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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
    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 of Inline Variables
    3.2 Using Inline Variables
    3.3 constexpr now implies inline
    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
    6.2 Passing Copies of this to Lambdas
    6.3 Capturing by 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 Usage 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 Strings as Template Parameters
    12.1 Using Strings in Templates
    12.2 Afternotes
    13 Placeholder Types like auto as Template Parameters
    13.1 Using auto as Template Parameter
    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 of 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 Inhomogeneous Collections with std::variant
    16.4.1 Geometric Objects with std::variant
    16.4.2 Other Inhomogeneous 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 to std::string
    19.2 Using String Views
    19.3 Using String Views Similar to Strings
    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 API's
    19.5.1 Using String Views to Initialize Strings
    19.5.2 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 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 Type Traits Extensions
    21.1 Type Traits Suffix _v
    21.2 New Type Traits
    21.3 std::bool_constant<>
    21.4 std::void_t<>
    21.5 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 New Algorithms for Parallel Processing
    22.6.1 reduce()
    22.7 Parallel Algorithms in Detail
    22.8 Afternotes
    23 Substring and Subsequence Searchers
    23.1 Using Substring Searchers
    23.1.1 Using Searchers with search()
    23.1.2 Using Searchers Directly
    23.2 Using General Subsequence Searchers
    23.3 Using Searcher Predicates
    23.4 Afternotes
    24 Other Utility Functions and Algorithms
    24.1 size(), empty(), and data()
    24.1.1 Generic size() Function
    24.1.2 Generic empty() Function
    24.1.3 Generic data() Function
    24.2 as_const()
    24.2.1 Capturing by Const Reference
    24.3 clamp()
    24.4 sample()
    24.5 for_each_n()
    24.6 Afternotes
    25 Container Extensions
    25.1 Container-Support of Incomplete Types
    25.2 Node Handles
    25.3 Afternotes
    26 Multi-Threading and Concurrency
    26.1 Supplementary Mutexes and Locks
    26.1.1 std::scoped_lock
    26.1.2 std::shared_mutex
    26.2 is_always_lock_free() for Atomics
    26.3 Cache-Line Sizes
    26.4 Afternotes
    Part V: Expert Utilities
    27 Polymorphic Memory Resources (PMR)
    27.1 Using Standard Memory Resources
    27.1.1 Motivating Example
    27.1.2 Standard Memory Resources
    27.1.3 Standard Memory Resources in Detail
    27.2 Defining Custom Memory Resources
    27.2.1 Equality of Memory Resources
    27.3 Providing Memory Resources Support for Custom Types
    27.3.1 Definition of a PMR Type
    27.3.2 Usage of a PMR Type
    27.3.3 Dealing with the Different Types
    27.4 Afternotes
    28 new and delete with Over-Aligned Data
    28.1 Using new with Alignments
    28.1.1 Distinct Dynamic/Heap Memory Arenas
    28.1.2 Passing the Alignment with the new Expression
    28.2 Implementing operator new() for Aligned Memory
    28.2.1 Implementing Aligned Allocation Before C++17
    28.2.2 Implementing Type-Specific operator new()
    28.3 Implementing Global operator new()
    28.3.1 Backward Incompatibilities
    28.4 Tracking all ::new Calls
    28.5 Afternotes
    29 Other Library Improvements for Experts
    29.1 Low-Level Conversions between Character Sequences and Numeric Values
    29.1.1 Example Usage
    29.1.2 Floating-Point Round-Trip Support
    29.2 Afternotes
    Glossary
    B
    F
    I
    P
    S
    V
    Index
    A
    B
    C
    D
    E
    F
    G
    H
    I
    J
    L
    M
    N
    O
    P
    R
    S
    T
    U
    V
    W
    X
    Josuttis: C++17 2019/02/16 18:57 page i Nicolai M. Josuttis C++17 - The Complete Guide
    Josuttis: C++17 2019/02/16 18:57 page ii C++17 - The Complete Guide Nicolai M. Josuttis This book is for sale at http://leanpub.com/cpp17. This version was published on 2019/02/16. This is a Leanpub book. Leanpub empowers authors and publishers with the Lean Publish- ing 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. © 2019 by Nicolai Josuttis. All rights reserved. This book was typeset by Nicolai M. Josuttis using the LATEX document processing system.
    Josuttis: C++17 2019/02/16 18:57 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . 1.3 Providing a Tuple-Like API for Structured Bindings . . . . . . . . . . . . . . . . . . . . 1.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 if and switch with Initialization 2.1 if with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii xiii xiii xiv xvii xvii xviii xviii xix xix xix 1 3 4 8 8 9 9 11 18 19 19
    Josuttis: C++17 2019/02/16 18:57 page iv iv Contents switch with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 2.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Inline Variables 3.1 Motivation of Inline Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Using Inline Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . constexpr now implies inline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 constexpr Lambdas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 6.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Passing Copies of this to Lambdas 6.3 Capturing by Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 New Attributes and Attribute Features 7.1 Attribute [[nodiscard]] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 21 23 23 25 26 27 29 31 32 32 34 34 35 37 37 39 40 40 42 43 44 45 45 47 49 49 51 51
    Josuttis: C++17 2019/02/16 18:57 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 Usage 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 52 53 54 54 57 57 58 61 62 63 64 65 68 69 69 71 73 74 75 76 77 78 80 81 81 82 83 83 84 89
    Josuttis: C++17 2019/02/16 18:57 page vi vi Contents 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 . . . . . . . . . . . . . . . . . . . . . . . . . 91 92 94 94 97 10.3 Compile-Time if with Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 10.4 Using Compile-Time if Outside Templates . . . . . . . . . . . . . . . . . . . . . . . . . 100 10.5 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 11 Fold Expressions 103 11.1 Motivation for Fold Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 11.2 Using Fold Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 11.2.1 Dealing with Empty Parameter Packs . . . . . . . . . . . . . . . . . . . . . . . 106 11.2.2 Supported Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 11.2.3 Using Fold Expressions for Types . . . . . . . . . . . . . . . . . . . . . . . . . . 112 11.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 12 Dealing with Strings as Template Parameters 115 12.1 Using Strings in Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 12.2 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 13 Placeholder Types like auto as Template Parameters 117 13.1 Using auto as Template Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 13.1.1 Parameterizing Templates for Characters and Strings . . . . . . . . . . . . . . 118 13.1.2 Defining Metaprogramming Constants . . . . . . . . . . . . . . . . . . . . . . . 119 13.2 Using auto as Variable Template Parameter . . . . . . . . . . . . . . . . . . . . . . . . 120 13.3 Using decltype(auto) as Template Parameter . . . . . . . . . . . . . . . . . . . . . . 122 13.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 14 Extended Using Declarations 125 14.1 Using Variadic Using Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 14.2 Variadic Using Declarations for Inheriting Constructors . . . . . . . . . . . . . . . . . 126 14.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
    Josuttis: C++17 2019/02/16 18:57 page vii Contents Part III: New Library Components vii 129 15 std::optional<> 131 15.1 Using std::optional<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 15.1.1 Optional Return Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 15.1.2 Optional Arguments and Data Members . . . . . . . . . . . . . . . . . . . . . . 133 15.2 std::optional<> Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 135 15.2.1 std::optional<> Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 15.2.2 std::optional<> Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 15.3 Special Cases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 15.3.1 Optional of Boolean or Raw Pointer Values . . . . . . . . . . . . . . . . . . . . 140 15.3.2 Optional of Optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 15.4 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 16 std::variant<> 143 16.1 Motivation of std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 16.2 Using std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 16.3 std::variant<> Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 16.3.1 std::variant<> Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 16.3.2 std::variant<> Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 16.3.3 Visitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 16.3.4 Valueless by Exception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 16.4 Polymorphism and Inhomogeneous Collections with std::variant . . . . . . . . . 156 16.4.1 Geometric Objects with std::variant . . . . . . . . . . . . . . . . . . . . . . 156 16.4.2 Other Inhomogeneous Collections with std::variant . . . . . . . . . . . . 159 16.4.3 Comparing variant Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . 160 16.5 Special Cases with std::variant<> . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 16.5.1 Having Both bool and std::string Alternatives . . . . . . . . . . . . . . . 161 16.6 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 17 std::any 163 17.1 Using std::any . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 17.2 std::any Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 17.2.1 Any Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
    Josuttis: C++17 2019/02/16 18:57 page viii viii Contents 17.2.2 Any Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 17.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 18 std::byte 171 18.1 Using std::byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 18.2 std::byte Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 18.2.1 std::byte Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 18.2.2 std::byte Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 18.3 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 19 String Views 19.3.1 String View Considered Harmful 177 19.1 Differences to std::string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 19.2 Using String Views . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 19.3 Using String Views Similar to Strings . . . . . . . . . . . . . . . . . . . . . . . . . . 180 19.4 String View Types and Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 19.4.1 Concrete String View Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 19.4.2 String View Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 19.4.3 String View Support by Other Types . . . . . . . . . . . . . . . . . . . . . . . . 187 19.5 Using String Views in API’s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 19.5.1 Using String Views to Initialize Strings . . . . . . . . . . . . . . . . . . . . . . 188 19.5.2 Using String Views instead of Strings . . . . . . . . . . . . . . . . . . . . . . . 190 19.6 Afternotes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 20 The Filesystem Library 193 20.1 Basic Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 20.1.1 Print Attributes of a Passed Filesystem Path . . . . . . . . . . . . . . . . . . . 193 20.1.2 Switch Over Filesystem Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 20.1.3 Create Different Types of Files . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 20.1.4 Dealing with Filesystems Using Parallel Algorithms . . . . . . . . . . . . . . 202 20.2 Principles and Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 20.2.1 General Portability Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 20.2.2 Namespace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 20.2.3 Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
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