// Copyright (C) 2016 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only

/*!
    \page moc.html
    \title Using the Meta-Object Compiler (moc)
    \ingroup qttools
    \keyword moc

    The Meta-Object Compiler, \c moc, is the program that handles
    \l{Meta-Object System}{Qt's C++ extensions}.

    The \c moc tool reads a C++ header file. If it finds one or more
    class declarations that contain the Q_OBJECT macro, it
    produces a C++ source file containing the meta-object code for
    those classes. Among other things, meta-object code is required
    for the signals and slots mechanism, the run-time type information,
    and the dynamic property system.

    The C++ source file generated by \c moc must be compiled and
    linked with the implementation of the class.

    If you use \l qmake to create your makefiles, build rules will be
    included that call the moc when required, so you will not need to
    use the moc directly. For more background information on \c moc,
    see \l{Why Does Qt Use Moc for Signals and Slots?}

    \section1 Usage

    \c moc is typically used with an input file containing class
    declarations like this:

    \snippet snippets/moc/myclass1.h 0

    In addition to the signals and slots shown above, \c moc also
    implements object properties as in the next example. The
    Q_PROPERTY() macro declares an object property, while
    Q_ENUM() declares a list of enumeration types within the class
    to be usable inside the \l{Qt's Property System}{property
    system}.

    In the following example, we declare a property of the
    enumeration type \c Priority that is also called \c priority and
    has a get function \c priority() and a set function \c
    setPriority().

    \snippet snippets/moc/myclass2.h 0

    The Q_FLAGS() macro declares enums that are to be used
    as flags, i.e. OR'd together. Another macro, Q_CLASSINFO(),
    allows you to attach additional name/value pairs to the class's
    meta-object:

    \snippet snippets/moc/myclass3.h 0

    The output produced by \c moc must be compiled and linked, just
    like the other C++ code in your program; otherwise, the build
    will fail in the final link phase. If you use \c qmake, this is
    done automatically. Whenever \c qmake is run, it parses the
    project's header files and generates make rules to invoke \c moc
    for those files that contain a Q_OBJECT macro.

    If the class declaration is found in the file \c myclass.h, the
    moc output should be put in a file called \c moc_myclass.cpp.
    This file should then be compiled as usual, resulting in an
    object file, e.g., \c moc_myclass.obj on Windows. This object
    should then be included in the list of object files that are
    linked together in the final building phase of the program.

    \section1 Writing Make Rules for Invoking \c moc

    For anything but the simplest test programs, it is recommended
    that you automate running the \c{moc}. By adding some rules to
    your program's makefile, \c make can take care of running moc
    when necessary and handling the moc output.

    We recommend using the \l qmake makefile generation tool for
    building your makefiles. This tool generates a makefile that does
    all the necessary \c moc handling.

    If you want to create your makefiles yourself, here are some tips
    on how to include moc handling.

    For Q_OBJECT class declarations in header files, here is a
    useful makefile rule if you only use GNU make:

    \snippet snippets/code/doc_src_moc.qdoc 0

    If you want to write portably, you can use individual rules of
    the following form:

    \snippet snippets/code/doc_src_moc.qdoc 1

    You must also remember to add \c moc_foo.cpp to your \c SOURCES
    (substitute your favorite name) variable and \c moc_foo.o or \c
    moc_foo.obj to your \c OBJECTS variable.

    Both examples assume that \c $(DEFINES) and \c $(INCPATH) expand
    to the define and include path options that are passed to the C++
    compiler. These are required by \c moc to preprocess the source
    files.

    While we prefer to name our C++ source files \c .cpp, you can use
    any other extension, such as \c .C, \c .cc, \c .CC, \c .cxx, and
    \c .c++, if you prefer.

    For Q_OBJECT class declarations in implementation (\c .cpp)
    files, we suggest a makefile rule like this:

    \snippet snippets/code/doc_src_moc.qdoc 2

    This guarantees that make will run the moc before it compiles
    \c foo.cpp. You can then put

    \snippet snippets/code/doc_src_moc.cpp 3

    at the end of \c foo.cpp, where all the classes declared in that
    file are fully known.

    \section1 Command-Line Options

    Here are the command-line options supported by the moc:

    \table
    \header \li Option \li Description

    \row
    \li \c {-D<macro>[=<def>]}
    \li Define macro, with optional definition.

    \row
    \li \c{-E}
    \li Preprocess only; do not generate meta-object code.

    \row
    \li \c{-f[<file>]}
    \li Force the generation of an \c #include statement in the
    output. This is the default for header files whose extension
    starts with \c H or \c h. This option is useful if you have
    header files that do not follow the standard naming conventions.
    The \c <file> part is optional.

    \row
    \li \c{-Fdir}

    \li \macos. Add the framework directory \c{dir} to the head of
       the list of directories to be searched for header files. These
       directories are interleaved with those specified by -I options
       and are scanned in a left-to-right order (see the manpage for
       gcc). Normally, use -F /Library/Frameworks/

    \row
    \li \c{-h}
    \li Display the usage and the list of options.

    \row
    \li \c -i
    \li Do not generate an \c #include statement in the output.
    This may be used to run the moc on a C++ file containing one or
    more class declarations. You should then \c #include the meta-object
    code in the \c .cpp file.

    \row
    \li \c {-I<dir>}
    \li Add dir to the include path for header files.

    \row
    \li \c{-M<key=value>}
    \li Append additional meta data to plugins. If a class
        has Q_PLUGIN_METADATA specified, the key-value pair will be added
        to its meta data. This will end up in the Json object that gets
        resolved for the plugin at run time (accessible from QPluginLoader).
        This argument is typically used for tagging static plugins with
        information resolved by the build system.

    \row
    \li \c -nw
    \li Do not generate any warnings. (Not recommended.)

    \row
    \li \c{-o<file>}
    \li Write output to \c <file> rather than to standard output.

    \row
    \li \c {-p<path>}
    \li Makes the moc prepend \c {<path>/} to the file name in the
    generated \c #include statement.

    \row
    \li \c{-U<macro>}
    \li Undefine macro.

    \row
    \li \c{@<file>}
    \li Read additional command-line options from \c{<file>}.
    Each line of the file is treated as a single option. Empty lines
    are ignored. Note that this option is not supported within the
    options file itself (i.e. an options file can't "include" another
    file).

    \row
    \li \c {-v}
    \li Display \c{moc}'s version number.

    \endtable

    You can explicitly tell the moc not to parse parts of a header
    file. \c moc defines the preprocessor symbol \c Q_MOC_RUN. Any
    code surrounded by

    \snippet snippets/code/doc_src_moc.cpp 4

    is skipped by the \c moc.

    \section1 Diagnostics

    \c moc will warn you about a number of dangerous or illegal
    constructs in the Q_OBJECT class declarations.

    If you get linkage errors in the final building phase of your
    program, saying that \c YourClass::className() is undefined or
    that \c YourClass lacks a vtable, something has been done wrong.
    Most often, you have forgotten to compile or \c #include the
    moc-generated C++ code, or (in the former case) include that
    object file in the link command. If you use \c qmake, try
    rerunning it to update your makefile. This should do the trick.

    \section1 Build Systems

    \section2 Including header moc files

    qmake and CMake behave differently with regards to including header moc
    files.

    To illustrate this with an example, suppose that you have two headers with
    corresponding source files: \c a.h, \c a.cpp, \c b.h, and \c b.cpp. Each
    header has a Q_OBJECT macro:

    \code
    // a.h
    class A : public QObject
    {
        Q_OBJECT

        public:
            // ...
    };
    \endcode

    \code
    // a.cpp
    #include "a.h"

    // ...

    #include "moc_a.cpp"
    \endcode

    \code
    // b.h
    class B : public QObject
    {
        Q_OBJECT

        public:
            // ...
    };
    \endcode

    \code
    // b.cpp
    #include "b.h"

    // ...

    #include "moc_b.cpp"
    \endcode

    With qmake, if you don't include the moc-generated file
    (\c{moc_a.cpp}/\c{moc_b.cpp}), \c a.cpp, \c b.cpp, \c moc_a.cpp, and \c moc_b.cpp
    will be compiled separately. This can result in slower builds.
    If you include the moc generated files, only a.cpp and b.cpp will need to
    be compiled, as the moc generated code is included in those files.

    With CMake, if you don't include the files, an single additional
    file is generated by moc (let's call it \c cmake.cpp for the sake of the
    example). \c cmake.cpp would include both \c moc_a.cpp and \c moc_b.cpp.
    Including the moc-generated file is still allowed with CMake, but it's
    not necessary.

    For more information on CMake's moc support regarding this topic, see
    \l {Including header moc files in sources}.

    \section1 Limitations

    \c moc does not handle all of C++. The main problem is that class
    templates cannot have the Q_OBJECT macro. Here is an example:

    \snippet snippets/code/doc_src_moc.cpp 5

    The following constructs are illegal. All of
    them have alternatives which we think are usually better, so
    removing these limitations is not a high priority for us.

    \section2 Multiple Inheritance Requires QObject to Be First

    If you are using multiple inheritance, \c moc assumes that the
    first inherited class is a subclass of QObject. Also, be sure
    that only the first inherited class is a QObject.

    \snippet snippets/code/doc_src_moc.cpp 6

    Virtual inheritance with QObject is \e not supported.

    \section2 Function Pointers Cannot Be Signal or Slot Parameters

    In most cases where you would consider using function pointers as
    signal or slot parameters, we think inheritance is a better
    alternative. Here is an example of illegal syntax:

    \snippet snippets/code/doc_src_moc.cpp 7

    You can work around this restriction like this:

    \snippet snippets/code/doc_src_moc.cpp 8

    It may sometimes be even better to replace the function pointer
    with inheritance and virtual functions.

    \section2 Enums and Typedefs Must Be Fully Qualified for Signal and Slot Parameters

    When checking the signatures of its arguments, QObject::connect()
    compares the data types literally. Thus,
    \l{Qt::Alignment}{Alignment} and \l{Qt::Alignment} are treated as
    two distinct types. To work around this limitation, make sure to
    fully qualify the data types when declaring signals and slots,
    and when establishing connections. For example:

    \snippet snippets/code/doc_src_moc.cpp 9

    \section2 Nested Classes Cannot Have Signals or Slots

    Here's an example of the offending construct:

    \snippet snippets/code/doc_src_moc.cpp 11

    \section2 Signal/Slot return types cannot be references

    Signals and slots can have return types, but signals or slots returning references
    will be treated as returning void.

    \section2 Only Signals and Slots May Appear in the \c signals and \c slots Sections of a Class

    \c moc will complain if you try to put other constructs in the \c
    signals or \c slots sections of a class than signals and slots.

    \sa {Meta-Object System}, {Signals and Slots}, {Qt's Property System}
*/
