C++ Templates: The Complete Guide. 2 Ed

Title Page....2

Copyright Page....3

Dedication....5

Contents....6

Preface....24

Acknowledgments for the Second Edition....26

Acknowledgments for the First Edition....29

About This Book....32

What You Should Know Before Reading This Book....33

Overall Structure of the Book....34

How to Read This Book....35

Some Remarks About Programming Style....35

The C++11, C++14, and C++17 Standards....38

Example Code and Additional Information....39

Feedback....40

Part I: The Basics....41

1 Function Templates....43

1.1 A First Look at Function Templates....43

1.1.1 Defining the Template....43

1.1.2 Using the Template....45

1.1.3 Two-Phase Translation....47

1.2 Template Argument Deduction....48

1.3 Multiple Template Parameters....50

1.3.1 Template Parameters for Return Types....51

1.3.2 Deducing the Return Type....53

1.3.3 Return Type as Common Type....55

1.4 Default Template Arguments....56

1.5 Overloading Function Templates....58

1.6 But, Shouldn’t We …?....64

1.6.1 Pass by Value or by Reference?....65

1.6.2 Why Not inline?....65

1.6.3 Why Not constexpr?....66

1.7 Summary....67

2 Class Templates....70

2.1 Implementation of Class Template Stack....70

2.1.1 Declaration of Class Templates....71

2.1.2 Implementation of Member Functions....73

2.2 Use of Class Template Stack....75

2.3 Partial Usage of Class Templates....77

2.3.1 Concepts....78

2.4 Friends....79

2.5 Specializations of Class Templates....81

2.6 Partial Specialization....83

2.7 Default Class Template Arguments....86

2.8 Type Aliases....88

2.9 Class Template Argument Deduction....91

2.10 Templatized Aggregates....96

2.11 Summary....97

3 Nontype Template Parameters....99

3.1 Nontype Class Template Parameters....99

3.2 Nontype Function Template Parameters....102

3.3 Restrictions for Nontype Template Parameters....104

3.4 Template Parameter Type auto....106

3.5 Summary....110

4 Variadic Templates....112

4.1 Variadic Templates....112

4.1.1 Variadic Templates by Example....112

4.1.2 Overloading Variadic and Nonvariadic Templates....114

4.1.3 Operator sizeof…....115

4.2 Fold Expressions....116

4.3 Application of Variadic Templates....119

4.4 Variadic Class Templates and Variadic Expressions....121

4.4.1 Variadic Expressions....121

4.4.2 Variadic Indices....122

4.4.3 Variadic Class Templates....123

4.4.4 Variadic Deduction Guides....124

4.4.5 Variadic Base Classes and using....125

4.5 Summary....127

5 Tricky Basics....128

5.1 Keyword typename....128

5.2 Zero Initialization....130

5.3 Using this->....132

5.4 Templates for Raw Arrays and String Literals....133

5.5 Member Templates....136

5.5.1 The .template Construct....144

5.5.2 Generic Lambdas and Member Templates....144

5.6 Variable Templates....145

5.7 Template Template Parameters....148

5.8 Summary....155

6 Move Semantics and enable_if<>....157

6.1 Perfect Forwarding....157

6.2 Special Member Function Templates....161

6.3 Disable Templates with enable_if<>....165

6.4 Using enable_if<>....167

6.5 Using Concepts to Simplify enable_if<> Expressions....172

6.6 Summary....174

7 By Value or by Reference?....176

7.1 Passing by Value....177

7.2 Passing by Reference....180

7.2.1 Passing by Constant Reference....180

7.2.2 Passing by Nonconstant Reference....182

7.2.3 Passing by Forwarding Reference....184

7.3 Using std::ref() and std::cref()....186

7.4 Dealing with String Literals and Raw Arrays....188

7.4.1 Special Implementations for String Literals and Raw Arrays....190

7.5 Dealing with Return Values....191

7.6 Recommended Template Parameter Declarations....194

7.7 Summary....197

8 Compile-Time Programming....200

8.1 Template Metaprogramming....201

8.2 Computing with constexpr....203

8.3 Execution Path Selection with Partial Specialization....205

8.4 SFINAE (Substitution Failure Is Not An Error)....207

8.4.1 Expression SFINAE with decltype....212

8.5 Compile-Time if....214

8.6 Summary....216

9 Using Templates in Practice....218

9.1 The Inclusion Model....218

9.1.1 Linker Errors....218

9.1.2 Templates in Header Files....221

9.2 Templates and inline....223

9.3 Precompiled Headers....224

9.4 Decoding the Error Novel....226

9.5 Afternotes....237

9.6 Summary....238

10 Basic Template Terminology....239

10.1 “Class Template” or “Template Class”?....239

10.2 Substitution, Instantiation, and Specialization....240

10.3 Declarations versus Definitions....241

10.3.1 Complete versus Incomplete Types....243

10.4 The One-Definition Rule....244

10.5 Template Arguments versus Template Parameters....244

10.6 Summary....246

11 Generic Libraries....248

11.1 Callables....248

11.1.1 Supporting Function Objects....249

11.1.2 Dealing with Member Functions and Additional Arguments....252

11.1.3 Wrapping Function Calls....255

11.2 Other Utilities to Implement Generic Libraries....257

11.2.1 Type Traits....258

11.2.2 std::addressof()....260

11.2.3 std::declval()....261

11.3 Perfect Forwarding Temporaries....262

11.4 References as Template Parameters....263

11.5 Defer Evaluations....267

11.6 Things to Consider When Writing Generic Libraries....269

11.7 Summary....270

Part II: Templates in Depth....272

12 Fundamentals in Depth....273

12.1 Parameterized Declarations....273

12.1.1 Virtual Member Functions....279

12.1.2 Linkage of Templates....280

12.1.3 Primary Templates....282

12.2 Template Parameters....283

12.2.1 Type Parameters....283

12.2.2 Nontype Parameters....284

12.2.3 Template Template Parameters....286

12.2.4 Template Parameter Packs....288

12.2.5 Default Template Arguments....290

12.3 Template Arguments....293

12.3.1 Function Template Arguments....293

12.3.2 Type Arguments....296

12.3.3 Nontype Arguments....296

12.3.4 Template Template Arguments....300

12.3.5 Equivalence....302

12.4 Variadic Templates....304

12.4.1 Pack Expansions....305

12.4.2 Where Can Pack Expansions Occur?....307

12.4.3 Function Parameter Packs....310

12.4.4 Multiple and Nested Pack Expansions....312

12.4.5 Zero-Length Pack Expansions....314

12.4.6 Fold Expressions....315

12.5 Friends....317

12.5.1 Friend Classes of Class Templates....317

12.5.2 Friend Functions of Class Templates....319

12.5.3 Friend Templates....321

12.6 Afternotes....322

13 Names in Templates....326

13.1 Name Taxonomy....326

13.2 Looking Up Names....330

13.2.1 Argument-Dependent Lookup....332

13.2.2 Argument-Dependent Lookup of Friend Declarations....335

13.2.3 Injected Class Names....336

13.2.4 Current Instantiations....338

13.3 Parsing Templates....340

13.3.1 Context Sensitivity in Nontemplates....341

13.3.2 Dependent Names of Types....345

13.3.3 Dependent Names of Templates....348

13.3.4 Dependent Names in Using Declarations....349

13.3.5 ADL and Explicit Template Arguments....352

13.3.6 Dependent Expressions....353

13.3.7 Compiler Errors....356

13.4 Inheritance and Class Templates....356

13.4.1 Nondependent Base Classes....357

13.4.2 Dependent Base Classes....358

13.5 Afternotes....362

14 Instantiation....367

14.1 On-Demand Instantiation....367

14.2 Lazy Instantiation....370

14.2.1 Partial and Full Instantiation....370

14.2.2 Instantiated Components....372

14.3 The C++ Instantiation Model....375

14.3.1 Two-Phase Lookup....375

14.3.2 Points of Instantiation....377

14.3.3 The Inclusion Model....381

14.4 Implementation Schemes....382

14.4.1 Greedy Instantiation....384

14.4.2 Queried Instantiation....386

14.4.3 Iterated Instantiation....388

14.5 Explicit Instantiation....389

14.5.1 Manual Instantiation....390

14.5.2 Explicit Instantiation Declarations....393

14.6 Compile-Time if Statements....394

14.7 In the Standard Library....396

14.8 Afternotes....397

15 Template Argument Deduction....402

15.1 The Deduction Process....402

15.2 Deduced Contexts....405

15.3 Special Deduction Situations....407

15.4 Initializer Lists....409

15.5 Parameter Packs....410

15.5.1 Literal Operator Templates....412

15.6 Rvalue References....413

15.6.1 Reference Collapsing Rules....414

15.6.2 Forwarding References....415

15.6.3 Perfect Forwarding....417

15.6.4 Deduction Surprises....421

15.7 SFINAE (Substitution Failure Is Not An Error)....422

15.7.1 Immediate Context....424

15.8 Limitations of Deduction....426

15.8.1 Allowable Argument Conversions....427

15.8.2 Class Template Arguments....429

15.8.3 Default Call Arguments....429

15.8.4 Exception Specifications....430

15.9 Explicit Function Template Arguments....431

15.10 Deduction from Initializers and Expressions....435

15.10.1 The auto Type Specifier....436

15.10.2 Expressing the Type of an Expression with decltype....442

15.10.3 decltype(auto)....446

15.10.4 Special Situations for auto Deduction....449

15.10.5 Structured Bindings....453

15.10.6 Generic Lambdas....458

15.11 Alias Templates....461

15.12 Class Template Argument Deduction....463

15.12.1 Deduction Guides....464

15.12.2 Implicit Deduction Guides....466

15.12.3 Other Subtleties....469

15.13 Afternotes....473

16 Specialization and Overloading....478

16.1 When “Generic Code” Doesn’t Quite Cut It....478

16.1.1 Transparent Customization....479

16.1.2 Semantic Transparency....481

16.2 Overloading Function Templates....483

16.2.1 Signatures....485

16.2.2 Partial Ordering of Overloaded Function Templates....488

16.2.3 Formal Ordering Rules....489

16.2.4 Templates and Nontemplates....491

16.2.5 Variadic Function Templates....495

16.3 Explicit Specialization....498

16.3.1 Full Class Template Specialization....499

16.3.2 Full Function Template Specialization....503

16.3.3 Full Variable Template Specialization....505

16.3.4 Full Member Specialization....506

16.4 Partial Class Template Specialization....509

16.5 Partial Variable Template Specialization....514

16.6 Afternotes....515

17 Future Directions....518

17.1 Relaxed typename Rules....519

17.2 Generalized Nontype Template Parameters....520

17.3 Partial Specialization of Function Templates....523

17.4 Named Template Arguments....525

17.5 Overloaded Class Templates....526

17.6 Deduction for Nonfinal Pack Expansions....527

17.7 Regularization of void....529

17.8 Type Checking for Templates....530

17.9 Reflective Metaprogramming....533

17.10 Pack Facilities....535

17.11 Modules....536

Part III: Templates and Design....539

18 The Polymorphic Power of Templates....540

18.1 Dynamic Polymorphism....540

18.2 Static Polymorphism....544

18.3 Dynamic versus Static Polymorphism....548

18.4 Using Concepts....550

18.5 New Forms of Design Patterns....553

18.6 Generic Programming....554

18.7 Afternotes....558

19 Implementing Traits....561

19.1 An Example: Accumulating a Sequence....562

19.1.1 Fixed Traits....562

19.1.2 Value Traits....566

19.1.3 Parameterized Traits....571

19.2 Traits versus Policies and Policy Classes....572

19.2.1 Traits and Policies: What’s the Difference?....574

19.2.2 Member Templates versus Template Template Parameters....576

19.2.3 Combining Multiple Policies and/or Traits....578

19.2.4 Accumulation with General Iterators....578

19.3 Type Functions....580

19.3.1 Element Types....581

19.3.2 Transformation Traits....584

19.3.3 Predicate Traits....591

19.3.4 Result Type Traits....595

19.4 SFINAE-Based Traits....598

19.4.1 SFINAE Out Function Overloads....599

19.4.2 SFINAE Out Partial Specializations....604

19.4.3 Using Generic Lambdas for SFINAE....606

19.4.4 SFINAE-Friendly Traits....610

19.5 IsConvertibleT....616

19.6 Detecting Members....620

19.6.1 Detecting Member Types....620

19.6.2 Detecting Arbitrary Member Types....622

19.6.3 Detecting Nontype Members....624

19.6.4 Using Generic Lambdas to Detect Members....629

19.7 Other Traits Techniques....632

19.7.1 If-Then-Else....632

19.7.2 Detecting Nonthrowing Operations....636

19.7.3 Traits Convenience....639

19.8 Type Classification....642

19.8.1 Determining Fundamental Types....643

19.8.2 Determining Compound Types....645

19.8.3 Identifying Function Types....650

19.8.4 Determining Class Types....652

19.8.5 Determining Enumeration Types....653

19.9 Policy Traits....654

19.9.1 Read-Only Parameter Types....655

19.10 In the Standard Library....658

19.11 Afternotes....660

20 Overloading on Type Properties....666

20.1 Algorithm Specialization....667

20.2 Tag Dispatching....669

20.3 Enabling/Disabling Function Templates....671

20.3.1 Providing Multiple Specializations....674

20.3.2 Where Does the EnableIf Go?....676

20.3.3 Compile-Time if....678

20.3.4 Concepts....680

20.4 Class Specialization....681

20.4.1 Enabling/Disabling Class Templates....682

20.4.2 Tag Dispatching for Class Templates....683

20.5 Instantiation-Safe Templates....687

20.6 In the Standard Library....693

20.7 Afternotes....695

21 Templates and Inheritance....697

21.1 The Empty Base Class Optimization (EBCO)....697

21.1.1 Layout Principles....698

21.1.2 Members as Base Classes....701

21.2 The Curiously Recurring Template Pattern (CRTP)....704

21.2.1 The Barton-Nackman Trick....707

21.2.2 Operator Implementations....710

21.2.3 Facades....712

21.3 Mixins....720

21.3.1 Curious Mixins....723

21.3.2 Parameterized Virtuality....723

21.4 Named Template Arguments....725

21.5 Afternotes....729

22 Bridging Static and Dynamic Polymorphism....732

22.1 Function Objects, Pointers, and std::function<>....732

22.2 Generalized Function Pointers....735

22.3 Bridge Interface....738

22.4 Type Erasure....739

22.5 Optional Bridging....740

22.6 Performance Considerations....744

22.7 Afternotes....744

23 Metaprogramming....746

23.1 The State of Modern C++ Metaprogramming....746

23.1.1 Value Metaprogramming....746

23.1.2 Type Metaprogramming....747

23.1.3 Hybrid Metaprogramming....748

23.1.4 Hybrid Metaprogramming for Unit Types....750

23.2 The Dimensions of Reflective Metaprogramming....752

23.3 The Cost of Recursive Instantiation....753

23.3.1 Tracking All Instantiations....755

23.4 Computational Completeness....756

23.5 Recursive Instantiation versus Recursive Template Arguments....757

23.6 Enumeration Values versus Static Constants....758

23.7 Afternotes....759

24 Typelists....762

24.1 Anatomy of a Typelist....762

24.2 Typelist Algorithms....765

24.2.1 Indexing....765

24.2.2 Finding the Best Match....767

24.2.3 Appending to a Typelist....770

24.2.4 Reversing a Typelist....773

24.2.5 Transforming a Typelist....774

24.2.6 Accumulating Typelists....776

24.2.7 Insertion Sort....779

24.3 Nontype Typelists....783

24.3.1 Deducible Nontype Parameters....786

24.4 Optimizing Algorithms with Pack Expansions....787

24.5 Cons-style Typelists....789

24.6 Afternotes....792

25 Tuples....794

25.1 Basic Tuple Design....795

25.1.1 Storage....795

25.1.2 Construction....797

25.2 Basic Tuple Operations....799

25.2.1 Comparison....800

25.2.2 Output....800

25.3 Tuple Algorithms....801

25.3.1 Tuples as Typelists....802

25.3.2 Adding to and Removing from a Tuple....803

25.3.3 Reversing a Tuple....805

25.3.4 Index Lists....806

25.3.5 Reversal with Index Lists....808

25.3.6 Shuffle and Select....810

25.4 Expanding Tuples....814

25.5 Optimizing Tuple....816

25.5.1 Tuples and the EBCO....816

25.5.2 Constant-time get()....821

25.6 Tuple Subscript....822

25.7 Afternotes....825

26 Discriminated Unions....828

26.1 Storage....829

26.2 Design....832

26.3 Value Query and Extraction....836

26.4 Element Initialization, Assignment and Destruction....837

26.4.1 Initialization....838

26.4.2 Destruction....839

26.4.3 Assignment....840

26.5 Visitors....846

26.5.1 Visit Result Type....849

26.5.2 Common Result Type....851

26.6 Variant Initialization and Assignment....854

26.7 Afternotes....858

27 Expression Templates....862

27.1 Temporaries and Split Loops....863

27.2 Encoding Expressions in Template Arguments....868

27.2.1 Operands of the Expression Templates....869

27.2.2 The Array Type....873

27.2.3 The Operators....876

27.2.4 Review....877

27.2.5 Expression Templates Assignments....880

27.3 Performance and Limitations of Expression Templates....882

27.4 Afternotes....883

28 Debugging Templates....887

28.1 Shallow Instantiation....888

28.2 Static Assertions....891

28.3 Archetypes....892

28.4 Tracers....895

28.5 Oracles....900

28.6 Afternotes....901

Appendixes....903

A The One-Definition Rule....903

A.1 Translation Units....903

A.2 Declarations and Definitions....904

A.3 The One-Definition Rule in Detail....906

A.3.1 One-per-Program Constraints....906

A.3.2 One-per-Translation Unit Constraints....909

A.3.3 Cross-Translation Unit Equivalence Constraints....911

B Value Categories....916

B.1 Traditional Lvalues and Rvalues....916

B.1.1 Lvalue-to-Rvalue Conversions....918

B.2 Value Categories Since C++11....918

B.2.1 Temporary Materialization....921

B.3 Checking Value Categories with decltype....923

B.4 Reference Types....924

C Overload Resolution....926

C.1 When Does Overload Resolution Kick In?....927

C.2 Simplified Overload Resolution....928

C.2.1 The Implied Argument for Member Functions....931

C.2.2 Refining the Perfect Match....933

C.3 Overloading Details....935

C.3.1 Prefer Nontemplates or More Specialized Templates....936

C.3.2 Conversion Sequences....936

C.3.3 Pointer Conversions....938

C.3.4 Initializer Lists....939

C.3.5 Functors and Surrogate Functions....943

C.3.6 Other Overloading Contexts....944

D Standard Type Utilities....947

D.1 Using Type Traits....947

D.1.1 std::integral_constant and std::bool_constant....947

D.1.2 Things You Should Know When Using Traits....949

D.2 Primary and Composite Type Categories....949

D.2.1 Testing for the Primary Type Category....950

D.2.2 Test for Composite Type Categories....953

D.3 Type Properties and Operations....954

D.3.1 Other Type Properties....955

D.3.2 Test for Specific Operations....962

D.3.3 Relationships Between Types....968

D.4 Type Construction....970

D.5 Other Traits....973

D.6 Combining Type Traits....975

D.7 Other Utilities....977

E Concepts....979

E.1 Using Concepts....980

E.2 Defining Concepts....983

E.3 Overloading on Constraints....985

E.3.1 Constraint Subsumption....986

E.3.2 Constraints and Tag Dispatching....988

E.4 Concept Tips....989

E.4.1 Testing Concepts....989

E.4.2 Concept Granularity....989

E.4.3 Binary Compatibility....991

Bibliography....1020

Forums....1020

Books and Web Sites....1021

Glossary....1033

Index....1050

Templates are among the most powerful features of C++, but they remain misunderstood and underutilized, even as the C++ language and development community have advanced. In C++ Templates, Second Edition, three pioneering C++ experts show why, when, and how to use modern templates to build software that’s cleaner, faster, more efficient, and easier to maintain.

Now extensively updated for the C++11, C++14, and C++17 standards, this new edition presents state-of-the-art techniques for a wider spectrum of applications. The authors provide authoritative explanations of all new language features that either improve templates or interact with them, including variadic templates, generic lambdas, class template argument deduction, compile-time if, forwarding references, and user-defined literals. They also deeply delve into fundamental language concepts (like value categories) and fully cover all standard type traits.

The book starts with an insightful tutorial on basic concepts and relevant language features. The remainder of the book serves as a comprehensive reference, focusing first on language details and then on coding techniques, advanced applications, and sophisticated idioms. Throughout, examples clearly illustrate abstract concepts and demonstrate best practices for exploiting all that C++ templates can do.

• Understand exactly how templates behave, and avoid common pitfalls
• Use templates to write more efficient, flexible, and maintainable software
• Master today’s most effective idioms and techniques
• Reuse source code without compromising performance or safety
• Benefit from utilities for generic programming in the C++ Standard Library
• Preview the upcoming concepts feature