C++ Templates

David Vandevoorde started programming C++ in the late 1980s. After obtaining a PhD from the Rensselaer Polytechnic Institute, he became technical lead of Hewlett-Packard's C++ compiler team. In 1999 he joined the Edison Design Group (EDG), whose C++ compiler technology is widely recognized as the industry's most advanced. He is an active member of the C++ Standard Committee and a moderator of the newsgroup comp.lang.c++.moderated (which he co-founded). He is the author of C++ Solutions, the companion to The C++ Programming Language, 3rd Edition. Nicolai M. Josuttis is well known for his best-selling de-facto standard bookThe C++ Standard Library - A Tutorial and Reference. He is an independent technical consultant who designs object-oriented software for the telecommunications, traffic, finance, and manufacturing industries. He is an active member of the C++ Standard Committee and a partner at System Bauhaus, a German group of prominent object-oriented system development experts. Josuttis has written several other books on object-oriented programming and C++. Douglas Gregor is Senior Swift/C++/Objective-C Compiler Engineer at Apple. He holds a PhD in computer science from Rensselaer Polytechnic Institute, and did post-doctoral work at Indiana University.

Preface xxiii

Acknowledgments for the Second Edition xxv

Acknowledgments for the First Edition xxvii

About This Book xxix







Part I: The Basics 1



Chapter 1: Function Templates 3



1.1 A First Look at Function Templates 3

1.2 Template Argument Deduction 7

1.3 Multiple Template Parameters 9

1.4 Default Template Arguments 13

1.5 Overloading Function Templates 15

1.6 But, Shouldn't We . . . ? 20

1.7 Summary 21



Chapter 2: Class Templates 23

2.1 Implementation of Class Template Stack 23

2.2 Use of Class Template Stack 27

2.3 Partial Usage of Class Templates 29

2.4 Friends 30

2.5 Specializations of Class Templates 31

2.6 Partial Specialization 33

2.7 Default Class Template Arguments 36

2.8 Type Aliases38

2.9 Class Template Argument Deduction 40

2.10 Templatized Aggregates 43

2.11 Summary 44



Chapter 3: Nontype Template Parameters 45

3.1 Nontype Class Template Parameters 45

3.2 Nontype Function Template Parameters 48

3.3 Restrictions for Nontype Template Parameters 49

3.4 Template Parameter Type auto 50

3.5 Summary 54



Chapter 4: Variadic Templates 55

4.1 Variadic Templates 55

4.2 Fold Expressions 58

4.3 Application of Variadic Templates 60

4.4 Variadic Class Templates and Variadic Expressions 61

4.5 Summary 66



Chapter 5: Tricky Basics 67

5.1 Keyword typename 67

5.2 Zero Initialization 68

5.3 Using this-> 70

5.4 Templates for Raw Arrays and String Literals 71

5.5 Member Templates 74

5.6 Variable Templates 80

5.7 Template Template Parameters 83

5.8 Summary 89



Chapter 6: Move Semantics and enable_if<> 91

6.1 Perfect Forwarding 91

6.2 Special Member Function Templates 95

6.3 Disable Templates with enable_if<> 98

6.4 Using enable_if<> 99

6.5 Using Concepts to Simplify enable_if<> Expressions 103

6.6 Summary 104



Chapter 7: By Value or by Reference? 105

7.1 Passing by Value 106

7.2 Passing by Reference 108

7.3 Using std::ref() and std::cref() 112

7.4 Dealing with String Literals and Raw Arrays 115

7.5 Dealing with Return Values 117

7.6 Recommended Template Parameter Declarations 118

7.7 Summary 121



Chapter 8: Compile-Time Programming 123

8.1 Template Metaprogramming 123

8.2 Computing with constexpr 125

8.3 Execution Path Selection with Partial Specialization 127

8.4 SFINAE (Substitution Failure Is Not An Error) 129

8.5 Compile-Time if 134

8.6 Summary 135



Chapter 9: Using Templates in Practice 137

9.1 The Inclusion Model 137

9.2 Templates and inline 140

9.3 Precompiled Headers 141

9.4 Decoding the Error Novel 143

9.5 Afternotes 149

9.6 Summary 150



Chapter 10: Basic Template Terminology 151

10.1 "Class Template" or "Template Class"? 151

10.2 Substitution, Instantiation, and Specialization 152

10.3 Declarations versus Definitions 153

10.4 The One-Definition Rule 154

10.5 Template Arguments versus Template Parameters 155

10.6 Summary 156



Chapter 11: Generic Libraries 157

11.1 Callables 157

11.2 Other Utilities to Implement Generic Libraries 164

11.3 Perfect Forwarding Temporaries 167

11.4 References as Template Parameters 167

11.5 Defer Evaluations 171

11.6 Things to Consider When Writing Generic Libraries 172

11.7 Summary 173





Part II: Templates in Depth 175



Chapter 12: Fundamentals in Depth 177



12.1 Parameterized Declarations 177

12.2 Template Parameters 185

12.3 Template Arguments 192

12.4 Variadic Templates 200

12.5 Friends 209

12.6 Afternotes 213



Chapter 13: Names in Templates 215

13.1 Name Taxonomy 215

13.2 Looking Up Names 217

13.3 Parsing Templates 224

13.4 Inheritance and Class Templates 236

13.5 Afternotes 240



Chapter 14: Instantiation 243

14.1 On-Demand Instantiation 243

14.2 Lazy Instantiation 245

14.3 The C++ Instantiation Model 249

14.4 Implementation Schemes 255

14.5 Explicit Instantiation 260

14.6 Compile-Time if Statements 263

14.7 In the Standard Library 265

14.8 Afternotes 266



Chapter 15: Template Argument Deduction 269

15.1 The Deduction Process 269

15.2 Deduced Contexts 271

15.3 Special Deduction Situations 273

15.4 Initializer Lists 274

15.5 Parameter Packs 275

15.6 Rvalue References 277

15.7 SFINAE (Substitution Failure Is Not An Error) 284

15.8 Limitations of Deduction 286

15.9 Explicit Function Template Arguments 291

15.10 Deduction from Initializers and Expressions 293

15.11 Alias Templates 312

15.12 Class Template Argument Deduction 313

15.13 Afternotes 321



Chapter 16: Specialization and Overloading 323

16.1 When "Generic Code" Doesn't Quite Cut It 323

16.2 Overloading Function Templates 326

16.3 Explicit Specialization 338

16.4 Partial Class Template Specialization 347

16.5 Partial Variable Template Specialization 351

16.6 Afternotes 352



Chapter 17: Future Directions 353

17.1 Relaxed typename Rules 354

17.2 Generalized Nontype Template Parameters 354

17.3 Partial Specialization of Function Templates 356

17.4 Named Template Arguments 358

17.5 Overloaded Class Templates 359

17.6 Deduction for Nonfinal Pack Expansions 360

17.7 Regularization of void 361

17.8 Type Checking for Templates 361

17.9 Reflective Metaprogramming 363

17.10 Pack Facilities 365

17.11 Modules 366





Part III: Templates and Design 367



Chapter 18: The Polymorphic Power of Templates 369



18.1 Dynamic Polymorphism 369

18.2 Static Polymorphism 372

18.3 Dynamic versus Static Polymorphism 375

18.4 Using Concepts 377

18.5 New Forms of Design Patterns 379

18.6 Generic Programming 380

18.7 Afternotes 383



Chapter 19: Implementing Traits 385

19.1 An Example: Accumulating a Sequence 385

19.2 Traits versus Policies and Policy Classes 394

19.3 Type Functions 401

19.4 SFINAE-Based Traits 416

19.5 IsConvertibleT 428

19.6 Detecting Members 431

19.7 Other Traits Techniques 440

19.8 Type Classification 448

19.9 Policy Traits 458

19.10 In the Standard Library 461

19.11 Afternotes 462



Chapter 20: Overloading on Type Properties 465

20.1 Algorithm Specialization 465

20.2 Tag Dispatching 467

20.3 Enabling/Disabling Function Templates 469

20.4 Class Specialization 477

20.5 Instantiation-Safe Templates 482

20.6 In the Standard Library 487

20.7 Afternotes 488



Chapter 21: Templates and Inheritance 489

21.1 The Empty Base Class Optimization (EBCO) 489

21.2 The Curiously Recurring Template Pattern (CRTP) 495

21.3 Mixins 508

21.4 Named Template Arguments 512

21.5 Afternotes 515



Chapter 22: Bridging Static and Dynamic Polymorphism 517

22.1 Function Objects, Pointers, and std::function<> 517

22.2 Generalized Function Pointers 519

22.3 Bridge Interface 522

22.4 Type Erasure 523

22.5 Optional Bridging 525

22.6 Performance Considerations 527

22.7 Afternotes 528



Chapter 23: Metaprogramming 529

23.1 The State of Modern C++ Metaprogramming 529

23.2 The Dimensions of Reflective Metaprogramming 537

23.3 The Cost of Recursive Instantiation 539

23.4 Computational Completeness 542

23.5 Recursive Instantiation versus Recursive Template Arguments 542

23.6 Enumeration Values versus Static Constants 543

23.7 Afternotes 545



Chapter 24: Typelists 549

24.1 Anatomy of a Typelist 549

24.2 Typelist Algorithms 551

24.3 Nontype Typelists 566

24.4 Optimizing Algorithms with Pack Expansions 569

24.5 Cons-style Typelists 571

24.6 Afternotes 573



Chapter 25: Tuples 575

25.1 Basic Tuple Design 576

25.2 Basic Tuple Operations 579

25.3 Tuple Algorithms 581

25.4 Expanding Tuples 592

25.5 Optimizing Tuple 593

25.6 Tuple Subscript 599

25.7 Afternotes 601



Chapter 26: Discriminated Unions 603

26.1 Storage 604

26.2 Design 606

26.3 Value Query and Extraction 610

26.4 Element Initialization, Assignment and Destruction 611

26.5 Visitors 617

26.6 Variant Initialization and Assignment 624

26.7 Afternotes 628



Chapter 27: Expression Templates 629

27.1 Temporaries and Split Loops 630

27.2 Encoding Expressions in Template Arguments 635

27.3 Performance and Limitations of Expression Templates 646

27.4 Afternotes 647



Chapter 28: Debugging Templates 651

28.1 Shallow Instantiation 652

28.2 Static Assertions 654

28.3 Archetypes 655

28.4 Tracers 657

28.5 Oracles 662

28.6 Afternotes 662





Appendixes 663



Appendix A: The One-Definition Rule 663



A.1 Translation Units 663

A.2 Declarations and Definitions 664

A.3 The One-Definition Rule in Detail 665



Appendix B: Value Categories 673

B.1 Traditional Lvalues and Rvalues 673

B.2 Value Categories Since C++11 674

B.3 Checking Value Categories with decltype 678

B.4 Reference Types 679



Appendix C: Overload Resolution 681

C.1 When Does Overload Resolution Kick In? 681

C.2 Simplified Overload Resolution 682

C.3 Overloading Details 688



Appendix D: Standard Type Utilities 697

D.1 Using Type Traits 697

D.2 Primary and Composite Type Categories 702

D.3 Type Properties and Operations 709

D.4 Type Construction 728

D.5 Other Traits 732

D.6 Combining Type Traits 734

D.7 Other Utilities 737



Appendix E: Concepts 739

E.1 Using Concepts 739

E.2 Defining Concepts 742

E.3 Overloading on Constraints 743

E.4 Concept Tips 746





Bibliography 749

Glossary 759

Index 771