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    C++17 - The Complete Guide
    Contents
    Preface
    About This Book
    What You Should Know Before Reading This Book
    Overall Structure of the Book
    How to Read This Book
    Error Terminology
    The C++17 Standard
    Example Code and Additional Information
    Feedback
    Part I: Basic Language Features
    1 Structured Bindings
    1.1 Structured Bindings in Detail
    1.2 Where Structured Bindings Can Be Used
    1.2.1 Structures and Classes
    1.2.2 Raw Arrays
    1.2.3 std::pair, std::tuple, and std::array
    1.3 Providing a Tuple-Like API for Structured Bindings
    1.4 Afternotes
    2 if and switch with Initialization
    2.1 if with Initialization
    2.2 switch with Initialization
    2.3 Afternotes
    3 Inline Variables
    3.1 Motivation for Inline Variables
    3.2 Using Inline Variables
    3.3 constexpr Now Implies inline For Static Members
    3.4 Inline Variables and thread_local
    3.5 Afternotes
    4 Aggregate Extensions
    4.1 Motivation for Extended Aggregate Initialization
    4.2 Using Extended Aggregate Initialization
    4.3 Definition of Aggregates
    4.4 Backward Incompatibilities
    4.5 Afternotes
    5 Mandatory Copy Elision or Passing Unmaterialized Objects
    5.1 Motivation for Mandatory Copy Elision for Temporaries
    5.2 Benefit of Mandatory Copy Elision for Temporaries
    5.3 Clarified Value Categories
    5.3.1 Value Categories
    5.3.2 Value Categories Since C++17
    5.4 Unmaterialized Return Value Passing
    5.5 Afternotes
    6 Lambda Extensions
    6.1 constexpr Lambdas
    6.1.1 Using constexpr Lambdas
    6.2 Passing Copies of this to Lambdas
    6.3 Capturing by const Reference
    6.4 Afternotes
    7 New Attributes and Attribute Features
    7.1 Attribute [[nodiscard]]
    7.2 Attribute [[maybe_unused]]
    7.3 Attribute [[fallthrough]]
    7.4 General Attribute Extensions
    7.5 Afternotes
    8 Other Language Features
    8.1 Nested Namespaces
    8.2 Defined Expression Evaluation Order
    8.3 Relaxed Enum Initialization from Integral Values
    8.4 Fixed Direct List Initialization with auto
    8.5 Hexadecimal Floating-Point Literals
    8.6 UTF-8 Character Literals
    8.7 Exception Specifications as Part of the Type
    8.8 Single-Argument static_assert
    8.9 Preprocessor Condition __has_include
    8.10 Afternotes
    Part II: Template Features
    9 Class Template Argument Deduction
    9.1 Use of Class Template Argument Deduction
    9.1.1 Copying by Default
    9.1.2 Deducing the Type of Lambdas
    9.1.3 No Partial Class Template Argument Deduction
    9.1.4 Class Template Argument Deduction Instead of Convenience Functions
    9.2 Deduction Guides
    9.2.1 Using Deduction Guides to Force Decay
    9.2.2 Non-Template Deduction Guides
    9.2.3 Deduction Guides versus Constructors
    9.2.4 Explicit Deduction Guides
    9.2.5 Deduction Guides for Aggregates
    9.2.6 Standard Deduction Guides
    9.3 Afternotes
    10 Compile-Time if
    10.1 Motivation for Compile-Time if
    10.2 Using Compile-Time if
    10.2.1 Caveats for Compile-Time if
    10.2.2 Other Compile-Time if Examples
    10.3 Compile-Time if with Initialization
    10.4 Using Compile-Time if Outside Templates
    10.5 Afternotes
    11 Fold Expressions
    11.1 Motivation for Fold Expressions
    11.2 Using Fold Expressions
    11.2.1 Dealing with Empty Parameter Packs
    11.2.2 Supported Operators
    11.2.3 Using Fold Expressions for Types
    11.3 Afternotes
    12 Dealing with String Literals as Template Parameters
    12.1 Using Strings in Templates
    12.2 Afternotes
    13 Placeholder Types like auto as Template Parameters
    13.1 Using auto for Template Parameters
    13.1.1 Parameterizing Templates for Characters and Strings
    13.1.2 Defining Metaprogramming Constants
    13.2 Using auto as Variable Template Parameter
    13.3 Using decltype(auto) as Template Parameter
    13.4 Afternotes
    14 Extended Using Declarations
    14.1 Using Variadic Using Declarations
    14.2 Variadic Using Declarations for Inheriting Constructors
    14.3 Afternotes
    Part III: New Library Components
    15 std::optional<>
    15.1 Using std::optional<>
    15.1.1 Optional Return Values
    15.1.2 Optional Arguments and Data Members
    15.2 std::optional<> Types and Operations
    15.2.1 std::optional<> Types
    15.2.2 std::optional<> Operations
    15.3 Special Cases
    15.3.1 Optional of Boolean or Raw Pointer Values
    15.3.2 Optional of Optional
    15.4 Afternotes
    16 std::variant<>
    16.1 Motivation for std::variant<>
    16.2 Using std::variant<>
    16.3 std::variant<> Types and Operations
    16.3.1 std::variant<> Types
    16.3.2 std::variant<> Operations
    16.3.3 Visitors
    16.3.4 Valueless by Exception
    16.4 Polymorphism and Heterogeneous Collections with std::variant
    16.4.1 Geometric Objects with std::variant
    16.4.2 Other Heterogeneous Collections with std::variant
    16.4.3 Comparing variant Polymorphism
    16.5 Special Cases with std::variant<>
    16.5.1 Having Both bool and std::string Alternatives
    16.6 Afternotes
    17 std::any
    17.1 Using std::any
    17.2 std::any Types and Operations
    17.2.1 Any Types
    17.2.2 Any Operations
    17.3 Afternotes
    18 std::byte
    18.1 Using std::byte
    18.2 std::byte Types and Operations
    18.2.1 std::byte Types
    18.2.2 std::byte Operations
    18.3 Afternotes
    19 String Views
    19.1 Differences Compared to std::string
    19.2 Using String Views
    19.3 Using String Views as Parameters
    19.3.1 String View Considered Harmful
    19.4 String View Types and Operations
    19.4.1 Concrete String View Types
    19.4.2 String View Operations
    19.4.3 String View Support by Other Types
    19.5 Using String Views in APIs
    19.5.1 Using String Views instead of Strings
    19.6 Afternotes
    20 The Filesystem Library
    20.1 Basic Examples
    20.1.1 Print Attributes of a Passed Filesystem Path
    20.1.2 Switch Over Filesystem Types
    20.1.3 Create Different Types of Files
    20.1.4 Dealing with Filesystems Using Parallel Algorithms
    20.2 Principles and Terminology
    20.2.1 General Portability Disclaimer
    20.2.2 Namespace
    20.2.3 Paths
    20.2.4 Normalization
    20.2.5 Member Function versus Free-Standing Functions
    20.2.6 Error Handling
    20.2.7 File Types
    20.3 Path Operations
    20.3.1 Path Creation
    20.3.2 Path Inspection
    20.3.3 Path I/O and Conversions
    20.3.4 Conversions Between Native and Generic Format
    20.3.5 Path Modifications
    20.3.6 Path Comparisons
    20.3.7 Other Path Operations
    20.4 Filesystem Operations
    20.4.1 File Attributes
    20.4.2 File Status
    20.4.3 Permissions
    20.4.4 Filesystem Modifications
    20.4.5 Symbolic Links and Filesystem-Dependent Path Conversions
    20.4.6 Other Filesystem Operations
    20.5 Iterating Over Directories
    20.5.1 Directory Entries
    20.6 Afternotes
    Part IV: Library Extensions and Modifications
    21 Extensions of Type Traits
    21.1 Type Traits Suffix _v
    21.2 New Type Traits
    21.3 Afternotes
    22 Parallel STL Algorithms
    22.1 Using Parallel Algorithms
    22.1.1 Using a Parallel for_each()
    22.1.2 Using a Parallel sort()
    22.2 Execution Policies
    22.3 Exception Handling
    22.4 Benefit of Not Using Parallel Algorithms
    22.5 Overview of Parallel Algorithms
    22.6 Motivation for New Algorithms for Parallel Processing
    22.6.1 reduce()
    22.7 Afternotes
    23 New STL Algorithms in Detail
    23.1 std::for_each_n()
    23.2 New Numeric STL Algorithms
    23.2.1 std::reduce()
    23.2.2 std::transform_reduce()
    23.2.3 std::inclusive_scan() and std::exclusive_scan()
    23.2.4 std::transform_inclusive_scan() and std::transform_exclusive_scan()
    23.3 Afternotes
    24 Substring and Subsequence Searchers
    24.1 Using Substring Searchers
    24.1.1 Using Searchers with search()
    24.1.2 Using Searchers Directly
    24.2 Using General Subsequence Searchers
    24.3 Using Searcher Predicates
    24.4 Afternotes
    25 Other Utility Functions and Algorithms
    25.1 size(), empty(), and data()
    25.1.1 Generic size() Function
    25.1.2 Generic empty() Function
    25.1.3 Generic data() Function
    25.2 as_const()
    25.2.1 Capturing by Const Reference
    25.3 clamp()
    25.4 sample()
    25.5 Afternotes
    26 Container and String Extensions
    26.1 Node Handles
    26.1.1 Modifying a Key
    26.1.2 Moving Nodes Between Containers
    26.1.3 Merging Containers
    26.2 Emplace Improvements
    26.2.1 Return Type of Emplace Functions
    26.2.2 try_emplace() and insert_or_assign() for Maps
    26.2.3 try_emplace()
    26.2.4 insert_or_assign()
    26.3 Container Support for Incomplete Types
    26.4 String Improvements
    26.5 Afternotes
    27 Multi-Threading and Concurrency
    27.1 Supplementary Mutexes and Locks
    27.1.1 std::scoped_lock
    27.1.2 std::shared_mutex
    27.2 is_always_lock_free for Atomics
    27.3 Cache Line Sizes
    27.4 Afternotes
    28 Other Small Library Features and Modifications
    28.1 std::uncaught_exceptions()
    28.2 Shared Pointer Improvements
    28.2.1 Special handling for Shared Pointers to Raw C Arrays
    28.2.2 reinterpret_pointer_cast for Shared Pointers
    28.2.3 weak_type for Shared Pointers
    28.2.4 weak_from_this for Shared Pointers
    28.3 Numeric Extensions
    28.3.1 Greatest Common Divisor and Least Common Multiple
    28.3.2 Three-Argument Overloads of std::hypot()
    28.3.3 Mathematical Special Functions
    28.4 chrono Extensions
    28.5 constexpr Extensions and Fixes
    28.6 noexcept Extensions and Fixes
    28.7 Afternotes
    Part V: Expert Utilities
    29 Polymorphic Memory Resources (PMR)
    29.1 Using Standard Memory Resources
    29.1.1 Motivating Example
    29.1.2 Standard Memory Resources
    29.1.3 Standard Memory Resources in Detail
    29.2 Defining Custom Memory Resources
    29.2.1 Equality of Memory Resources
    29.3 Providing Memory Resource Support for Custom Types
    29.3.1 Definition of a PMR Type
    29.3.2 Using a PMR Type
    29.3.3 Dealing with the Different Types
    29.4 Afternotes
    30 new and delete with Over-Aligned Data
    30.1 Using new with Alignments
    30.1.1 Distinct Dynamic/Heap Memory Arenas
    30.1.2 Passing the Alignment with the new Expression
    30.2 Implementing operator new() for Aligned Memory
    30.2.1 Implementing Aligned Allocation Before C++17
    30.2.2 Implementing Type-Specific operator new()
    30.3 Implementing Global operator new()
    30.3.1 Backward Incompatibilities
    30.4 Tracking All ::new Calls
    30.5 Afternotes
    31 std::to_chars() and std::from_chars()
    31.1 Motivation for Low-Level Conversions between Character Sequences and Numeric Values
    31.2 Example Usage
    31.2.1 from_chars()
    31.2.2 to_chars()
    31.3 Floating-Point Round-Trip Support
    31.4 Afternotes
    32 std::launder()
    32.1 Motivation for std::launder()
    32.2 How launder() Solves the Problem
    32.3 Why/When launder() Does Not Work
    32.4 Afternotes
    33 Improvements for Implementing Generic Code
    33.1 std::invoke<>()
    33.2 std::bool_constant<>
    33.3 std::void_t<>
    33.4 Afternotes
    Part VI: Final General Hints
    34 Common C++17 Settings
    34.1 Value of __cplusplus
    34.2 Compatibility to C11
    34.3 Dealing with Signal Handlers
    34.4 Forward Progress Guarantees
    34.5 Afternotes
    35 Deprecated and Removed Features
    35.1 Deprecated and Removed Core Language Features
    35.1.1 Throw Specifications
    35.1.2 Keyword register
    35.1.3 Disable ++ for bool
    35.1.4 Trigraphs
    35.1.5 Definition/Redeclaration of static constexpr Members
    35.2 Deprecated and Removed Library Features
    35.2.1 auto_ptr
    35.2.2 Algorithm random_shuffle()
    35.2.3 unary_function and binary_function
    35.2.4 ptr_fun(), mem_fun(), and Binders
    35.2.5 Allocator Support for std::function<>
    35.2.6 Deprecated IOStream Aliases
    35.2.7 Deprecated Library Features
    35.3 Afternotes
    Glossary
    B
    F
    I
    P
    S
    V
    Index
    A
    B
    C
    D
    E
    F
    G
    H
    I
    J
    K
    L
    M
    N
    O
    P
    R
    S
    T
    U
    V
    W
    X
    Josuttis: C++17 2019/12/20 17:30 page i Nicolai M. Josuttis C++17 - The Complete Guide First Edition
    Josuttis: C++17 2019/12/20 17:30 page ii C++17 - The Complete Guide First Edition Nicolai M. Josuttis This version was published on 2019-12-20. © 2019 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/cpp17. 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.
    Josuttis: C++17 2019/12/20 17:30 page iii Contents Preface Versions of This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . About This Book What You Should Know Before Reading This Book . . . . . . . . . . . . . . . . . . . . . . . . . . Overall Structure of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . How to Read This Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Error Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The C++17 Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Example Code and Additional Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part I: Basic Language Features 1 Structured Bindings 1.1 Structured Bindings in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Where Structured Bindings Can Be Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structures and Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Raw Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 1.2.3 std::pair, std::tuple, and std::array . . . . . . . . . . . . . . . . . . . . . . . Providing a Tuple-Like API for Structured Bindings . . . . . . . . . . . . . . . . . . . . . . . . 1.3 1.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 if and switch with Initialization 2.1 if with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii xv xv xvii xix xix xx xx xx xxi xxi xxi 1 3 4 7 8 9 9 11 19 21 21
    Josuttis: C++17 2019/12/20 17:30 page iv iv Contents 2.2 switch with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Inline Variables 3.1 Motivation for Inline Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Using Inline Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . constexpr Now Implies inline For Static Members . . . . . . . . . . . . . . . . . . . . . . 3.3 3.4 Inline Variables and thread_local . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Aggregate Extensions 4.1 Motivation for Extended Aggregate Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Using Extended Aggregate Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Definition of Aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Backward Incompatibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Mandatory Copy Elision or Passing Unmaterialized Objects 5.1 Motivation for Mandatory Copy Elision for Temporaries . . . . . . . . . . . . . . . . . . . . . 5.2 Benefit of Mandatory Copy Elision for Temporaries . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Clarified Value Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Value Categories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Value Categories Since C++17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Unmaterialized Return Value Passing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Lambda Extensions 6.1 constexpr Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1 Using constexpr Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Passing Copies of this to Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Capturing by const Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 New Attributes and Attribute Features 7.1 Attribute [[nodiscard]] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 24 25 25 27 28 29 31 33 34 34 36 36 37 39 39 41 42 42 44 45 46 47 47 49 50 53 53 55 55
    Josuttis: C++17 2019/12/20 17:30 page v Contents 7.2 Attribute [[maybe_unused]] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Attribute [[fallthrough]] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 General Attribute Extensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Other Language Features 8.1 Nested Namespaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Defined Expression Evaluation Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Relaxed Enum Initialization from Integral Values . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Fixed Direct List Initialization with auto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Hexadecimal Floating-Point Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 UTF-8 Character Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Exception Specifications as Part of the Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 8.8 Single-Argument static_assert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Preprocessor Condition __has_include . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.10 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Part II: Template Features 9 Class Template Argument Deduction Copying by Default 9.1 Use of Class Template Argument Deduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.2 Deducing the Type of Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.3 No Partial Class Template Argument Deduction . . . . . . . . . . . . . . . . . . . . . Class Template Argument Deduction Instead of Convenience Functions . . . . . . 9.1.4 9.2 Deduction Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Using Deduction Guides to Force Decay . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Non-Template Deduction Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3 Deduction Guides versus Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.4 Explicit Deduction Guides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.5 Deduction Guides for Aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.6 9.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standard Deduction Guides v 57 58 58 59 61 61 62 65 66 67 68 69 72 73 73 75 77 77 79 80 81 83 84 85 86 86 87 88 89 93
    Josuttis: C++17 2019/12/20 17:30 page vi vi 10 Compile-Time if 10.1 Motivation for Compile-Time if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Using Compile-Time if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Caveats for Compile-Time if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Other Compile-Time if Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Compile-Time if with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Using Compile-Time if Outside Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Fold Expressions 11.1 Motivation for Fold Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Using Fold Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Dealing with Empty Parameter Packs . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 Supported Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 Using Fold Expressions for Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Dealing with String Literals as Template Parameters 12.1 Using Strings in Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Placeholder Types like auto as Template Parameters 13.1 Using auto for Template Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.1 Parameterizing Templates for Characters and Strings . . . . . . . . . . . . . . . . . . 13.1.2 Defining Metaprogramming Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Using auto as Variable Template Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Using decltype(auto) as Template Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Extended Using Declarations 14.1 Using Variadic Using Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Variadic Using Declarations for Inheriting Constructors . . . . . . . . . . . . . . . . . . . . . 14.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Contents 95 96 98 98 101 103 104 105 107 108 108 109 112 117 118 119 119 120 121 121 122 123 124 126 127 129 129 130 132
    Josuttis: C++17 2019/12/20 17:30 page vii Contents Part III: New Library Components 15 std::optional<> 15.1 Using std::optional<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.1 Optional Return Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.1.2 Optional Arguments and Data Members . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 std::optional<> Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.1 std::optional<> Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 std::optional<> Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Special Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.1 Optional of Boolean or Raw Pointer Values . . . . . . . . . . . . . . . . . . . . . . . . 15.3.2 Optional of Optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 std::variant<> 16.1 Motivation for std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Using std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3 std::variant<> Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.1 std::variant<> Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.2 std::variant<> Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.3 Visitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.4 Valueless by Exception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4 Polymorphism and Heterogeneous Collections with std::variant . . . . . . . . . . . . . . 16.4.1 Geometric Objects with std::variant . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.2 Other Heterogeneous Collections with std::variant . . . . . . . . . . . . . . . . . 16.4.3 Comparing variant Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5 Special Cases with std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5.1 Having Both bool and std::string Alternatives . . . . . . . . . . . . . . . . . . . 16.6 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 std::any 17.1 Using std::any . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.2 std::any Types and Operations 17.2.1 Any Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.2.2 Any Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 133 135 135 135 137 139 139 139 146 146 146 147 149 149 150 152 152 153 157 161 162 163 165 167 168 168 168 171 171 174 174 174
    Josuttis: C++17 2019/12/20 17:30 page viii viii Contents 17.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 18 std::byte 18.1 Using std::byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2 std::byte Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.1 std::byte Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.2.2 std::byte Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 String Views 19.3.1 String View Considered Harmful 19.1 Differences Compared to std::string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.2 Using String Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.3 Using String Views as Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.4 String View Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.4.1 Concrete String View Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.4.2 String View Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.4.3 String View Support by Other Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.5 Using String Views in APIs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.5.1 Using String Views instead of Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . 19.6 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 The Filesystem Library 20.1 Basic Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.1.1 Print Attributes of a Passed Filesystem Path . . . . . . . . . . . . . . . . . . . . . . . 20.1.2 Switch Over Filesystem Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.1.3 Create Different Types of Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.1.4 Dealing with Filesystems Using Parallel Algorithms . . . . . . . . . . . . . . . . . . 20.2 Principles and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.1 General Portability Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.2 Namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.3 Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.4 Normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.5 Member Function versus Free-Standing Functions . . . . . . . . . . . . . . . . . . . . 20.2.6 Error Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 179 181 181 182 184 185 185 186 186 188 192 192 193 196 196 197 198 201 201 201 204 206 210 210 210 211 211 212 213 214
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