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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../preprocessor/../coroutine/generator.html below:

std::generator - cppreference.com

template< class Ref, class V = void, class Allocator = void > class generator : public ranges::view_interface<generator<Ref, V, Allocator>> (1) (since C++23) (2) (since C++23) 1)

The class template

std::generator

presents a

view

of the elements yielded by the evaluation of a

coroutine

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.

[edit] 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.

[edit] Member types

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

[edit] 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 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*) [edit] Member functions constructs a generator object
(public member function) [edit] effectively destroys the entire stack of yielded generators
(public member function) [edit] assigns a generator object
(public member function) [edit] resumes the initially suspended coroutine and returns an iterator to its handle
(public member function) [edit] returns std::default_sentinel
(public member function) [edit] Inherited from std::ranges::view_interface 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>) [edit] returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit] returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit] 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>) [edit] [edit] Nested classes the promise type
(public member class) the iterator type
(exposition-only member class*) [edit] Notes [edit] 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},
        //                            â
        //            âââââââââââââââââ´âââââââââââââââââ
        //            â                                â
                    {'B', tree + 3, tree + 4},       {'F', tree + 5, tree + 6},
        //            â                                â
        //  âââââââââââ´ââââââââââââââ      âââââââââââââ´ââââââââââââââ
        //  â                       â      â                         â
          {'A'},                  {'C'}, {'E'},                    {'G'}
    };
 
    for (char x : tree->traverse_inorder())
        std::cout << x << ' ';
    std::cout << '\n';
}

Output:

[edit] References

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

26.8 Range generators [coro.generator]

[edit] See also creates a coroutine handle that has no observable effects when resumed or destroyed
(function) [edit]

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