使用 moc (元對象編譯器)

元對象編譯器 moc 程序用於處理 Qt 的 C++ 擴展 .

The moc 工具讀取 C++ 頭文件。若找到一個或多個類聲明包含 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 moc must be compiled and linked with the implementation of the class.

Both qmake and CMake generate makefiles with build rules that will invoke moc accordingly, so you will not need to use the moc 直接。 qmake will add these build rules by default, whereas with CMake, you can use the AUTOMOC property to handle moc automatically. For more background information on moc ，見 Qt 為什麼將 MOC 用於信號/槽？

用法

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

class MyClass : public QObject
{
    Q_OBJECT
public:
    MyClass(QObject *parent = 0);
    ~MyClass();
signals:
    void mySignal();
public slots:
    void mySlot();
};

In addition to the signals and slots shown above, 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 特性係統 .

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

class MyClass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(Priority priority READ priority WRITE setPriority)
    Q_ENUMS(Priority)
public:
    enum Priority { High, Low, VeryHigh, VeryLow };
    MyClass(QObject *parent = 0);
    ~MyClass();
    void setPriority(Priority priority) { m_priority = priority; }
    Priority priority() const { return m_priority; }
private:
    Priority m_priority;
};

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:

class MyClass : public QObject
{
    Q_OBJECT
    Q_CLASSINFO("Author", "Oscar Peterson")
    Q_CLASSINFO("Status", "Active")
public:
    MyClass(QObject *parent = 0);
    ~MyClass();
};

The output produced by 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 qmake , this is done automatically. Whenever qmake is run, it parses the project's header files and generates make rules to invoke moc for those files that contain a Q_OBJECT macro. Similarly, when setting AUTOMOC to ON , CMake will scan the header and source files at build time and invoke moc accordingly.

If the class declaration is found in the file myclass.h , the moc output should be put in a file called moc_myclass.cpp . This file should then be compiled as usual, resulting in an object file, e.g., 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.

編寫 Make 規則為援引 `moc`

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

可以使用 CMake or qmake to generate makefiles that does all the necessary moc 處理。

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:

moc_%.cpp: %.h
        moc $(DEFINES) $(INCPATH) $< -o $@

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

moc_foo.cpp: foo.h
        moc $(DEFINES) $(INCPATH) $< -o $@

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

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

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

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

foo.o: foo.moc
foo.moc: foo.cpp
        moc $(DEFINES) $(INCPATH) -i $< -o $@

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

#include "foo.moc"

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

命令行選項

這裏是 moc 支持的命令行選項：

選項	描述
`-o<file>`	寫入輸齣到 `<file>` 而不是到標準輸齣。
`-f[<file>]`	Force the generation of an `#include` statement in the output. This is the default for header files whose extension starts with `H` or `h` . This option is useful if you have header files that do not follow the standard naming conventions. The `<file>` part is optional.
`-i`	Do not generate an `#include` statement in the output. This may be used to run the moc on on a C++ file containing one or more class declarations. You should then `#include` the meta-object code in the `.cpp` 文件。
`-nw`	不生成任何警告 (不推薦)。
`-p<path>`	Makes the moc prepend `<path>/` to the file name in the generated `#include` 語句。
`-I<dir>`	添加 dir 添加到 Header (頭) 文件的 include 路徑。
`-E`	Preprocess only; do not generate meta-object code.
`-D<macro>[=<def>]`	定義宏采用可選定義。
`-U<macro>`	Undefine 宏。
`-M<key=value>`	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.
`@<file>`	Read additional command-line options from `<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).
`-h`	顯示用法和選項列錶。
`-v`	顯示 `moc` 的版本號。
`-Fdir`	macOS. Add the framework directory `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/

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

#ifndef Q_MOC_RUN
    ...
#endif

is skipped by the moc .

診斷

moc will warn you about a number of dangerous or illegal constructs in the Q_OBJECT 類聲明。

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

局限性

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

class SomeTemplate<int> : public QFrame
{
    Q_OBJECT
    ...
signals:
    void mySignal(int);
};

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.

多重繼承要求 QObject 是第 1 個

若正使用多繼承， moc assumes that the first inherited class is a subclass of QObject . Also, be sure that only the first inherited class is a QObject .

// correct
class SomeClass : public QObject, public OtherClass
{
    ...
};

虛擬繼承采用 QObject is not 被支持。

函數指針不可以是信號或槽參數

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:

class SomeClass : public QObject
{
    Q_OBJECT
public slots:
    void apply(void (*apply)(List *, void *), char *); // WRONG
};

You can work around this restriction like this:

typedef void (*ApplyFunction)(List *, void *);
class SomeClass : public QObject
{
    Q_OBJECT
public slots:
    void apply(ApplyFunction, char *);
};

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

枚舉和類型定義必須完全閤格對於信號和槽參數

當校驗其自變量的簽名時， QObject::connect () 會逐字比較數據類型。因此， Alignment and 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:

class MyClass : public QObject
{
    Q_OBJECT
    enum Error {
        ConnectionRefused,
        RemoteHostClosed,
        UnknownError
    };
signals:
    void stateChanged(MyClass::Error error);
};

嵌套類不能有信號或槽

這裏是令人不快構造的範例：

class A
{
public:
    class B
    {
        Q_OBJECT
    public slots:   // WRONG
        void b();
    };
};

信號/槽返迴類型不能是引用

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

僅信號和槽可以齣現在 `signals` and `slots` 類區間

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

另請參閱 元對象係統 , 信號和槽，和 Qt 的特性係統 .