NAME¶

std::generator - std::generator

Synopsis¶

Defined in header <generator>
template<

class Ref,
class V = void, (1) (since C++23)
class Allocator = void >
class generator

: public ranges::view_interface<generator<Ref, V, Allocator>>
namespace pmr {

template< class Ref, class V = void >
using generator = (2) (since C++23)
std::generator<Ref, V, std::pmr::polymorphic_allocator<>>;

}

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 (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
std::conditional_t<std::is_void_v<V>, std::remove_cvref_t<Ref>,
value (private) V>;.
(exposition-only member type*)
reference std::conditional_t<std::is_void_v<V>, Ref&&, Ref>;.
(private) (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 objects¶

Member name Definition
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.
active_ * When co_yield ranges::elements_of(rng) is evaluated in a generator
(private) 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_ a handle of type std::coroutine_handle<promise_type>
(private) (exposition-only member object*)

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)
resumes the initially suspended coroutine and returns an iterator to
begin 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
(C++20) sized_range or forward_range.
(public member function of std::ranges::view_interface<D>)
cbegin returns a constant iterator to the beginning of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
cend returns a sentinel for the constant iterator of the range.
(C++23) (public member function of std::ranges::view_interface<D>)
operator bool returns whether the derived view is not empty. Provided if
(C++20) 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_inorder() const
{
if (left)
for (const T& x : left->traverse_inorder())
co_yield x;

co_yield value;
if (right)
for (const T& x : right->traverse_inorder())
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_inorder())
std::cout << x << ' ';
std::cout << '\n';
}

Output:¶

A B C D E F G

References¶

* C++23 standard (ISO/IEC 14882:2023):

* 26.8 Range generators [coro.generator]

See also¶

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