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Why is it important to add an include for .moc file in a Qt cpp source code?

This is a common step used in several Qt samples, including this one: http://doc.qt.io/qt-5/qttestlib-tutorial1-example.html; where the line #include "testqstring.moc" should be included in the end of the file.

I don't understand exactly why this is necessary.

Jarod42
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Daniel Santos
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    You don't need to do that if you declare your `Q_OBJECT` classes in a separate header file. Otherwise it's missing all the additional features that make a `QObject` a, well, `QObject`. – Zeta Jan 21 '16 at 16:52

2 Answers2

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It's necessary if you define QObject subclasses with the Q_OBJECT macro in a .cpp file. When you do so:

  1. qmake must generate rules inside your Makefile to invoke moc on that .cpp file.

    That special (hackish?) inclusion triggers qmake to do so, and tells it which would be moc's output file (teststring.moc) when invoked on your .cpp.

  2. In order to compile moc's output (which is still a bunch of C++ code) the compiler must see your class definition. Otherwise, it will complain that there's no such thing as YourClass::staticMetaObject and similar, because it has no idea that YourClass exists.

    Typically one defines classes featuring Q_OBJECT in a header file. moc then adds a #include "header.h" into its generated output, and this means moc's output can be happily compiled.

    But what if your class definition is inside a .cpp? You can't #include a .cpp file in moc's output, as that would give you tons of redefinition errors.

    Instead, you #include moc's output in your .cpp, so that it gets compiled together and everyone is happy. (This means qmake will only emit one rule saying to run moc, but not another rule telling the compiler to compile moc's output.)

From 2. you can also also desume that defining classes with Q_OBJECT in a .h does not require any special inclusion.

peppe
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4

Make an addition:

When Q_OBJECT in xxx.h, after qmake, the system will generate a moc_xxx.cpp with xxx.h in it.

When Q_OBJECT in xxx.cpp, after qmake, the system will generate a xxx.moc, and you need to add the .moc file into the .cpp file.

StephennQin
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