How to mark something in Qt as obsolete(deprecated)?

Question

Is there Q_OBSOLETE or Q_DEPRECATED in C++ with Qt 4.7?

Or is there a similar C++ macro or keyword?

Teevens

What do you want such a macro or keyword to do? – Wet

To issue a warning if deprecated constructs are found in use. See en.wikipedia.org/wiki/Deprecation for better explanation. – Teevens

Answer 1

2023:

Yes, the Qt is defined QT_DEPRECATED and QT_DEPRECATED_X macros for this purpose:

QT_DEPRECATED void myOldFunc();
QT_DEPRECATED_X("use myNewFunc instead") void myOldFunc2();

difference is just the text message.
Also the C++14 presents [[deprecated("text")]] attribute as standard. It seems that this attribute is used under the hood if you use C++ 14+.

Facture

But what's the difference between Q_DECL_DEPRECATED (suggested by the top answer, dating back to 2012) and Q_DEPRECATED? By the way, are you sure it's Q_DEPRECATED? In the docs I found the macros QT_DEPRECATED and QT_DEPRECATED_X("message") (notice: QT, not Q). – Pereyra

@FabiosaysReinstateMonica, take a look at the qglobal.h for differences between that two macros. Some other macros rely on the QT_DEPRECATED. Also, yeah! You right. I had a misspelling about the Q_DEPRECATED. – Facture

Answer 2

If you use Q_DECL_DEPRECATED you should get the outcome you are looking for e.g.:

Q_DECL_DEPRECATED void foo();

Albany

I my case, I do use the attribute after the function name void foo() Q_DECL_DEPRECATED; – Peebles

Answer 3

Just use the

#warning 

directive

although is not C++ standard is quite unlikely you will encounter a compiler that does not support it (see this SO question).

Alp

Windows / MSVC is a fairly common target with Qt, so I have to disagree with "quite unlikely." – Franchot

Answer 4

1. Pull the real function out of public scope.
2. Create another function with the same name in public scope.
3. Insert your warning/fail code in that function.
4. Call the original with the new.

Demonolatry

Can't emit a compile time warning that way. – Tails

Answer 5

You might want to do something similiar yourself:

#ifdef Q_TREAT_OBSOLETE_AS_ERRORS
#define Q_OBSOLETE(X) \
        BOOST_STATIC_ASSERT(false); \
        X

#else 
#define Q_OBSOLETE(X) X
#endif

This construction simply substitutes some deprecated code / part of code if there is no Q_TREAT_OBSOLETE_AS_ERRORS defined and generates compilation-time error otherwise.

Note that BOOST_STATIC_ASSERT has no scope limitations, so does the Q_OBSOLETE macro.

Probably this is not the best way to solve your problem and actually I'm not sure this is useful.

You might just mark the code as @obsolete or simply point it out in the comments.

Dissect

This doesn't work. This will cause compile errors if the deprecated code is compiled, whether or not that code is called from somewhere else. – Tails

@Ken Well, it's obviously that you don't have a way to check if some code chunk is actually being called. This static assertions thing simply helps drawing attention to the compilation of deprecated features. Actually, all this Q_OBSOLETE stuff seems pretty unnecessary for me. – Dissect

he's asking (I think) because he's used to Java, which can annotate functions as deprecated, and give you a compiler warning when you try to use them. – Tails

Answer 6

By "deprecated constructs", you really mean "deprecated member functions". You're asking for a compile-time warning to draw your attention to the call site of any deprecated function.

This isn't possible in any reasonable way in standard C++, and I don't see any attributes in G++ that would support this either. Qt can't really add a feature like that if the compiler doesn't have some support for it already.

However, Microsoft Visual C++ supports an __declspec(deprecated) extension, and I would imagine it's possible to write a compiler plugin for G++ 4.5 that adds a similar feature.

Tails

It seems I missed __attribute__((deprecated)) in G++. – Tails

By the way in c++14 and so there's the [[deprecated]] keyword that you can add to some functions en.cppreference.com/w/cpp/language/attributes – Vallievalliere