std::generator

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Coroutine support

Coroutine traits

coroutine_traits (C++20)

Coroutine handle

coroutine_handle (C++20)

No-op coroutines

noop_coroutine_promise (C++20)

noop_coroutine (C++20)

Trivial awaitables

suspend_never (C++20)

suspend_always (C++20)

Range generators

generator (C++23)

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while concat_viewviews::concat (C++26)(C++26)	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

std::generator

Member functions

generator::generator

generator::~generator

generator::operator=

generator::begin

generator::end

promise_type

generator::promise_type::get_return_object

generator::promise_type::initial_suspend

generator::promise_type::final_suspend

generator::promise_type::yield_value

generator::promise_type::return_void

generator::promise_type::unhandled_exception

generator::promise_type::operator new

generator::promise_type::operator delete

Iterator

generator::iterator::iterator

generator::iterator::operator=

generator::iterator::operator*

generator::iterator::operator++ generator::iterator::operator++(int)

operator==(generator::iterator, std::default_sentinel_t)

 

Defined in header <generator>
template<     class Ref,     class V = void,     class Allocator = void > class generator     : public ranges::view_interface<generator<Ref, V, Allocator>>	(1)	(since C++23)
namespace pmr {     template< class Ref, class V = void >     using generator =         std::generator<Ref, V, std::pmr::polymorphic_allocator<>>; }	(2)	(since C++23)

1) The class template std::generator presents a view of the elements yielded by the evaluation of a coroutine.

2) Convenience alias template for the generator using the polymorphic allocator.

A std::generator generates a sequence of elements by repeatedly resuming the coroutine from which it was returned. Each time a co_yield statement is evaluated, the coroutine produces one element of the sequence. When the co_yield statement is of the form co_yield ranges::elements_of(rng), each element of the range rng is successively produced as an element of the sequence.

std::generator models view and input_range.

The behavior of a program that adds a specialization for std::generator is undefined.

Data members

Name	Description
active_ (private)	Internally, each active instance of std::generator is associated with a stack (handled as if by object of type std::unique_ptr<std::stack<std::coroutine_handle<>>>). When begin is called, a new stack is created and the generator is added to the stack. When co_yield ranges::elements_of(rng) is evaluated in a generator body, rng is converted to a generator and added to the stack that contains the enclosing generator. When a generator iterator is incremented, the coroutine at the top of the associated stack is resumed. When a generator finishes (i.e. when final_suspend is called), it is removed from the stack. (exposition-only member object*)
coroutine_ (private)	std::coroutine_handle<promise_type> (exposition-only member object*)

Template parameters

Ref	-	the reference type (ranges::range_reference_t) of the generator. If V is void, both the reference type and the value type are inferred from Ref
V	-	the value type (range_value_t) of the generator, or void
Allocator	-	an allocator type or void

If Allocator is not void, then the behavior is undefined if Allocator does not meet the Allocator requirements.

Member types

Member type	Definition
value (private)	std::conditional_t<std::is_void_v<V>, std::remove_cvref_t<Ref>, V>;. (exposition-only member type*)
reference (private)	std::conditional_t<std::is_void_v<V>, Ref&&, Ref>;. (exposition-only member type*)
yielded	std::conditional_t<std::is_reference_v<reference>, reference, const reference&>.

Type requirements

- std::allocator_traits<Allocator>::pointer is a pointer type.

- value is a cv-unqualified object type.

- reference is either a reference type, or a cv-unqualified object type that models copy_constructible.

- Let /*RRef*/ denote:std::remove_reference_t</*reference*/>&&, if /*reference*/ is a reference type, and /*reference*/ otherwise. std::common_reference_with</*reference*/&&, /*value*/&> is modeled. std::common_reference_with</*reference*/&&, /*RRef*/&&> is modeled. std::common_reference_with</*RRef*/&&, const /*value*/&> is modeled.

The program is ill-formed if any of these type requirements is not satisfied.

Member functions

(constructor)	constructs a generator object (public member function)
(destructor)	effectively destroys the entire stack of yielded generators (public member function)
operator=	assigns a generator object (public member function)
begin	resumes the initially suspended coroutine and returns an iterator to its handle (public member function)
end	returns std::default_sentinel (public member function)
Inherited from std::ranges::view_interface
empty	returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function of std::ranges::view_interface<D>)
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of std::ranges::view_interface<D>)
operator bool	returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of std::ranges::view_interface<D>)

Nested classes

promise_type	the promise type (public member class)
iterator	the iterator type (exposition-only member class*)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_generator	202207L	(C++23)	std::generator – synchronous coroutine generator for ranges

Example

Can be tried in Compiler Explorer

Run this code

#include <generator>
#include <iostream>
 
template<typename T>
struct Tree
{
    T value;
    Tree *left{}, *right{};
 
    std::generator<const T&> traverse_preorder() const
    {
        if (left)
            for (const T& x : left->traverse_preorder())
                co_yield x;
 
        co_yield value;
        if (right)
            for (const T& x : right->traverse_preorder())
                co_yield x;
    }
};
 
int main()
{
    Tree<char> tree[]
    {
                                    {'D', tree + 1, tree + 2},
        //                            │
        //            ┌───────────────┴────────────────┐
        //            │                                │
                    {'B', tree + 3, tree + 4},       {'F', tree + 5, tree + 6},
        //            │                                │
        //  ┌─────────┴─────────────┐      ┌───────────┴─────────────┐
        //  │                       │      │                         │
          {'A'},                  {'C'}, {'E'},                    {'G'}
    };
 
    for (char x : tree->traverse_preorder())
        std::cout << x << ' ';
    std::cout << '\n';
}

Output:

A B C D E F G

References

• C++23 standard (ISO/IEC 14882:2024):

• 26.8 Range generators [coro.generator]

See also

noop_coroutine (C++20)

creates a coroutine handle that has no observable effects when resumed or destroyed (function)