[Development] Upgrading the sources to C++11 keywords (Q_NULLPTR, etc.)

Bo Thorsen bo at vikingsoft.eu
Mon Feb 9 12:05:52 CET 2015

On 02/09/2015 09:54 AM, Smith Martin wrote:
> "In the vast majority of cases, a sorted vector is much faster, due to
>     locality-of-reference. A RB-tree is optimized for wildly mixing insertions,
>     lookups, and removals. When you can batch updates and separate them from
>     lookups, time-wise, then a sorted vector is usually preferrable."
> This is the kind of thing we should add to the documentation, but can you elaborate? I mean, illustrate the meaning of "locality of reference," "wildly mixing insertions, lookups, and removals," and "batch updates and separate them from lookups, time-wise."

IMHO: A lot of this is information you find in a CS course or in a book. 
Not in the Qt documentation.

That said, there is certainly room for more info on performance impact 
in the Qt documentation. The discussion a while ago about what calls 
removes the effect of reserve() was one example.


> ________________________________________
> From: development-bounces+martin.smith=theqtcompany.com at qt-project.org <development-bounces+martin.smith=theqtcompany.com at qt-project.org> on behalf of Marc Mutz <marc.mutz at kdab.com>
> Sent: Sunday, February 8, 2015 2:28 PM
> To: development at qt-project.org
> Cc: Thiago Macieira
> Subject: Re: [Development] Upgrading the sources to C++11 keywords      (Q_NULLPTR, etc.)
> Hi,
> Sorry for being late, didn't see the thread before.
> On Thursday 08 January 2015 23:33:34 Thiago Macieira wrote:
>> I think it's time to institute a policy that we should fix our sources to
>> use the new C++11 keywords. I'd like to propose the following.
> I totally agree, with the following amendments:
> 1. override - before adding an overriding function to a class, add
>     Q_DECL_OVERRIDE to all members in a separate commit, to avoid said
>     warning.
> 2. noexcept/nothrow - the only difference between the two is that nothrow may
>     expand to throw() on compilers that implement semantics close to noexcept
>     for that, and not the C++98 standard behaviour. This is currently only the
>     case for MSVC, even though I believe GCC has a switch to emulate MSVC here.
>     The semantic difference currently, is: when you violate noexcept, you're
>     getting C++11 behaviour (std::terminate is called) or the exception leaves
>     the function. If you violate nothrow, you're enterin undefined bahaviour.
>     So only use nothrow if functions _cannot possibly_ throw. If you want to
>     say "I'm fine with errors in this function terminating the process", which
>     you should be very carefully considering (it should be the exception), you
>     must use noexcept instead. Obviously, if you need conditionally-noexcept,
>     you must use noexcept_expr.
>     Talking about warnings: there's -Wnoexcept, which warns when a
>     conditionally-noexcept function turns noexecpt(false) because of a function
>     that isn't marked noexcept and where the compiler can prove it doesn't
>     throw. That's a bit of a mouthful, but this, too, should be added to the
>     headersclean check.
>     Talking about narrow contracts: A function has a narrow contract if it has
>     preconditions (on it's arguments, or *this). If you have preciditions, you
>     may, in debug mode, assert them as an aid to the user of your function.
>     Assertions may be turned by the user (there's also a movement behind John
>     Lakos, yes, _the_ John Lakos that essentially gave us the d-pointer, to
>     make this standard functionality), into exception throwing (which I had
>     personally good experience with during Kleopatra development). But if your
>     users do this, they expect to receive those exceptions, and not terminate
>     the program without a hint that an assertion was triggered).
>     That's why functions with narrow contracts should not be noexcept.
>     Aside: of course, you can often drop preconditions by tightening the
>     interface of the function. E.g. instead of taking a naked pointer, you
>     could take a non_null_ptr<T>, which would explode when constructed with a
>     nullptr, thus making sure that every successfully constructed instance is
>     actually representing a non-null pointer. Another technique is to use
>     enums, because the standard says that you cannot load a value from an enum
>     variable that does not correspond to one of the enumerated values. Doing
>     otherwise constitutes undefined behaviour, and compilers are getting oh-so-
>     good at exploiting UB that your only chance is to learn about and avoid
>     them.
> 3. nullptr - On top of the warning, which I wasn't aware about, I find the
>     code easier to read. It's a mouthful, but it's what everyone will be using
>     five years from now, so we might as well start it now. I treat this as a
>     whitespace error, meaning I correct it whenever I touch a line of code for
>     unrelated changes.
> I would add the following, unrelated to C++11, but found all over the place in
> Qt, and it's next to impossible to root out: Algorithmic ineffciency. That's a
> large blob, but the most important instances of it are:
> a. Not marking types as movable or primitive. We might actually want to have
>     a policy to mark complex types explicitly as complex, to allow easier
>     fixing of missing declarations.
>     The rule here should be that every new class or struct that may at some
>     point be held in a container must have a q_declare_typeinfo. Rationale:
>     it's impossible to add them after release, since changing the typeinfo
>     changes the memory layout of QList, making the change BIC.
> b. Using QLists when they are not optimally efficient. For a QList to be
>     optimally efficient, the type T must be movable (or primitive) and sizeof T
>     == sizeof (void*) (yes, different on 32 and 64-bit platforms!). For a QList
>     to be acceptable (actually, it's not, but it's at least not horribly
>     inefficient), replace == sizeof(void*) with <= sizeof(void*).
>     The rule should be that you need to accompany any use of QList that isn't
>     already mandated by existing APIs (QList<QVariant>, say), by a
>     static_assert that QList is optimally efficient. There are patches in the
>     pipeline to make this easier than checking QTypeInfo yourself.
>     Of course, when QList is not efficient, you should use a QVector instead.
> c. Using QMap. As Alex Stepanov put it: every use of a map should be discussed
>     in a face-to-face meeting with your manager. Since we don't have that, I'd
>     change this to: Everyone wishing to use a QMap should implement one before
>     using it for the first time. Then you'd see what you inflict on the world.
>     In the vast majority of cases, a sorted vector is much faster, due to
>     locality-of-reference. A RB-tree is optimized for wildly mixing insertions,
>     lookups, and removals. When you can batch updates and separate them from
>     lookups, time-wise, then a sorted vector is usually preferrable.
> d. Algorithmic complexity. Avoid O(n²) like the plague. Anthing worse than
>     that should get a big fat comment saying why it's necessary (like: this
>     optimisation algorithm is equivalent to the knapsack problem, so I need to
>     use this exponential algorithm, because a) we need the global optimum, not
>     a local one, and b) the set is always less than four elements.
> I could go on and on with this. Like using a set for de-duplication, then
> converting to a list and sorting that (because, surprise, QSet is
> std::unordered_set), but there's really no substitute to understanding what
> you're doing and no set of rules, however large, will give you that. Using std
> algorithms would, though, as far as they go.
> I'm sorry, this has become so much longer than planned...
> Thanks for reading up to here. May your code be the better for it,
> Marc
> --
> Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
> KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
> www.kdab.com || Germany +49-30-521325470 || Sweden (HQ) +46-563-540090
> KDAB - Qt Experts - Platform-Independent Software Solutions
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Bo Thorsen,
Director, Viking Software.

Viking Software
Qt and C++ developers for hire

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