C++17: Latest Features Coming To 33-Year-Old Programming Language

The C++17 is taken standing and putting new features to the classic programming language. This significant update is designed to create C++ an easier language to work alongside and delivers effective technical requirements.


During the latest criteria meeting in Oulu, Finland, the actual and final features list of C++ is finalised. This means that, C++17 has turned into a major release similar to C++98 and C++11.

The latest C++ 17 standards are launching new features such as structured bindings and if initialisers, creating this classic programming language “feature complete”.

The C++ community is calling C++17 the beginning of a new era designed to take robust technical specifications. The non-C++17 features will likely be revealed later as add-ons in the future releases.



Here’s the list of the highest features that were voted during the meeting:
 

Templates and Generic Code

• Template argument deduction for class templates
  
  • Like how functions deduce template arguments, now constructors can deduce the template arguments of the class
• template <auto>
  
  • Represents a value of any (non-type template argument) type
• Non-type template arguments fixes
• template<template<class...>typename bob> struct foo {}
• ( Folding + ... + expressions )
• auto x{8}; is an int
• constexpr if
  
  • Much requested feature to simplify almost-generic code

Lambda

• constexpr lambdas
  
  • Lambdas are implicitly constexpr if they qualify
• Capturing *this in lambdas
  
  • [*this]{ std::cout << could << " be " << useful << '\n'; }

Attributes

• [[fallthrough]], [[nodiscard]], [[maybe_unused]] attributes
• [[attributes]] on namespaces and enum { erator[[s]] }

Syntax cleanup

• Inline variables
  
  • Like inline functions
  • Compiler picks where the instance is instantiated?
• namespace A::B
• Simple static_assert(expression); with no string

Cleaner multi-return and flow control

• Structured bindings
  
  • Basically, first-class std::tie with auto
  • Example:
    • const auto [it, inserted] = map.insert( {"foo", bar} );
    • Creates variables it and inserted with deduced type from the pair that map::insert returns.
  • Works with tuple/pair-likes & std::arrays and relatively flat structs
• if (init; condition) and switch (init; condition)
  
  • if (const auto [it, inserted] = map.insert( {"foo", bar} ); inserted)
  • Extends the if(decl) to cases where decl isn't convertible-to-bool sensibly
• Generalizing range-based for loops
  
  • Appears to be mostly support for sentinals, or end iterators that are not the same type as begin iterators, which helps with null-terminated loops and the like.

Misc

• Hexadecimal float point literals
• Dynamic memory allocation for over-aligned data
• Guaranteed copy elision (r1 accepted, not r0: have not found r1 on 'net yet)
  
  • Finally!
• A weaker version of fixed order-of-evaluation for expressions
  
  • "See Chandler's tweet and Alisdair's tweet" (cannot find Alisdair's tweet, linked to doc found in Chandler's tweet)
  • Not including function arguments, but function argument evaluation interleaving now banned
  • Makes a bunch of broken code work mostly
• Forward progress guarantees (FPG) (also, FPGs for parallel algorithms)
  
  • I think this is saying "the implementation may not stall threads forever"?
• u8'U', u8'T', u8'F', u8'8' character literals.
• "noexcept" in type system
• __has_include
  
  • Test if a header file include would be an error
• Arrays of pointer conversion fixes
• inherited constructors fixes to some corner cases.

Library additions:

Data types

• std::variant<Ts...>
  
  • Almost-always non-empty last I checked?
  • Tagged union type
• std::optional
  
  • Maybe holds one of something
• std::any
  
  • Holds one of anything
• std::string_view
  
  • std::string like reference-to-character-array or substring
• std::shared_mutex
  
  • Untimed, which can be more efficient if you don't need it.

Invoke stuff

• std::invoke
  • Call any callable (function pointer, function, member pointer) with one syntax. From the standard INVOKE concept.
• std::apply
  • Takes a function-like and a tuple, and unpacks the tuple into the call.
• std::is_callable
• std::make_from_tuple, std::apply applied to object construction

File System TS v1

• [class.path]
• [class.filesystem.error]
• [class.file_status]
• [class.directory_entry]
• [class.directory_iterator] and [class.recursive_directory_iterator]
• [fs.ops.funcs]

Parallelism TS v1

• The linked paper from 2014, may be out of date
• Parallel versions of std algorithms, and related machinery

(parts of) Library Fundamentals TS v1 not covered above or below

• [func.searchers] and [alg.search]
  • A searching algorithm and techniques
• [pmr]
  
  • Polymorphic allocator, like std::function for allocators
  • And some standard memory resources to go with it.
• std::sample, sampling from a range?

Container Improvements

• try_emplace and insert_or_assign
• Splicing for map<>, unordered_map<>, set<>, and unordered_set<>
  
  • Paper from 2013, so maybe newer proposal accepted?
• non-const .data() for string.
• non-member std::size, std::empty, std::data
• Minimal incomplete type support in containers
• Contiguous iterator "concept"
• constexpr iterators

Other std datatype improvements:

• {} construction of std::tuple and other improvements
• atomic<T>.is_always_lockfree()
• lock_guard<Mutexes...>
  • Saves some std::lock pain when locking more than one mutex at a time.
• unique_ptr<T[]> fixes and other unique_ptr tweaks.
• TriviallyCopyable reference_wrapper, can be performance boost

Misc

• C++17 library is based on C11 instead of C99
• Reserved std[0-9]+ for future standard libraries
• destroy(_at|_n), uninitialized_move(_n), uninitialized_value_construct(_n), uninitialized_default_construct(_n)
• Special math functions
• hardware_*_interference_size
• std::clamp()
  • std::clamp( a, b, c ) == std::max( b, std::min( a, c ) ) roughly
• std::uncaught_exceptions
  • Required if you want to only throw if safe from destructors
• std::as_const
• std::bool_constant<bool>
• A whole bunch of _v template variables
• std::void_t<T>
  • Surprisingly useful when writing templates
• std::owner_less<void>
  • like std::less<void>, but for smart pointers to sort based on contents
• std::chrono polish
• std::conjunction, std::disjunction, std::negation exposed
• std::not_fn
• Rules for noexcept within std
• std::is_contiguous_layout, useful for efficient hashing
• std::to_chars, high performance number conversion
• std::default_order, indirection over std::less.

Isocpp.org has an even more detailed list of documents, a few of which I looted for on top. Now it is not up-to-date exactly what is approved into C++17.
these slides had some features missing elsewhere.


Despite "what was eliminated" wasn't questioned, this is a short list of a few things ((mainly?) earlier deprecated) being removed in C++17 from C++:

Removed:

• register
• bool b; ++b;
• trigraphs
• ios aliases
• auto_ptr, old <functional>, random_shuffle
• allocators in std::function

Spec changes:

• exception specs and throw expressions

You can check out Reddit to obtain the detailed information of the new modifications arriving with C++17.


In conclusion, C++17 is actually a major update that includes plenty of smaller features which are likely to help make this language a lot more understandable.

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