std::generator

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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

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

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
-
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.

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

Data members

Member Definition
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
  • 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

 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
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>)
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

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)
            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]

See also

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