std::generator

Defined in header <generator>

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)

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.

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 (`ranges::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	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.

Data members

Member Description

Member	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 `promise_type::final_suspend` is called), it is removed from the stack. (exposition-only member object*)
`coroutine_` (private)	a handle of type std::coroutine_handle<promise_type> (exposition-only member object*)

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 promise_type::final_suspend is called), it is removed from the stack.
(exposition-only member object*)

coroutine_ (private) a handle of type std::coroutine_handle<promise_type>
(exposition-only member object*)

Member functions

(constructor)	constructs a `generator` object (public member function)
(destructor)	effectively destroys the entire stack of yielded `generator`s (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 only 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 only 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

#include <generator>
#include <iostream>

template<typename T>
struct Tree
{
    T value;
    Tree *left{}, *right{};

    std::generator<const T&> traverse_inorder() const
    {
        if (left)
            co_yield std::ranges::elements_of(left->traverse_inorder());

        co_yield value;

        if (right)
            co_yield std::ranges::elements_of(right->traverse_inorder());
    }
};

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_inorder())
        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]

ranges::elements_of (C++23)	tags a range to be treated as a sequence rather than a single value (class template)
noop_coroutine (C++20)	creates a coroutine handle that has no observable effects when resumed or destroyed (function)

std::generator

Template parameters

Member types

Data members

Member functions

Inherited from `std::ranges::view_interface`

Nested classes

Notes

Example

References

See also

std::generator

Template parameters

Member types

Data members

Member functions

Inherited from std::ranges::view_interface

Nested classes

Notes

Example

References

See also

Inherited from `std::ranges::view_interface`