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Nicolai M. Josuttis
C++20 - The Complete Guide
First Edition
To the nation of Ukraine
and all the children, women, men, and soldiers
killed in a terrible war driven by a Russian dictator
C++20 - The Complete Guide
First Edition
Nicolai M. Josuttis
This version was published on 2022-10-30.
© 2022 by Nicolai Josuttis. All rights reserved.
This publication is protected by copyright, and permission must be obtained from the author prior
to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any
means, electronic, mechanical, photocopying, recording, or likewise.
This book was typeset by Nicolai M. Josuttis using the LATEX document processing system.
This book is for sale at http://leanpub.com/cpp20.
This is a Leanpub book. Leanpub empowers authors and publishers with the Lean Publishing
process. Lean Publishing is the act of publishing an in-progress ebook using lightweight tools and
many iterations to get reader feedback, pivot until you have the right book, and build traction once
you do.
Contents
Preface
xix
An Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xix
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xix
Versions of This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xx
About This Book
1
xxiii
What You Should Know Before Reading This Book . . . . . . . . . . . . . . . . . . . . . . . . . .
xxiii
Overall Structure of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxiii
How to Read This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxiv
The Way I Implement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxiv
The C++ Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxv
Example Code and Additional Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxv
Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xxvi
Comparisons and Operator <=>
1
1.1
Motivation for Operator<=> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1.1
Defining Comparison Operators Before C++20 . . . . . . . . . . . . . . . . . . . . .
1
1.1.2
Defining Comparison Operators Since C++20 . . . . . . . . . . . . . . . . . . . . . .
4
Defining and Using Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.2.1
Using Operator<=> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.2.2
Comparison Category Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
1.2.3
Using Comparison Categories with operator<=> . . . . . . . . . . . . . . . . . . . .
9
1.2.4
Calling Operator <=> Directly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
1.2.5
Dealing with Multiple Ordering Criteria . . . . . . . . . . . . . . . . . . . . . . . . . .
11
Defining operator<=> and operator== . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
1.3.1
14
1.2
1.3
Defaulted operator== and operator<=> . . . . . . . . . . . . . . . . . . . . . . . .
iii
iv
Contents
1.3.2
Defaulted operator<=> Implies Defaulted operator== . . . . . . . . . . . . . . .
14
1.3.3
Implementation of the Defaulted operator<=> . . . . . . . . . . . . . . . . . . . . .
16
1.4
Overload Resolution with Rewritten Expressions . . . . . . . . . . . . . . . . . . . . . . . . . .
17
1.5
Using Operator <=> in Generic Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.6
1.7
2
compare_three_way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1.5.2
Algorithm lexicographical_compare_three_way() . . . . . . . . . . . . . . .
19
Compatibility Issues with the Comparison Operators . . . . . . . . . . . . . . . . . . . . . . .
21
1.6.1
Delegating Free-Standing Comparison Operators . . . . . . . . . . . . . . . . . . . .
21
1.6.2
Inheritance with Protected Members . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
Placeholder Types for Function Parameters
25
2.1
auto for Parameters of Ordinary Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
2.1.1
26
2.2
2.3
2.4
3
1.5.1
auto for Parameters of Member Functions . . . . . . . . . . . . . . . . . . . . . . . .
Using auto for Parameters in Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.2.1
Deferred Type Checks with auto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.2.2
auto Functions versus Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
auto for Parameters in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3.1
Basic Constraints for auto Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
2.3.2
Combining Template and auto Parameters . . . . . . . . . . . . . . . . . . . . . . . .
31
Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
Concepts, Requirements, and Constraints
3.1
3.2
3.3
33
Motivating Example of Concepts and Requirements . . . . . . . . . . . . . . . . . . . . . . . .
33
3.1.1
Improving the Template Step by Step . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
3.1.2
A Complete Example with Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
Where Constraints and Concepts Can Be Used . . . . . . . . . . . . . . . . . . . . . . . . . . .
40
3.2.1
Constraining Alias Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.2.2
Constraining Variable Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
41
3.2.3
Constraining Member Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
3.2.4
Constraining Non-Type Template Parameters . . . . . . . . . . . . . . . . . . . . . . .
43
Typical Applications of Concepts and Constraints in Practice . . . . . . . . . . . . . . . . . .
44
3.3.1
Using Concepts to Understand Code and Error Messages . . . . . . . . . . . . . . .
44
3.3.2
Using Concepts to Disable Generic Code . . . . . . . . . . . . . . . . . . . . . . . . .
47
3.3.3
Using Requirements to Call Different Functions . . . . . . . . . . . . . . . . . . . . .
51
Contents
3.4
3.5
3.6
4
3.3.4
The Example as a Whole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
56
3.3.5
Former Workarounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
Semantic Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.4.1
Examples of Semantic Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
Design Guidelines for Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
3.5.1
Concepts Should Group Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
3.5.2
Define Concepts with Care . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
3.5.3
Concepts versus Type Traits and Boolean Expressions . . . . . . . . . . . . . . . . .
62
Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
Concepts, Requirements, and Constraints in Detail
65
4.1
Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
4.2
requires Clauses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
4.2.1
Using && and || in requires Clauses . . . . . . . . . . . . . . . . . . . . . . . . . . .
66
4.3
Ad-hoc Boolean Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
67
4.4
requires Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
68
4.4.1
Simple Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
69
4.4.2
Type Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
4.4.3
Compound Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
73
4.4.4
Nested Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
Concepts in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.5.1
Defining Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.5.2
Special Abilities of Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
75
4.5.3
Concepts for Non-Type Template Parameters . . . . . . . . . . . . . . . . . . . . . . .
76
4.6
Using Concepts as Type Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
4.7
Subsuming Constraints with Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
4.7.1
Indirect Subsumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
82
4.7.2
Defining Commutative Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
83
4.5
5
v
Standard Concepts in Detail
87
5.1
Overview of All Standard Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
5.1.1
Header Files and Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
87
5.1.2
Standard Concepts Subsume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
90
Language-Related Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
5.2.1
91
5.2
Arithmetic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vi
Contents
5.3
5.4
5.5
6
5.2.2
Object Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
5.2.3
Concepts for Relationships between Types . . . . . . . . . . . . . . . . . . . . . . . .
93
5.2.4
Comparison Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
94
Concepts for Iterators and Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
5.3.1
Concepts for Ranges and Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
96
5.3.2
Concepts for Pointer-Like Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100
5.3.3
Concepts for Iterators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
102
5.3.4
Iterator Concepts for Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
105
Concepts for Callables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
5.4.1
Basic Concepts for Callables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
106
5.4.2
Concepts for Callables Used by Iterators . . . . . . . . . . . . . . . . . . . . . . . . .
109
Auxiliary Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
111
5.5.1
Concepts for Specific Type Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . .
111
5.5.2
Concepts for Incrementable Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
112
Ranges and Views
115
6.1
A Tour of Ranges and Views Using Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .
116
6.1.1
Passing Containers to Algorithms as Ranges . . . . . . . . . . . . . . . . . . . . . . .
116
6.1.2
Constraints and Utilities for Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . .
118
6.1.3
Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
6.1.4
Sentinels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
126
6.1.5
Range Definitions with Sentinels and Counts . . . . . . . . . . . . . . . . . . . . . . .
129
6.1.6
Projections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
133
6.1.7
Utilities for Implementing Code for Ranges . . . . . . . . . . . . . . . . . . . . . . . .
133
6.1.8
Limitations and Drawbacks of Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . .
135
6.2
6.3
6.4
Borrowed Iterators and Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
136
6.2.1
Borrowed Iterators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
136
6.2.2
Borrowed Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
138
Using Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
140
6.3.1
Views on Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
142
6.3.2
Lazy Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
143
6.3.3
Caching in Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
146
6.3.4
Performance Issues with Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149
Views on Ranges That Are Destroyed or Modified . . . . . . . . . . . . . . . . . . . . . . . . .
151
6.4.1
151
Lifetime Dependencies Between Views and Their Ranges . . . . . . . . . . . . . . .
Contents
6.4.2
Views with Write Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.4.3
Views on Ranges That Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
6.4.4
Copying Views Might Change Behavior . . . . . . . . . . . . . . . . . . . . . . . . . .
154
Views and const . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
155
6.5.1
Generic Code for Both Containers and Views . . . . . . . . . . . . . . . . . . . . . . .
156
6.5.2
Views May Remove the Propagation of const . . . . . . . . . . . . . . . . . . . . . .
163
6.5.3
Bringing Back Deep Constness to Views . . . . . . . . . . . . . . . . . . . . . . . . .
165
6.6
Summary of All Container Idioms Broken By Views . . . . . . . . . . . . . . . . . . . . . . .
168
6.7
Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
168
6.5
7
vii
Utilities for Ranges and Views
7.1
152
171
Key Utilities for Using Ranges as Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
171
7.1.1
std::views::all() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
172
7.1.2
std::views::counted() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
174
7.1.3
std::views::common() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
176
7.2
New Iterator Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
177
7.3
New Iterator and Sentinel Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
178
7.3.1
std::counted_iterator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
179
7.3.2
std::common_iterator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
179
7.3.3
std::default_sentinel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
180
7.3.4
std::unreachable_sentinel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
182
7.3.5
std::move_sentinel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
182
New Functions for Dealing with Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
183
7.4.1
Functions for Dealing with the Elements of Ranges (and Arrays) . . . . . . . . . . .
183
7.4.2
Functions for Dealing with Iterators . . . . . . . . . . . . . . . . . . . . . . . . . . . .
185
7.4.3
Functions for Swapping and Moving Elements/Values . . . . . . . . . . . . . . . . .
186
7.4.4
Functions for Comparisons of Values . . . . . . . . . . . . . . . . . . . . . . . . . . . .
188
New Type Functions/Utilities for Dealing with Ranges . . . . . . . . . . . . . . . . . . . . . .
189
7.5.1
Generic Types of Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
7.5.2
Generic Types of Iterators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
189
7.5.3
New Functional Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
190
7.5.4
Other New Types for Dealing with Iterators . . . . . . . . . . . . . . . . . . . . . . . .
191
Range Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
192
7.6.1
Benefits and Restrictions for Range Algorithms . . . . . . . . . . . . . . . . . . . . .
192
7.6.2
Algorithm Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
193
7.4
7.5
7.6
viii
8
Contents
View Types in Detail
199
8.1
Overview of All Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
199
8.1.1
Overview of Wrapping and Generating Views . . . . . . . . . . . . . . . . . . . . . .
199
8.1.2
Overview of Adapting Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
201
8.2
8.3
8.4
8.5
8.6
8.7
8.8
Base Class and Namespace of Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
202
8.2.1
Base Class for Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
202
8.2.2
Why Range Adaptors/Factories Have Their Own Namespace . . . . . . . . . . . . .
203
Source Views to External Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205
8.3.1
Subrange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
205
8.3.2
Ref View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
211
8.3.3
Owning View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
214
8.3.4
Common View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
218
Generating Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
8.4.1
Iota View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
221
8.4.2
Single View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
226
8.4.3
Empty View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
230
8.4.4
IStream View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
233
8.4.5
String View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
237
8.4.6
Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
239
Filtering Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
241
8.5.1
Take View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
241
8.5.2
Take-While View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
244
8.5.3
Drop View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
247
8.5.4
Drop-While View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
252
8.5.5
Filter View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
257
Transforming Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265
8.6.1
Transform View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
265
8.6.2
Elements View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
270
8.6.3
Keys and Values View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
276
Mutating Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
8.7.1
Reverse View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
Views for Multiple Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
284
8.8.1
Split and Lazy-Split View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
284
8.8.2
Join View . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
290
Contents
9
ix
Spans
295
9.1
295
Using Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1.1
Fixed and Dynamic Extent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
295
9.1.2
Example Using a Span with a Dynamic Extent . . . . . . . . . . . . . . . . . . . . . .
296
9.1.3
Example Using a Span with Non-const Elements . . . . . . . . . . . . . . . . . . . .
301
9.1.4
Example Using a Span with Fixed Extent . . . . . . . . . . . . . . . . . . . . . . . . .
303
9.1.5
Fixed vs. Dynamic Extent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
305
9.2
Spans Considered Harmful . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
306
9.3
Design Aspects of Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
306
9.3.1
Lifetime Dependencies of Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
307
9.3.2
Performance of Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
307
9.3.3
const Correctness of Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
308
9.3.4
Using Spans as Parameters in Generic Code . . . . . . . . . . . . . . . . . . . . . . .
310
Span Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
312
9.4.1
Span Operations and Member Types Overview . . . . . . . . . . . . . . . . . . . . . .
312
9.4.2
Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
314
9.4.3
Operations for Sub-Spans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
317
Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
318
9.4
9.5
10 Formatted Output
319
10.1 Formatted Output by Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
319
10.1.1 Using std::format() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
319
10.1.2 Using std::format_to_n() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
321
10.1.3 Using std::format_to() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
322
10.1.4 Using std::formatted_size() . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
322
10.2 Performance of the Formatting Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
322
10.2.1 Using std::vformat() and vformat_to() . . . . . . . . . . . . . . . . . . . . . .
323
10.3 Formatted Output in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
324
10.3.1 General Format of Format Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
324
10.3.2 Standard Format Specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
324
10.3.3 Width, Precision, and Fill Characters . . . . . . . . . . . . . . . . . . . . . . . . . . . .
325
10.3.4 Format/Type Specifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
326
10.4 Internationalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
329
10.5 Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
331
x
Contents
10.6 User-Defined Formatted Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
333
10.6.1 Basic Formatter API . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
333
10.6.2 Improved Parsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
335
10.6.3 Using Standard Formatters for User-Defined Formatters . . . . . . . . . . . . . . . .
337
10.6.4 Using Standard Formatters for Strings . . . . . . . . . . . . . . . . . . . . . . . . . . .
340
10.7 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
342
11 Dates and Timezones for <chrono>
343
11.1 Overview by Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
343
11.1.1 Scheduling a Meeting on the 5th of Every Month . . . . . . . . . . . . . . . . . . . .
343
11.1.2 Scheduling a Meeting on the Last Day of Every Month . . . . . . . . . . . . . . . . .
348
11.1.3 Scheduling a Meeting Every First Monday . . . . . . . . . . . . . . . . . . . . . . . .
350
11.1.4 Using Different Timezones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
352
11.2 Basic Chrono Concepts and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
357
11.3 Basic Chrono Extensions with C++20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
358
11.3.1 Duration Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
358
11.3.2 Clocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
359
11.3.3 Timepoint Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
360
11.3.4 Calendrical Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
361
11.3.5 Time Type hh_mm_ss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
364
11.3.6 Hours Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
366
11.4 I/O with Chrono Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
367
11.4.1 Default Output Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
367
11.4.2 Formatted Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
368
11.4.3 Locale-Dependent Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
371
11.4.4 Formatted Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
372
11.5 Using the Chrono Extensions in Practice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
378
11.5.1 Invalid Dates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
378
11.5.2 Dealing with months and years . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
380
11.5.3 Parsing Timepoints and Durations . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
382
11.6 Timezones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
385
11.6.1 Characteristics of Timezones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
385
11.6.2 The IANA Timezone Database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
385
11.6.3 Using Timezones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
387
11.6.4 Dealing with Timezone Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . .
390
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