C++ standard libraries extensions, version 2

Version 2 of the C++ Extensions for Library Fundamentals, ISO/IEC TS 19568:2017, defines the following new components for the C++ standard library, in addition to those defined in version 1 :

The C++ detection idiom

Defined in header <experimental/type_traits>
is_detected detected_t detected_or	alias templates that detect whether a template-id is well-formed (alias template)
nonesuch	class type returned by detected_t on failure (class)

A const-propagating wrapper for pointer-like objects

Defined in header <experimental/propagate_const>
propagate_const	a const-propagating wrapper for pointer-like objects (class template)

Non-owning pointers

Defined in header <experimental/memory>
observer_ptr	a class representing a non-owning pointer (class template)

std::array creation

Defined in header <experimental/array>
make_array (library fundamentals TS v2)	creates a std::array object whose size and optionally element type are deduced from the arguments (function template)

ostream_joiner

Defined in header <experimental/iterator>
ostream_joiner	an output iterator that writes successive elements into an output stream, separating adjacent elements with a delimiter (class template)

Simple random number generation facilities

A per-thread engine of type std::default_random_engine , initialized to an unpredictable state, is maintained for each thread and used by the functions below.

Defined in header <experimental/random>
randint	generates a random integer in the specified range (function template)
reseed	reseeds the per-thread random engine (function)
Defined in header <experimental/algorithm>
sample	selects n random elements from a sequence (function template)
shuffle	randomly re-orders elements in a range (function template)

Feature test macros

Defined in header <experimental/type_traits>
__cpp_lib_experimental_logical_traits	a value of at least 201511 indicates that the logical operator type traits are supported (macro constant)
__cpp_lib_experimental_detect	a value of at least 201505 indicates that the detection idiom is supported (macro constant)
Defined in header <experimental/propagate_const>
__cpp_lib_experimental_propagate_const	a value of at least 201505 indicates that propagate_const is supported (macro constant)
Defined in header <experimental/functional>
__cpp_lib_experimental_not_fn	a value of at least 201406 indicates that not_fn is supported (macro constant)
Defined in header <experimental/memory>
__cpp_lib_experimental_observer_ptr	a value of at least 201411 indicates that observer_ptr is supported (macro constant)
Defined in header <experimental/array>
__cpp_lib_experimental_make_array	a value of at least 201505 indicates that make_array is supported (macro constant)
Defined in header <experimental/iterator>
__cpp_lib_experimental_ostream_joiner	a value of at least 201411 indicates that ostream_joiner is supported (macro constant)
Defined in header <experimental/vector>
__cpp_lib_experimental_erase_if	a value of at least 201411 indicates that uniform container erasure is supported (macro constant)
Defined in header <experimental/numeric>
__cpp_lib_experimental_gcd_lcm	a value of at least 201411 indicates that gcd and lcm are supported (macro constant)
Defined in header <experimental/random>
__cpp_lib_experimental_randint	a value of at least 201511 indicates that randint and related functions are supported (macro constant)
Defined in header <experimental/source_location>
__cpp_lib_experimental_source_location	a value of at least 201505 indicates that source_location is supported (macro constant)

Merged into C++17

The following components of the library fundamentals v2 have been adopted into the C++17 standard.

Defined in header <experimental/type_traits>
void_t	variadic alias template for void (alias template)
Defined in header <experimental/type_traits>
conjunction	variadic logical AND metafunction (class template)
disjunction	variadic logical OR metafunction (class template)
negation	logical NOT metafunction (class template)
Defined in header <experimental/functional>
not_fn	creates a function object that returns the complement of the result of the function object it holds (function template)
Defined in header <experimental/numeric>
gcd	constexpr function template returning the greatest common divisor of two integers (function template)
lcm	constexpr function template returning the least common multiple of two integers (function template)

Merged into C++20

The following components of the library fundamentals v2 have been adopted into the C++20 standard.

Uniform container erasure

Defined in header <experimental/string>
erase (std::basic_string)	erases all elements equal to a specific value from a std::basic_string (function template)
erase_if (std::basic_string)	erases all elements satisfying a predicate from a std::basic_string (function template)
Defined in header <experimental/deque>
erase (std::deque)	erases all elements equal to a specific value from a std::deque (function template)
erase_if (std::deque)	erases all elements satisfying a predicate from a std::deque (function template)
Defined in header <experimental/vector>
erase (std::vector)	erases all elements equal to a specific value from a std::vector (function template)
erase_if (std::vector)	erases all elements satisfying a predicate from a std::vector (function template)
Defined in header <experimental/forward_list>
erase (std::forward_list)	erases all elements equal to a specific value from a std::forward_list (function template)
erase_if (std::forward_list)	erases all elements satisfying a predicate from a std::forward_list (function template)
Defined in header <experimental/list>
erase (std::list)	erases all elements equal to a specific value from a std::list (function template)
erase_if (std::list)	erases all elements satisfying a predicate from a std::list (function template)
Defined in header <experimental/map>
erase_if (std::map)	erases all elements satisfying a predicate from a std::map (function template)
erase_if (std::multimap)	erases all elements satisfying a predicate from a std::multimap (function template)
Defined in header <experimental/set>
erase_if (std::set)	erases all elements satisfying a predicate from a std::set (function template)
erase_if (std::multiset)	erases all elements satisfying a predicate from a std::multiset (function template)
Defined in header <experimental/unordered_map>
erase_if (std::unordered_map)	erases all elements satisfying a predicate from a std::unordered_map (function template)
erase_if (std::unordered_multimap)	erases all elements satisfying a predicate from a std::unordered_multimap (function template)
Defined in header <experimental/unordered_set>
erase_if (std::unordered_set)	erases all elements satisfying a predicate from a std::unordered_set (function template)
erase_if (std::unordered_multiset)	erases all elements satisfying a predicate from a std::unordered_multiset (function template)

Source code information capture

Defined in header <experimental/source_location>
source_location	a class representing information about the source code, such as file names, line numbers, and function names (class)

std::array conversion

Defined in header <experimental/array>
to_array	creates a std::array object from a built-in array (function template)