-
Notifications
You must be signed in to change notification settings - Fork 13
Expand file tree
/
Copy pathfutures-and-promises.html
More file actions
230 lines (204 loc) · 51.6 KB
/
futures-and-promises.html
File metadata and controls
230 lines (204 loc) · 51.6 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
<!DOCTYPE html>
<html lang="en-US">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>C++ | Futures and Promises</title>
<meta name="generator" content="VuePress 1.8.2">
<link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png">
<link rel="icon" type="image/png" sizes="16x16" href="/favicon-16x16.png">
<link rel="manifest" href="/site.webmanifest">
<link rel="apple-touch-icon" sizes="180x180" href="/apple-touch-icon.png">
<link rel="mask-icon" href="/safari-pinned-tab.svg" color="#5bbad5">
<meta name="description" content="std::future and std::promise, Deferred async example, std::packaged_task and std::future, std::future_error and std::future_errc, std::future and std::async, Async operation classes">
<meta property="og:site_name" content="DevTut">
<meta property="og:title" content="C++ | Futures and Promises">
<meta property="og:description" content="std::future and std::promise, Deferred async example, std::packaged_task and std::future, std::future_error and std::future_errc, std::future and std::async, Async operation classes">
<meta property="og:type" content="article">
<meta property="og:url" content="/cpp/futures-and-promises.html">
<meta property="og:image" content="/logo.png">
<meta name="twitter:title" content="C++ | Futures and Promises">
<meta name="twitter:description" content="std::future and std::promise, Deferred async example, std::packaged_task and std::future, std::future_error and std::future_errc, std::future and std::async, Async operation classes">
<meta name="twitter:url" content="/cpp/futures-and-promises.html">
<meta name="twitter:card" content="summary_large_image">
<meta name="twitter:image" content="/logo.png">
<meta name="theme-color" content="#ffffff">
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black">
<meta name="msapplication-TileImage" content="/mstile-150x150.png">
<meta name="msapplication-TileColor" content="#da532c">
<meta name="google-site-verification" content="76_rKXgwMVIjd-axJC_1zPV9OS4mEjvtgjYOWVkAdnQ">
<link rel="preload" href="/assets/css/0.styles.60619e34.css" as="style"><link rel="preload" href="/assets/js/app.1779e102.js" as="script"><link rel="preload" href="/assets/js/3.2cfa8016.js" as="script"><link rel="preload" href="/assets/js/659.adc1864e.js" as="script">
<link rel="stylesheet" href="/assets/css/0.styles.60619e34.css">
</head>
<body>
<div id="app" data-server-rendered="true"><div class="theme-container"><header class="navbar"><div class="sidebar-button"><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" role="img" viewBox="0 0 448 512" class="icon"><path fill="currentColor" d="M436 124H12c-6.627 0-12-5.373-12-12V80c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12zm0 160H12c-6.627 0-12-5.373-12-12v-32c0-6.627 5.373-12 12-12h424c6.627 0 12 5.373 12 12v32c0 6.627-5.373 12-12 12z"></path></svg></div> <a href="/" class="home-link router-link-active"><!----> <span class="site-name">DevTut</span></a> <div class="links"><form id="search-form" role="search" class="algolia-search-wrapper search-box"><input id="algolia-search-input" class="search-query"></form> <nav class="nav-links can-hide"> <a href="https://github.com/devtut/generate" target="_blank" rel="noopener noreferrer" class="repo-link">
GitHub
<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav></div></header> <div class="sidebar-mask"></div> <aside class="sidebar"><nav class="nav-links"> <a href="https://github.com/devtut/generate" target="_blank" rel="noopener noreferrer" class="repo-link">
GitHub
<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></nav> <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>C++</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/cpp/" aria-current="page" class="sidebar-link">Disclaimer</a></li><li><a href="/cpp/getting-started-with-cpp.html" class="sidebar-link">Getting started with C++</a></li><li><a href="/cpp/literals.html" class="sidebar-link">Literals</a></li><li><a href="/cpp/operator-precedence.html" class="sidebar-link">operator precedence</a></li><li><a href="/cpp/floating-point-arithmetic.html" class="sidebar-link">Floating Point Arithmetic</a></li><li><a href="/cpp/bit-operators.html" class="sidebar-link">Bit Operators</a></li><li><a href="/cpp/bit-manipulation.html" class="sidebar-link">Bit Manipulation</a></li><li><a href="/cpp/bit-fields.html" class="sidebar-link">Bit fields</a></li><li><a href="/cpp/arrays.html" class="sidebar-link">Arrays</a></li><li><a href="/cpp/iterators.html" class="sidebar-link">Iterators</a></li><li><a href="/cpp/basic-input-output-in-c.html" class="sidebar-link">Basic input/output in c++</a></li><li><a href="/cpp/loops.html" class="sidebar-link">Loops</a></li><li><a href="/cpp/file-i-o.html" class="sidebar-link">File I/O</a></li><li><a href="/cpp/cpp-streams.html" class="sidebar-link">C++ Streams</a></li><li><a href="/cpp/stream-manipulators.html" class="sidebar-link">Stream manipulators</a></li><li><a href="/cpp/flow-control.html" class="sidebar-link">Flow Control</a></li><li><a href="/cpp/metaprogramming.html" class="sidebar-link">Metaprogramming</a></li><li><a href="/cpp/const-keyword.html" class="sidebar-link">const keyword</a></li><li><a href="/cpp/mutable-keyword.html" class="sidebar-link">mutable keyword</a></li><li><a href="/cpp/friend-keyword.html" class="sidebar-link">Friend keyword</a></li><li><a href="/cpp/type-keywords.html" class="sidebar-link">Type Keywords</a></li><li><a href="/cpp/basic-type-keywords.html" class="sidebar-link">Basic Type Keywords</a></li><li><a href="/cpp/variable-declaration-keywords.html" class="sidebar-link">Variable Declaration Keywords</a></li><li><a href="/cpp/keywords.html" class="sidebar-link">Keywords</a></li><li><a href="/cpp/returning-several-values-from-a-function.html" class="sidebar-link">Returning several values from a function</a></li><li><a href="/cpp/polymorphism.html" class="sidebar-link">Polymorphism</a></li><li><a href="/cpp/references.html" class="sidebar-link">References</a></li><li><a href="/cpp/value-and-reference-semantics.html" class="sidebar-link">Value and Reference Semantics</a></li><li><a href="/cpp/c-function-call-by-value-vs-call-by-reference.html" class="sidebar-link">C++ function "call by value" vs. "call by reference"</a></li><li><a href="/cpp/copying-vs-assignment.html" class="sidebar-link">Copying vs Assignment</a></li><li><a href="/cpp/pointers.html" class="sidebar-link">Pointers</a></li><li><a href="/cpp/pointers-to-members.html" class="sidebar-link">Pointers to members</a></li><li><a href="/cpp/the-this-pointer.html" class="sidebar-link">The This Pointer</a></li><li><a href="/cpp/smart-pointers.html" class="sidebar-link">Smart Pointers</a></li><li><a href="/cpp/classes-structures.html" class="sidebar-link">Classes/Structures</a></li><li><a href="/cpp/function-overloading.html" class="sidebar-link">Function Overloading</a></li><li><a href="/cpp/operator-overloading.html" class="sidebar-link">Operator Overloading</a></li><li><a href="/cpp/function-template-overloading.html" class="sidebar-link">Function Template Overloading</a></li><li><a href="/cpp/virtual-member-functions.html" class="sidebar-link">Virtual Member Functions</a></li><li><a href="/cpp/inline-functions.html" class="sidebar-link">Inline functions</a></li><li><a href="/cpp/special-member-functions.html" class="sidebar-link">Special Member Functions</a></li><li><a href="/cpp/non-static-member-functions.html" class="sidebar-link">Non-Static Member Functions</a></li><li><a href="/cpp/constant-class-member-functions.html" class="sidebar-link">Constant class member functions</a></li><li><a href="/cpp/c-containers.html" class="sidebar-link">C++ Containers</a></li><li><a href="/cpp/namespaces.html" class="sidebar-link">Namespaces</a></li><li><a href="/cpp/header-files.html" class="sidebar-link">Header Files</a></li><li><a href="/cpp/using-declaration.html" class="sidebar-link">Using declaration</a></li><li><a href="/cpp/std-string.html" class="sidebar-link">std::string</a></li><li><a href="/cpp/std-array.html" class="sidebar-link">std::array</a></li><li><a href="/cpp/std-vector.html" class="sidebar-link">std::vector</a></li><li><a href="/cpp/std-map.html" class="sidebar-link">std::map</a></li><li><a href="/cpp/std-optional.html" class="sidebar-link">std::optional</a></li><li><a href="/cpp/std-function-to-wrap-any-element-that-is-callable.html" class="sidebar-link">std::function: To wrap any element that is callable</a></li><li><a href="/cpp/std-forward-list.html" class="sidebar-link">std::forward_list</a></li><li><a href="/cpp/std-pair.html" class="sidebar-link">std::pair</a></li><li><a href="/cpp/std-atomics.html" class="sidebar-link">std::atomics</a></li><li><a href="/cpp/std-variant.html" class="sidebar-link">std::variant</a></li><li><a href="/cpp/std-iomanip.html" class="sidebar-link">std::iomanip</a></li><li><a href="/cpp/std-any.html" class="sidebar-link">std::any</a></li><li><a href="/cpp/std-set-and-std-multiset.html" class="sidebar-link">std::set and std::multiset</a></li><li><a href="/cpp/std-integer-sequence.html" class="sidebar-link">std::integer_sequence</a></li><li><a href="/cpp/using-std-unordered-map.html" class="sidebar-link">Using std::unordered_map</a></li><li><a href="/cpp/standard-library-algorithms.html" class="sidebar-link">Standard Library Algorithms</a></li><li><a href="/cpp/the-iso-c-standard.html" class="sidebar-link">The ISO C++ Standard</a></li><li><a href="/cpp/inline-variables.html" class="sidebar-link">Inline variables</a></li><li><a href="/cpp/random-number-generation.html" class="sidebar-link">Random number generation</a></li><li><a href="/cpp/date-and-time-using-chrono-header.html" class="sidebar-link">Date and time using header</a></li><li><a href="/cpp/sorting.html" class="sidebar-link">Sorting</a></li><li><a href="/cpp/enumeration.html" class="sidebar-link">Enumeration</a></li><li><a href="/cpp/iteration.html" class="sidebar-link">Iteration</a></li><li><a href="/cpp/regular-expressions.html" class="sidebar-link">Regular expressions</a></li><li><a href="/cpp/implementation-defined-behavior.html" class="sidebar-link">Implementation-defined behavior</a></li><li><a href="/cpp/exceptions.html" class="sidebar-link">Exceptions</a></li><li><a href="/cpp/lambdas.html" class="sidebar-link">Lambdas</a></li><li><a href="/cpp/value-categories.html" class="sidebar-link">Value Categories</a></li><li><a href="/cpp/preprocessor.html" class="sidebar-link">Preprocessor</a></li><li><a href="/cpp/data-structures-in-c.html" class="sidebar-link">Data Structures in C++</a></li><li><a href="/cpp/templates.html" class="sidebar-link">Templates</a></li><li><a href="/cpp/expression-templates.html" class="sidebar-link">Expression templates</a></li><li><a href="/cpp/curiously-recurring-template-pattern-crtp.html" class="sidebar-link">Curiously Recurring Template Pattern (CRTP)</a></li><li><a href="/cpp/threading.html" class="sidebar-link">Threading</a></li><li><a href="/cpp/thread-synchronization-structures.html" class="sidebar-link">Thread synchronization structures</a></li><li><a href="/cpp/the-rule-of-three-five-and-zero.html" class="sidebar-link">The Rule of Three, Five, And Zero</a></li><li><a href="/cpp/raii-resource-acquisition-is-initialization.html" class="sidebar-link">RAII: Resource Acquisition Is Initialization</a></li><li><a href="/cpp/rtti-run-time-type-information.html" class="sidebar-link">RTTI: Run-Time Type Information</a></li><li><a href="/cpp/mutexes.html" class="sidebar-link">Mutexes</a></li><li><a href="/cpp/recursive-mutex.html" class="sidebar-link">Recursive Mutex</a></li><li><a href="/cpp/semaphore.html" class="sidebar-link">Semaphore</a></li><li><a href="/cpp/futures-and-promises.html" aria-current="page" class="active sidebar-link">Futures and Promises</a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#std-future-and-std-promise" class="sidebar-link">std::future and std::promise</a></li><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#deferred-async-example" class="sidebar-link">Deferred async example</a></li><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#std-packaged-task-and-std-future" class="sidebar-link">std::packaged_task and std::future</a></li><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#std-future-error-and-std-future-errc" class="sidebar-link">std::futureerror and std::futureerrc</a></li><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#std-future-and-std-async" class="sidebar-link">std::future and std::async</a></li><li class="sidebar-sub-header"><a href="/cpp/futures-and-promises.html#async-operation-classes" class="sidebar-link">Async operation classes</a></li></ul></li><li><a href="/cpp/atomic-types.html" class="sidebar-link">Atomic Types</a></li><li><a href="/cpp/type-erasure.html" class="sidebar-link">Type Erasure</a></li><li><a href="/cpp/explicit-type-conversions.html" class="sidebar-link">Explicit type conversions</a></li><li><a href="/cpp/unnamed-types.html" class="sidebar-link">Unnamed types</a></li><li><a href="/cpp/type-traits.html" class="sidebar-link">Type Traits</a></li><li><a href="/cpp/return-type-covariance.html" class="sidebar-link">Return Type Covariance</a></li><li><a href="/cpp/layout-of-object-types.html" class="sidebar-link">Layout of object types</a></li><li><a href="/cpp/type-inference.html" class="sidebar-link">Type Inference</a></li><li><a href="/cpp/typedef-and-type-aliases.html" class="sidebar-link">Typedef and type aliases</a></li><li><a href="/cpp/type-deduction.html" class="sidebar-link">type deduction</a></li><li><a href="/cpp/trailing-return-type.html" class="sidebar-link">Trailing return type</a></li><li><a href="/cpp/alignment.html" class="sidebar-link">Alignment</a></li><li><a href="/cpp/perfect-forwarding.html" class="sidebar-link">Perfect Forwarding</a></li><li><a href="/cpp/decltype.html" class="sidebar-link">decltype</a></li><li><a href="/cpp/sfinae-substitution-failure-is-not-an-error.html" class="sidebar-link">SFINAE (Substitution Failure Is Not An Error)</a></li><li><a href="/cpp/undefined-behavior.html" class="sidebar-link">Undefined Behavior</a></li><li><a href="/cpp/overload-resolution.html" class="sidebar-link">Overload resolution</a></li><li><a href="/cpp/move-semantics.html" class="sidebar-link">Move Semantics</a></li><li><a href="/cpp/pimpl-idiom.html" class="sidebar-link">Pimpl Idiom</a></li><li><a href="/cpp/auto.html" class="sidebar-link">auto</a></li><li><a href="/cpp/copy-elision.html" class="sidebar-link">Copy Elision</a></li><li><a href="/cpp/fold-expressions.html" class="sidebar-link">Fold Expressions</a></li><li><a href="/cpp/unions.html" class="sidebar-link">Unions</a></li><li><a href="/cpp/design-pattern-implementation-in-c.html" class="sidebar-link">Design pattern implementation in C++</a></li><li><a href="/cpp/singleton-design-pattern.html" class="sidebar-link">Singleton Design Pattern</a></li><li><a href="/cpp/user-defined-literals.html" class="sidebar-link">User-Defined Literals</a></li><li><a href="/cpp/memory-management.html" class="sidebar-link">Memory management</a></li><li><a href="/cpp/c-11-memory-model.html" class="sidebar-link">C++11 Memory Model</a></li><li><a href="/cpp/scopes.html" class="sidebar-link">Scopes</a></li><li><a href="/cpp/static-assert.html" class="sidebar-link">static_assert</a></li><li><a href="/cpp/constexpr.html" class="sidebar-link">constexpr</a></li><li><a href="/cpp/one-definition-rule-odr.html" class="sidebar-link">One Definition Rule (ODR)</a></li><li><a href="/cpp/unspecified-behavior.html" class="sidebar-link">Unspecified behavior</a></li><li><a href="/cpp/argument-dependent-name-lookup.html" class="sidebar-link">Argument Dependent Name Lookup</a></li><li><a href="/cpp/attributes.html" class="sidebar-link">Attributes</a></li><li><a href="/cpp/recursion-in-c.html" class="sidebar-link">Recursion in C++</a></li><li><a href="/cpp/arithmitic-metaprogramming.html" class="sidebar-link">Arithmitic Metaprogramming</a></li><li><a href="/cpp/callable-objects.html" class="sidebar-link">Callable Objects</a></li><li><a href="/cpp/client-server-examples.html" class="sidebar-link">Client server examples</a></li><li><a href="/cpp/const-correctness.html" class="sidebar-link">Const Correctness</a></li><li><a href="/cpp/parameter-packs.html" class="sidebar-link">Parameter packs</a></li><li><a href="/cpp/build-systems.html" class="sidebar-link">Build Systems</a></li><li><a href="/cpp/concurrency-with-openmp.html" class="sidebar-link">Concurrency With OpenMP</a></li><li><a href="/cpp/resource-management.html" class="sidebar-link">Resource Management</a></li><li><a href="/cpp/storage-class-specifiers.html" class="sidebar-link">Storage class specifiers</a></li><li><a href="/cpp/linkage-specifications.html" class="sidebar-link">Linkage specifications</a></li><li><a href="/cpp/digit-separators.html" class="sidebar-link">Digit separators</a></li><li><a href="/cpp/c-incompatibilities.html" class="sidebar-link">C incompatibilities</a></li><li><a href="/cpp/side-by-side-comparisons-of-classic-c-examples-solved-via-c-vs-c-11-vs-c-14-vs-c-17.html" class="sidebar-link">Side by Side Comparisons of classic C++ examples solved via C++ vs C++11 vs C++14 vs C++17</a></li><li><a href="/cpp/compiling-and-building.html" class="sidebar-link">Compiling and Building</a></li><li><a href="/cpp/common-compile-linker-errors-gcc.html" class="sidebar-link">Common compile/linker errors (GCC)</a></li><li><a href="/cpp/more-undefined-behaviors-in-c.html" class="sidebar-link">More undefined behaviors in C++</a></li><li><a href="/cpp/unit-testing-in-c.html" class="sidebar-link">Unit Testing in C++</a></li><li><a href="/cpp/c-debugging-and-debug-prevention-tools-techniques.html" class="sidebar-link">C++ Debugging and Debug-prevention Tools & Techniques</a></li><li><a href="/cpp/optimization-in-c.html" class="sidebar-link">Optimization in C++</a></li><li><a href="/cpp/optimization.html" class="sidebar-link">Optimization</a></li><li><a href="/cpp/profiling.html" class="sidebar-link">Profiling</a></li><li><a href="/cpp/refactoring-techniques.html" class="sidebar-link">Refactoring Techniques</a></li><li><a href="/cpp/internationalization-in-c.html" class="sidebar-link">Internationalization in C++</a></li><li><a href="/cpp/contributors.html" class="sidebar-link">The Contributors</a></li></ul></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="futures-and-promises"><a href="#futures-and-promises" class="header-anchor">#</a> Futures and Promises</h1> <p>Promises and Futures are used to ferry a single object from one thread to another.</p> <p>A <code>std::promise</code> object is set by the thread which generates the result.</p> <p>A <code>std::future</code> object can be used to retrieve a value, to test to see if a value is available, or to halt execution until the value is available.</p> <h2 id="std-future-and-std-promise"><a href="#std-future-and-std-promise" class="header-anchor">#</a> std::future and std::promise</h2> <p>The following example sets a promise to be consumed by another thread:</p> <div class="language- extra-class"><pre class="language-text"><code>
{
auto promise = std::promise<std::string>();
auto producer = std::thread([&]
{
promise.set_value("Hello World");
});
auto future = promise.get_future();
auto consumer = std::thread([&]
{
std::cout << future.get();
});
producer.join();
consumer.join();
}
</code></pre></div><h2 id="deferred-async-example"><a href="#deferred-async-example" class="header-anchor">#</a> Deferred async example</h2> <p>This code implements a version of <code>std::async</code>, but it behaves as if <code>async</code> were always called with the <code>deferred</code> launch policy. This function also does not have <code>async</code>'s special <code>future</code> behavior; the returned <code>future</code> can be destroyed without ever acquiring its value.</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">template</span><span class="token operator"><</span><span class="token keyword">typename</span> <span class="token class-name">F</span><span class="token operator">></span>
<span class="token keyword">auto</span> <span class="token function">async_deferred</span><span class="token punctuation">(</span>F<span class="token operator">&&</span> func<span class="token punctuation">)</span> <span class="token operator">-></span> std<span class="token double-colon punctuation">::</span>future<span class="token operator"><</span><span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token operator">></span>
<span class="token punctuation">{</span>
<span class="token keyword">using</span> result_type <span class="token operator">=</span> <span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> promise <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">promise</span><span class="token generic class-name"><span class="token operator"><</span>result_type<span class="token operator">></span></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> future <span class="token operator">=</span> promise<span class="token punctuation">.</span><span class="token function">get_future</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">thread</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token operator">=</span><span class="token punctuation">]</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>promise<span class="token operator"><</span>result_type<span class="token operator">></span><span class="token operator">&</span> promise<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">try</span>
<span class="token punctuation">{</span>
promise<span class="token punctuation">.</span><span class="token function">set_value</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// Note: Will not work with std::promise<void>. Needs some meta-template programming which is out of scope for this example.</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
promise<span class="token punctuation">.</span><span class="token function">set_exception</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">current_exception</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>promise<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">detach</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> future<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="std-packaged-task-and-std-future"><a href="#std-packaged-task-and-std-future" class="header-anchor">#</a> std::packaged_task and std::future</h2> <p><code>std::packaged_task</code> bundles a function and the associated promise for its return type:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">template</span><span class="token operator"><</span><span class="token keyword">typename</span> <span class="token class-name">F</span><span class="token operator">></span>
<span class="token keyword">auto</span> <span class="token function">async_deferred</span><span class="token punctuation">(</span>F<span class="token operator">&&</span> func<span class="token punctuation">)</span> <span class="token operator">-></span> std<span class="token double-colon punctuation">::</span>future<span class="token operator"><</span><span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token operator">></span>
<span class="token punctuation">{</span>
<span class="token keyword">auto</span> task <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">packaged_task</span><span class="token generic class-name"><span class="token operator"><</span><span class="token keyword">decltype</span><span class="token punctuation">(</span><span class="token function">func</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">></span></span></span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">forward</span><span class="token generic class-name"><span class="token operator"><</span>F<span class="token operator">></span></span></span><span class="token punctuation">(</span>func<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> future <span class="token operator">=</span> task<span class="token punctuation">.</span><span class="token function">get_future</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
std<span class="token double-colon punctuation">::</span><span class="token function">thread</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>task<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">detach</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>future<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>The thread starts running immediately. We can either detach it, or have join it at the end of the scope. When the function call to std::thread finishes, the result is ready.</p> <p>Note that this is slightly different from <code>std::async</code> where the returned <code>std::future</code> when destructed will actually <strong>block</strong> until the thread is finished.</p> <h2 id="std-future-error-and-std-future-errc"><a href="#std-future-error-and-std-future-errc" class="header-anchor">#</a> std::future_error and std::future_errc</h2> <p>If constraints for std::promise and std::future are not met an exception of type
std::future_error is thrown.</p> <p>The error code member in the exception is of type std::future_errc and values are as below, along with some test cases:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">enum</span> <span class="token keyword">class</span> <span class="token class-name">future_errc</span> <span class="token punctuation">{</span>
broken_promise <span class="token operator">=</span> <span class="token comment">/* the task is no longer shared */</span><span class="token punctuation">,</span>
future_already_retrieved <span class="token operator">=</span> <span class="token comment">/* the answer was already retrieved */</span><span class="token punctuation">,</span>
promise_already_satisfied <span class="token operator">=</span> <span class="token comment">/* the answer was stored already */</span><span class="token punctuation">,</span>
no_state <span class="token operator">=</span> <span class="token comment">/* access to a promise in non-shared state */</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
</code></pre></div><p><strong>Inactive promise:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">int</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>promise<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> pr<span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">// returns ok</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>Active promise, unused:</strong></p> <div class="language- extra-class"><pre class="language-text"><code>
int test()
{
std::promise<int> pr;
auto fut = pr.get_future(); //blocks indefinitely!
return 0;
}
</code></pre></div><p><strong>Double retrieval:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">int</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>promise<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> pr<span class="token punctuation">;</span>
<span class="token keyword">auto</span> fut1 <span class="token operator">=</span> pr<span class="token punctuation">.</span><span class="token function">get_future</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">try</span><span class="token punctuation">{</span>
<span class="token keyword">auto</span> fut2 <span class="token operator">=</span> pr<span class="token punctuation">.</span><span class="token function">get_future</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// second attempt to get future</span>
<span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token keyword">const</span> std<span class="token double-colon punctuation">::</span>future_error<span class="token operator">&</span> e<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
cout <span class="token operator"><<</span> e<span class="token punctuation">.</span><span class="token function">what</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><<</span> endl<span class="token punctuation">;</span> <span class="token comment">// Error: "The future has already been retrieved from the promise or packaged_task."</span>
<span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> fut2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><strong>Setting std::promise value twice:</strong></p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token keyword">int</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>promise<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> pr<span class="token punctuation">;</span>
<span class="token keyword">auto</span> fut <span class="token operator">=</span> pr<span class="token punctuation">.</span><span class="token function">get_future</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">try</span><span class="token punctuation">{</span>
std<span class="token double-colon punctuation">::</span>promise<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> <span class="token function">pr2</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span><span class="token function">move</span><span class="token punctuation">(</span>pr<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
pr2<span class="token punctuation">.</span><span class="token function">set_value</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
pr2<span class="token punctuation">.</span><span class="token function">set_value</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// second attempt to set promise throws exception</span>
<span class="token punctuation">}</span>
<span class="token keyword">catch</span><span class="token punctuation">(</span><span class="token keyword">const</span> std<span class="token double-colon punctuation">::</span>future_error<span class="token operator">&</span> e<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
cout <span class="token operator"><<</span> e<span class="token punctuation">.</span><span class="token function">what</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator"><<</span> endl<span class="token punctuation">;</span> <span class="token comment">// Error: "The state of the promise has already been set."</span>
<span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> fut<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="std-future-and-std-async"><a href="#std-future-and-std-async" class="header-anchor">#</a> std::future and std::async</h2> <p>In the following naive parallel merge sort example, <code>std::async</code> is used to launch multiple parallel merge_sort tasks. <code>std::future</code> is used to wait for the results and synchronize them:</p> <div class="language-cpp extra-class"><pre class="language-cpp"><code><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string"><iostream></span></span>
<span class="token keyword">using</span> <span class="token keyword">namespace</span> std<span class="token punctuation">;</span>
<span class="token keyword">void</span> <span class="token function">merge</span><span class="token punctuation">(</span><span class="token keyword">int</span> low<span class="token punctuation">,</span><span class="token keyword">int</span> mid<span class="token punctuation">,</span><span class="token keyword">int</span> high<span class="token punctuation">,</span> vector<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span><span class="token operator">&</span>num<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
vector<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span> <span class="token function">copy</span><span class="token punctuation">(</span>num<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">int</span> h<span class="token punctuation">,</span>i<span class="token punctuation">,</span>j<span class="token punctuation">,</span>k<span class="token punctuation">;</span>
h<span class="token operator">=</span>low<span class="token punctuation">;</span>
i<span class="token operator">=</span>low<span class="token punctuation">;</span>
j<span class="token operator">=</span>mid<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">while</span><span class="token punctuation">(</span><span class="token punctuation">(</span>h<span class="token operator"><=</span>mid<span class="token punctuation">)</span><span class="token operator">&&</span><span class="token punctuation">(</span>j<span class="token operator"><=</span>high<span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>num<span class="token punctuation">[</span>h<span class="token punctuation">]</span><span class="token operator"><=</span>num<span class="token punctuation">[</span>j<span class="token punctuation">]</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
copy<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">=</span>num<span class="token punctuation">[</span>h<span class="token punctuation">]</span><span class="token punctuation">;</span>
h<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span>
<span class="token punctuation">{</span>
copy<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">=</span>num<span class="token punctuation">[</span>j<span class="token punctuation">]</span><span class="token punctuation">;</span>
j<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
i<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>h<span class="token operator">></span>mid<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>k<span class="token operator">=</span>j<span class="token punctuation">;</span>k<span class="token operator"><=</span>high<span class="token punctuation">;</span>k<span class="token operator">++</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
copy<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">=</span>num<span class="token punctuation">[</span>k<span class="token punctuation">]</span><span class="token punctuation">;</span>
i<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">else</span>
<span class="token punctuation">{</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>k<span class="token operator">=</span>h<span class="token punctuation">;</span>k<span class="token operator"><=</span>mid<span class="token punctuation">;</span>k<span class="token operator">++</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
copy<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token operator">=</span>num<span class="token punctuation">[</span>k<span class="token punctuation">]</span><span class="token punctuation">;</span>
i<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>k<span class="token operator">=</span>low<span class="token punctuation">;</span>k<span class="token operator"><=</span>high<span class="token punctuation">;</span>k<span class="token operator">++</span><span class="token punctuation">)</span>
<span class="token function">swap</span><span class="token punctuation">(</span>num<span class="token punctuation">[</span>k<span class="token punctuation">]</span><span class="token punctuation">,</span>copy<span class="token punctuation">[</span>k<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">merge_sort</span><span class="token punctuation">(</span><span class="token keyword">int</span> low<span class="token punctuation">,</span><span class="token keyword">int</span> high<span class="token punctuation">,</span>vector<span class="token operator"><</span><span class="token keyword">int</span><span class="token operator">></span><span class="token operator">&</span> num<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token keyword">int</span> mid<span class="token punctuation">;</span>
<span class="token keyword">if</span><span class="token punctuation">(</span>low<span class="token operator"><</span>high<span class="token punctuation">)</span>
<span class="token punctuation">{</span>
mid <span class="token operator">=</span> low <span class="token operator">+</span> <span class="token punctuation">(</span>high<span class="token operator">-</span>low<span class="token punctuation">)</span><span class="token operator">/</span><span class="token number">2</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> future1 <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span><span class="token function">async</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>launch<span class="token double-colon punctuation">::</span>deferred<span class="token punctuation">,</span><span class="token punctuation">[</span><span class="token operator">&</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token function">merge_sort</span><span class="token punctuation">(</span>low<span class="token punctuation">,</span>mid<span class="token punctuation">,</span>num<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">auto</span> future2 <span class="token operator">=</span> std<span class="token double-colon punctuation">::</span><span class="token function">async</span><span class="token punctuation">(</span>std<span class="token double-colon punctuation">::</span>launch<span class="token double-colon punctuation">::</span>deferred<span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token operator">&</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
<span class="token function">merge_sort</span><span class="token punctuation">(</span>mid<span class="token operator">+</span><span class="token number">1</span><span class="token punctuation">,</span>high<span class="token punctuation">,</span>num<span class="token punctuation">)</span> <span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
future1<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
future2<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">merge</span><span class="token punctuation">(</span>low<span class="token punctuation">,</span>mid<span class="token punctuation">,</span>high<span class="token punctuation">,</span>num<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>Note: In the example <code>std::async</code> is launched with policy <code>std::launch_deferred</code>. This is to avoid a new thread being created in every call. In the case of our example, the calls to <code>std::async</code> are made out of order, the they synchronize at the calls for <code>std::future::get()</code>.</p> <p><code>std::launch_async</code> forces a new thread to be created in every call.</p> <p>The default policy is <code>std::launch::deferred| std::launch::async</code>, meaning the implementation determines the policy for creating new threads.</p> <h2 id="async-operation-classes"><a href="#async-operation-classes" class="header-anchor">#</a> Async operation classes</h2> <ul><li>std::async: performs an asynchronous operation.</li> <li>std::future: provides access to the result of an asynchronous operation.</li> <li>std::promise: packages the result of an asynchronous operation.</li> <li>std::packaged_task: bundles a function and the associated promise for its return type.</li></ul></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/cpp/futures-and-promises.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
←
<a href="/cpp/semaphore.html" class="prev">
Semaphore
</a></span> <span class="next"><a href="/cpp/atomic-types.html">
Atomic Types
</a>
→
</span></p></div> </main></div><div class="global-ui"><!----></div></div>
<script src="/assets/js/app.1779e102.js" defer></script><script src="/assets/js/3.2cfa8016.js" defer></script><script src="/assets/js/659.adc1864e.js" defer></script>
</body>
</html>