QtCS2015 ModernCpp

C++ session notes

Sunday June 7, 10:00 AM

Impact on supported compilers: VS2012 from 5.6. onwards and LTS release Discussed more in depth with the deprecation session

What kind of features do we want, and what version of C++? 11? 14? What are the minimum requirements, even if we don’t rely on the existance of the feature? Qt is 5-8 years behind on C++, due to its nature. Auto, constexpr, lambda’s, r-value ref, decltype, nullptr, … (missed the rest). From compiler detection Variadic templates? We don’t use too many templates at all… QNX is not a problem in terms of compatibility. Gcc needs 4.7 for decent support of modern cpp. OS/X on gcc is dead, use clang instead. For windows embedded (EC) rely on MSVC 2012.

Available for use then: Yes:

• Auto
• decltype
• nullptr
• r-value ref
• lambda
• class enum
• explicit overrides

No:

• uniform initialization and constexpr is broken in VS2013.
• Explicit conversions,
• = default,
• = deleted,
• inheriting constructors,
• noexcept

C++/11 needed for user applications as well then. It also lightens the maintenance burden.

When to use?

Don’t go overboard. We need some guidelines. Creator team has experience. 

r-value, deltype are no-brainers. Class enums are a bit more of a discussion. It needs testing, and do we want that in the API? There is an inconsistency with existing API. The Qt:: scope is a big collection that would have been clearer. Auto is for use in code, not in API. Use auto where the type is (implicity) also on the right side. Iterators is a prime example. Creator may be used as a rule of thumb: if creator can’t follow them, don’t use them.

Nullptr should be used in the headers, they need to be cleaner than the implementation. In source pointers: where it increases readability. Where it is not unclear, feel free to use 0 instead. It may cause some minor source incompatibilities. Don’t change existing code for the sake of this.  

Library use

We can’t detect the version, but we might require a minimum? Wishlist :

• initialize lists
• std::move
• std::function
• type traits (including enable_if),
• std::forward
• nulltpr_t
• std::chrono?
• shared_ptr
• unique_ptr. QScopedPointer might be made movable now.

New smart pointer session, or on the Mailing list?

TODO: We need to test this what is supported with the base line compilers.

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

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

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

We need to be a bit conservative in our public API for now, until we get some more experience. First learn, then slowly introduce.