C++20

New Features

Language

• concepts, with terse syntax
• modules
• designated initializers
• as a lambda capture
• template parameter lists on lambdas
• three-way comparison using the "spaceship operator", operator <=>
• initialization of an additional variable within a range-based for statement
• lambdas in unevaluated contexts
• default constructible and assignable stateless lambdas
• allow pack expansions in lambda init-capture
• string literals as template parameters
• removing the need for typename in certain circumstances
• new standard attributes no_unique_address, likely and unlikely
• conditional explicit, allowing the explicit modifier to be contingent on a boolean expression
• expanded constexpr: virtual functions, union, try and catch, dynamic_cast and typeid, std::pointer_traits
• immediate functions using the new consteval keyword
• signed integers are now defined to be represented using two's complement
• a revised memory model
• various improvements to structured bindings
• coroutines
• using on scoped enums
• constinit keyword

  Library

• ranges
• std::make_shared and std::allocate_shared for arrays
• atomic smart pointers
• std::to_address to convert a pointer to a raw pointer
• calendar and time-zone additions to
• std::span, providing a view to a contiguous array
• header
• std::bit_cast<> for type casting of object representations, with less verbosity than memcpy and more ability to exploit compiler internals
• feature test macros
• various constexpr library bits
• smart pointer creation with default initialization
• std::map::contains method

  Deprecation

• Use of comma operator in subscript expressions has been deprecated
• volatile has been deprecated

  New (and changed) keywords

History of changes

• concepts
• designated initializers
• as a lambda capture
• template parameter lists on lambdas
• std::make_shared and std::allocate_shared for arrays

• three-way comparison using the "spaceship operator", operator <=>
• initialization of an additional variable within a range-based for statement
• lambdas in unevaluated contexts
• default constructible and assignable stateless lambdas
• allow pack expansions in lambda init-capture
• string literals as template parameters
• atomic smart pointers
• std::to_address to convert a pointer to a raw pointer

• removing the need for typename in certain circumstances
• new standard attributes no_unique_address, likely and unlikely
• calendar and time-zone additions to
• std::span, providing a view to a contiguous array
• header

• contracts [|deferred to a later standard]
• feature test macros
• bit-casting of object representations, with less verbosity than memcpy and more ability to exploit compiler internals
• conditional explicit, allowing the explicit modifier to be contingent on a boolean expression
• constexpr virtual functions

• ranges
• concept terse syntax
• constexpr union, try and catch, dynamic_cast, typeid and std::pointer_traits.
• various constexpr library bits
• immediate functions using the new consteval keyword
• signed integers are now defined to be represented using two's complement
• refinements of the contracts facility
• a revised memory model
• Smart pointer creation with default initialization

• coroutines already experimentally supported in Clang 5
• modules experimentally supported in Clang 5 and Visual Studio 2015 Update 1 as well as GCC
• various improvements to structured bindings

• contracts have been removed
• use of comma operator in subscript expressions has been deprecated
• constexpr additions
• using scoped enums
• various changes to the spaceship operator
• DR: minor changes to modules
• constinit keyword
• changes to concepts
• volatile has been deprecated
• DR: nodiscard effects on constructors
• The new standard library concepts will not use PascalCase
• text formatting
• bit operations
• constexpr INVOKE
• math constants
• consistency additions to atomics
• add the <=> operator to the standard library
• header units for the standard library
• synchronization facilities
• std::source_location
• constexpr containers
• std::stop_token and joining thread

• Class Types in Non-Type Template Parameters : The restriction of no user-defined operator allowed has been removed as the meaning of template argument equality has been divorced from operator. This allows also for array members in class-type NTTP.
• Floating-point types, pointers and references and unions and union-like classes are now allowed as NTTP.
• Function identity now also includes trailing requires-clauses
• Constrained non-template functions have been removed
• <compare> is now available in freestanding implementations
• std::spans typedef was changed from index_type to size_type to be consistent with the rest of the standard library
• Concept traits have been renamed to follow the renaming of the concepts as a result from the Cologne meeting
• Several fixes and additions to ranges
• Initialization for std::atomic<T> has been changed to make it work with default and list initialization, std::latch and std::barrier can now report the maximum number of threads that the implementation supports through the new member function max
• std::weak_equality and std::strong_equality have been removed as they are not used anymore
• Algorithms in <numeric> have been made constexpr
• Missing feature-test macros for new or changed features of C++20 have been added

  Features published as Technical Specifications

• Parallelism TS v2
• Networking TS v1
• Reflection TS v1

  Features deferred to a later standard

• Contracts a new study group has been formed to work on a new proposal
• Reflection
• Metaclasses
• Executors
• Networking extensions, including async, basic I/O services, timers, buffers and buffer-oriented streams, sockets, and Internet protocols
• Properties
• Extended futures