[Development] Avoid QT_BEGIN_NAMESPACE..QT_END_NAMESPACE in Qt examples

[Kai Köhne]

Hi,

I suggest extending QUIP 13 [1] with a paragraph explaining that we shouldn't pre-declare Qt types in Qt examples:

https://codereview.qt-project.org/c/meta/quips/+/503709

For the reasoning: While it's good practice to pre-declare referenced types in header files in our tools and Qt library sources, the need to deal with 'çustom namespaced' Qt (QT_BEGIN_HEADER, QT_END_HEADER, QT_FORWARD_DECLARE_CLASS) makes this rather ugly for Qt examples. It furthermore gives bad precedence to Qt users, who might 'cargo-cult' the magic macros in their sources. Finally, as the Qt examples are rather small, the benefits of this optimization are neglectable.

Happy to hear any feedback, either here or, preferably, in the Gerrit commit.

Kai

[1]: https://contribute.qt-project.org/quips/13

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Kai Köhne, Director R&D | The Qt Company

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