Difference between revisions of "cpp/coroutine/generator"

Latest revision as of 21:38, 12 November 2024

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.

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.

[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

(constructor)	constructs a `generator` object (public member function) [edit]
(destructor)	effectively destroys the entire stack of yielded `generator`s (public member function) [edit]
operator=	assigns a `generator` object (public member function) [edit]
begin	resumes the initially suspended coroutine and returns an iterator to its handle (public member function) [edit]
end	returns std::default_sentinel (public member function) [edit]
Inherited from std::ranges::view_interface
empty	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>`) [edit]
cbegin (C++23)	returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`) [edit]
cend (C++23)	returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`) [edit]
operator bool	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>`) [edit]

[edit] Nested classes

promise_type	the promise type (public member class)
iterator	the iterator type (exposition-only member class*)

[edit] Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_generator`	202207L	(C++23)	`std::generator` – synchronous coroutine generator for ranges

[edit] 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

[edit] References

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

26.8 Range generators [coro.generator]

[edit] See also

noop_coroutine

(C++20)

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

@@ Line 7: / Line 7: @@
      class Ref,
      class V = void,
-     class Allocator = void
+     class Allocator = void >
->
+class generator
-class generator : public ranges::view_interface<generator<Ref, V, Allocator>>
+    : public ranges::view_interface<generator<Ref, V, Allocator>>
 }}
 {{dcl|num=2|since=c++23|1=
 namespace pmr {
-template< class Ref, class V = void >
+    template< class Ref, class V = void >
-using generator =
+    using generator =
-    std::generator<Ref, V, std::pmr::polymorphic_allocator<{{void}}Ref>>;
+        std::generator<Ref, V, std::pmr::polymorphic_allocator<>>;
 }
 }}
@@ Line 22: / Line 22: @@
 @1@ The class template {{tt|std::generator}} presents a {{lconcept|view}} of the elements yielded by the evaluation of a {{lt|cpp/language/coroutines|coroutine}}.
-@2@ Convenience alias template for the {{tt|generator}} using the [[cpp/memory/polymorphic_allocator|polymorphic allocator]].
+@2@ Convenience alias template for the {{tt|generator}} using the {{lt|cpp/memory/polymorphic allocator}}.
 A {{tt|std::generator}} generates a sequence of elements by repeatedly resuming the coroutine from which it was returned.
-Each time a {{c|co_yield}} statement is evaluated, the coroutine produces one element of the sequence.
+Each time a {{c/core|co_yield}} statement is evaluated, the coroutine produces one element of the sequence.
-When the {{c|co_yield}} statement is of the form {{c|co_yield ranges::elements_of(rng)}}, each element of the {{lconcept|range}} {{tt|rng}} is successively produced as an element of the sequence.
+When the {{c/core|co_yield}} statement is of the form {{c|co_yield ranges::elements_of(rng)}}, each element of the {{lconcept|range}} {{c|rng}} is successively produced as an element of the sequence.
 {{tt|std::generator}} models {{lconcept|view}} and {{lconcept|input_range}}.
 The behavior of a program that adds a specialization for {{tt|std::generator}} is undefined.
-Internally, each active instance of {{tt|std::generator}} is associated with a stack.
-* When {{rlt|begin}} is called, a new stack is created and the generator is added to the stack.
-* When {{c|co_yield ranges::elements_of(rng)}} is evaluated in a generator body, {{c|rng}} is converted to a generator and added to the stack that contains the enclosing generator.
-* When a generator iterator is {{rl|iterator#increment|incremented}}, the coroutine at the top of the associated stack is resumed.
-* When a generator finishes (i.e. when {{rlt|promise_type/final_suspend|final_suspend}} is called), it is removed from the stack.
 ===Template parameters===
 {{par begin}}
-{{par|Ref|a reference type}}
+{{par|Ref|the reference type ({{lc|ranges::range_reference_t}}) of the generator. If {{tt|V}} is {{c/core|void}}, both the reference type and the value type are inferred from {{tt|Ref}}}}
-{{par|V|a value type or {{c|void}}}}
+{{par|V|the value type ({{lc|ranges::range_value_t}}) of the generator, or {{c/core|void}}}}
-{{par|Allocator|an allocator type or {{c|void}}}}
+{{par|Allocator|an allocator type or {{c/core|void}}}}
 {{par end}}
-If {{tt|Allocator}} is not {{c|void}}, then:
+If {{tt|Allocator}} is not {{c/core|void}}, then the behavior is undefined if {{tt|Allocator}} does not meet the {{named req|Allocator}} requirements.
-* {{c|std::allocator_traits<Allocator>::pointer}} is a pointer type;
-* {{tt|Allocator}} meets the {{named req|LegacyAllocator}} requirements.
 ===Member types===
 {{dsc begin}}
-{{dsc hitem|Member type|Definition}}
+{{dsc hitem|Member|Definition}}
-{{dsc|{{tt|yielded}} {{mark since c++23}}|{{c|std::conditional_t<std::is_reference_v<T>, T, const T&>}}}}
+{{dsc expos mem type|value|private=yes|{{c/core|std::conditional_t<std::is_void_v<V>, std::remove_cvref_t<{{void}}Ref>, V>;}}}}
-{{dsc|{{c|/*value*/}} {{mark|private}}{{mark since c++23}}|{{c|std::conditional_t<std::is_void_v<V>, std::remove_cvref_t<{{void}}Ref>, V>;}}.<br>{{c|/*value*/}} is a cv-unqualified object type. The name is for exposition only.}}
+{{dsc expos mem type|reference|private=yes|{{c/core|std::conditional_t<std::is_void_v<V>, Ref&&, Ref>;}}}}
-{{dsc|{{c|/*reference*/}} {{mark|private}}{{mark since c++23}}|{{c|std::conditional_t<std::is_void_v<V>, Ref&&, Ref>;}}.<br>{{c|/*reference*/}} is either a reference type, or a cv-unqualified object type that models {{lconcept|copy_constructible}}. The name is for exposition only.}}
+{{dsc|{{tt|yielded}}|{{box/core|{{c/core|std::conditional_t<std::is_reference_v<}}{{tti|reference}}{{sep}}{{c/core|>,}}{{nbspt|1}}{{tti|reference}}{{c/core|, const}}{{nbspt|1}}{{tti|reference}}{{sep}}{{c/core|&>}}}}}}
 {{dsc end}}
-Let {{tt|RRef}} denote {{c|std::remove_reference_t</*reference*/>&&}} if {{c|/*reference*/}} is a reference type, and {{c|/*reference*/}} otherwise.
+{{par begin}}
-The following concepts are modeled:
+{{par hreq}}
-* {{c|std::common_reference_with</*reference*/&&, /*value*/&>}},
+{{par req|{{c/core|std::allocator_traits<Allocator>::pointer}} is a pointer type.}}
-* {{c|std::common_reference_with</*reference*/&&, RRef&&>}}, and
+{{par req|{{tti|value}} is a cv-unqualified object type.}}
-* {{c|std::common_reference_with<RRef&&, const /*value*/&>}}.
+{{par req|{{tti|reference}} is either a reference type, or a cv-unqualified object type that models {{lconcept|copy_constructible}}.}}
+{{par req|Let {{tti|RRef}} denote {{box/core|{{c/core|std::remove_reference_t<}}{{tti|reference}}{{sep}}{{c/core|>&&}}}}, if {{tti|reference}} is a reference type, and {{tti|reference}} otherwise.
+* {{box/core|{{c/core|std::common_reference_with<}}{{tti|reference}}{{sep}}{{c/core|&&,}}{{nbspt|1}}{{tti|value}}{{sep}}{{c/core|&>}}}} is modeled.
+* {{box/core|{{c/core|std::common_reference_with<}}{{tti|reference}}{{sep}}{{c/core|&&,}}{{nbspt|1}}{{tti|RRef}}{{sep}}{{c/core|&&>}}}} is modeled.
+* {{box/core|{{c/core|std::common_reference_with<}}{{tti|RRef}}{{sep}}{{c/core|&&, const}}{{nbspt|1}}{{tti|value}}{{sep}}{{c/core|&>}}}} is modeled.
+}}
+{{par end}}
+The program is ill-formed if any of these type requirements is not satisfied.
+===Data members===
+{{dsc begin}}
+{{dsc hitem|Member|Definition}}
+{{dsc expos mem obj|active_|private=yes|
+Internally, each active instance of {{tt|std::generator}} is associated with a stack (handled as if by object of type {{c/core|std::unique_ptr<std::stack<std::coroutine_handle<>>>}}).
+* When {{rlt|begin}} is called, a new stack is created and the generator is added to the stack.
+* When {{c|co_yield ranges::elements_of(rng)}} is evaluated in a generator body, {{c|rng}} is converted to a generator and added to the stack that contains the enclosing generator.
+* When a generator iterator is {{rl|iterator#increment|incremented}}, the coroutine at the top of the associated stack is resumed.
+* When a generator finishes (i.e. when {{l2tt|cpp/coroutine/generator/promise_type/final_suspend}} is called), it is removed from the stack.}}
+{{dsc expos mem obj|coroutine_|private=yes|a handle of type {{c/core|std::coroutine_handle<promise_type>}}}}
+{{dsc end}}
 ===Member functions===
@@ Line 76: / Line 87: @@
 ===Nested classes===
 {{dsc begin}}
-{{dsc mem class|cpp/coroutine/generator/promise|the promise type|notes={{mark c++23}}}}
+{{dsc mem class|cpp/coroutine/generator/promise_type|the promise type}}
-{{dsc expos mem class|cpp/coroutine/generator/iterator|the iterator type|notes={{mark c++23}}}}
+{{dsc expos mem class|cpp/coroutine/generator/iterator|the iterator type}}
 {{dsc end}}
@@ Line 87: / Line 98: @@
 |code=
 #include <generator>
-#include <ranges>
 #include <iostream>
-std::generator<char> letters(char first)
+template<typename T>
+struct Tree
 {
-     for (;; co_yield first++);
+     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()
 {
-     for (const char ch : letters('a') {{!}} std::views::take(032))
+     Tree<char> tree[]
-         std::cout << ch << ' ';
+    {
+                                    {'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 h i j k l m n o p q r s t u v w x y z
+A B C D E F G
 }}
+===References===
+{{ref std c++23}}
+{{ref std|title=Range generators|id=coro.generator|section=26.8}}
+{{ref std end}}
 ===See also===

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Coroutine traits
coroutine_traits (C++20)
Coroutine handle
coroutine_handle (C++20)
No-op coroutines
noop_coroutine_promise (C++20)
noop_coroutine (C++20)
Trivial awaitables
suspend_never (C++20)
suspend_always (C++20)
Range generators
generator (C++23)

Member functions
generator::generator
generator::~generator
generator::operator=
generator::begin
generator::end
promise_type
generator::promise_type::get_return_object
generator::promise_type::initial_suspend
generator::promise_type::final_suspend
generator::promise_type::yield_value
generator::promise_type::return_void
generator::promise_type::unhandled_exception
generator::promise_type::operator new
generator::promise_type::operator delete
Iterator
generator::iterator::iterator
generator::iterator::operator=
generator::iterator::operator*
generator::iterator::operator++ generator::iterator::operator++(int)
operator==(generator::iterator, std::default_sentinel_t)

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

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Difference between revisions of "cpp/coroutine/generator"

Latest revision as of 21:38, 12 November 2024

Contents

[edit] Template parameters

[edit] Member types