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std::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)
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:

Member types

Member type Definition
value (private) std::conditional_t<std::is_void_v<V>, std::remove_cvref_t<Ref>, V>;.
value is a cv-unqualified object type.
(exposition-only member type*)
reference (private) std::conditional_t<std::is_void_v<V>, Ref&&, Ref>;.
reference is either a reference type, or a cv-unqualified object type that models copy_constructible.
(exposition-only member type*)
yielded std::conditional_t<std::is_reference_v<reference>, reference, const reference&>.

Let /*RRef*/ denote:

The following concepts are modeled:

Member functions

constructs a generator object
(public member function)
effectively destroys the entire stack of yielded generators
(public member function)
assigns a generator object
(public member function)
resumes the initially suspended coroutine and returns an iterator to its handle
(public member function)
returns std::default_sentinel
(public member function)
Inherited from std::ranges::view_interface
(C++20)
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>)
(C++23)
returns a constant iterator to the beginning of the range.
(public member function of std::ranges::view_interface<D>)
(C++23)
returns a sentinel for the constant iterator of the range.
(public member function of std::ranges::view_interface<D>)
(C++20)
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

the promise type
(public member class)
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>
#include <ranges>
 
std::generator<char> letters(char first)
{
    for (;; co_yield first++);
}
 
int main()
{
    for (const char ch : letters('a') | std::views::take(26))
        std::cout << ch << ' ';
    std::cout << '\n';
}

Output:

a b c d e f g h i j k l m n o p q r s t u v w x y z

References

  • C++23 standard (ISO/IEC 14882:2023):
    • 26.8 Range generators [coro.generator]

See also

(C++20)
creates a coroutine handle that has no observable effects when resumed or destroyed
(function)

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