[Development] QtCS: Notes from Modern C++ session

[Marc Mutz]

Hi Andre,

thanks for the write-up!

On Friday 12 June 2015 08:08:51 André Somers wrote:
> Available for use then:

range-for?
variadic macros (these we already use in tests/ and no-one complained so far).

> No for now: std::for_each (issues with leaks)

Which leaks?

> For now, don’t put std lib ABI into Qt ABI, except for nulltpr_t.

Too late: QException inherits std::exception (for a looong time already), and 
by virtue of various exported subclasses of QVector and QList, we export ABI 
that contains std::vector and std::list (toStdList(), toStdVector()). We 
probably also export ABI using std::string, because I'm sure someone somewhere 
has inherited an exported class from QString...

std::list and std::string are BiC in GCC, probably each at different releases, 
and probably between C++98 and 11 builds.

FTR: Making std::list::size() O(1) in C++11 was a huge mistake, IMO. It 
pessimises the one use-case I have _ever_ used std::list for: splicing. Maybe 
forward_list is a replacement now...

> For lambda’s: how to deal with captures? They are dangerous. Default to 
> capture by value, capture by reference or pointer is problem.

The first thing *I* think when thinking lambdas is: code bloat. Please be 
_very_ careful. Every lambda has it's own unique type, even if they are token-
for-token identical. Using the same one on a template makes the whole template 
code duplicate. And I don't know any compiler that does a merge of binary 
identical code (does anyone?).

So, when a lambda is used in more than one place, please consider making it a 
namespace-level auto variable (only possible if it doesn't use local captures) 
or, yes, go the extra mile and write the function object explicitly.

Then again, using a lambda instead of a member-function private slot tends to 
leads to more compact executable code (at least than the SLOT()-based 
connect()).

Thanks,
Marc

-- 
Marc Mutz <marc.mutz at kdab.com> | Senior Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
Tel: +49-30-521325470
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