library, we add:

    \code
        DEFINES += MYSHAREDLIB_LIBRARY
    \endcode

    In each header of the library, we specify the following:

    \code
        #include "mysharedlib_global.h"

        MYSHAREDLIB_EXPORT void foo();
        class MYSHAREDLIB_EXPORT MyClass...
    \endcode
    This ensures that the right macro is seen by both library and clients. We
    also use this technique in Qt's sources.


    \section1 Header File Considerations

    Typically, clients will include only the public header files of shared
    libraries. These libraries might be installed in a different location, when
    deployed. Therefore, it is important to exclude other internal header files
    that were used when building the shared library.

    For example, the library might provide a class that wraps a hardware device
    and contains a handle to that device, provided by some 3rd-party library:

    \code
        #include <footronics/device.h>

        class MyDevice {
        private:
            FOOTRONICS_DEVICE_HANDLE handle;
        };
    \endcode  

    A similar situation arises with forms created by Qt Designer when using
    aggregation or multiple inheritance:

    \code
        #include "ui_widget.h"

        class MyWidget : public QWidget {
        private:
            Ui::MyWidget m_ui;
        };
    \endcode  

    When deploying the library, there should be no dependency to the internal
    headers \c{footronics/device.h} or \c{ui_widget.h}.

    This can be avoided by making use of the \e{Pointer to implementation}
    idiom described in various C++ programming books. For classes with
    \e{value semantics}, consider using QSharedDataPointer.


    \section1  Binary compatibility

    For clients loading a shared library, to work correctly, the memory
    layout of the classes being used must match exactly the memory layout of
    the library version that was used to compile the client. In other words,
    the library found by the client at runtime must be \e{binary compatible}
    with the version used at compile time.

    This is usually not a problem if the client is a self-contained software
    package that ships all the libraries it needs.

    However, if the client application relies on a shared library that belongs
    to a different installation package or to the operating system, then we
    need to think of a versioning scheme for shared libraries and decide at
    which level \e{Binary compatibility} is to be maintained. For example, Qt
    libraries of the same \e{major version number} are guaranteed to be binary
    compatible.

    Maintaining \e{Binary compatibility} places some restrictions on the changes
    you can make to the classes. A good explanation can be found at
    \l{http://techbase.kde.org/Policies/Binary_Compatibility_Issues_With_C++}
    {KDE - Policies/Binary Compatibility Issues With C++}. These issues should
    be considered right from the start of library design.
    We recommend that the principle of \e{Information hiding} and the
    \e{Pointer to implementation} technique be used wherever possible.
*/