C++ Initialization Story. A Guide Through All Initialization Options and Related C++ Areas

276 Pages·2022·2.304 MB·English

by  Bartłomiej Filipek

C++ Initialization Story
A Guide Through All Initialization Options and Related
C++ Areas
Bartłomiej Filipek
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©2021-2022BartłomiejFilipek
Contents
AbouttheBook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Whyshouldyoureadthisbook? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Learningobjectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Thestructureofthebook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Whoisthisbookfor? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Prerequisites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Readerfeedback&errata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
Examplecode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Codelicense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
Specialsections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
AbouttheAuthor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
RevisionHistory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix
1. LocalVariablesandSimpleTypes . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Startingwithsimpletypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Settingvaluestozero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Initializationwithaggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Defaultdatamemberinitialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. InitializationWithConstructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Asimpleclasstype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Basicsofconstructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Bodyofaconstructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
AddingconstructorstoDataPacket . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
CONTENTS
Compiler-generateddefaultconstructors . . . . . . . . . . . . . . . . . . . . . . . . 26
Explicitconstructorsandconversions . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Differencebetweendirectandcopyinitialization . . . . . . . . . . . . . . . . . . . 32
Implicitconversionandconvertingconstructors . . . . . . . . . . . . . . . . . . . . 34
Constructorsummary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3. CopyandMoveOperations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Copyconstructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Moveconstructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Distinguishingfromassignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Addingdebugloggingtoconstructors . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Trivialclassesanduser-provideddefaultconstructors . . . . . . . . . . . . . . . . . 59
4. DelegatingandInheritingConstructors . . . . . . . . . . . . . . . . . . . . . . . 64
Delegatingconstructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Inheritance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Inheritingconstructors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5. Destructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Objectsallocatedontheheap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Destructorsanddatamembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Virtualdestructorsandpolymorphism. . . . . . . . . . . . . . . . . . . . . . . . . . 80
Partiallycreatedobjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Acompiler-generateddestructor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Summaryandusecases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6. TypeDeductionandInitialization . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Deductionwithauto. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Rulesforautotypededuction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Deductionwithdecltype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Printingtypeinfo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
StructuredbindingsinC++17 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Lifetimeextension,references,andloops . . . . . . . . . . . . . . . . . . . . . . . . 103
AlmostAlwaysAuto . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
7. QuizfromChapters1…6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
CONTENTS
8. Non-StaticDataMemberInitialization . . . . . . . . . . . . . . . . . . . . . . . . 112
Howitworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
OtherformsofNSDMI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
CopyconstructorandNSDMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
MoveconstructorandNSDMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
C++14changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
C++20changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
LimitationsofNSDMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
NSDMI:AdvantagesandDisadvantages. . . . . . . . . . . . . . . . . . . . . . . . . 129
NSDMIsummary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
9. ContainersasDataMembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Thebasics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Usingstd::initializerlist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Exampleimplementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Thecostofcopyingelements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Someinconvenience-non-copyabletypes . . . . . . . . . . . . . . . . . . . . . . . 146
Moreoptions(advanced) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
10. Non-regularDataMembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Constantnon-staticdatamembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Pointersasdatamembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Smartpointersasdatamembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Referencesasdatamembers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
11. Non-localobjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Storagedurationandlinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Initializationofnon-localstaticobjects . . . . . . . . . . . . . . . . . . . . . . . . . 180
constinitinC++20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
Staticvariablesinafunctionscope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
Aboutstaticdatamembers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Motivationforinlinevariables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
Globalinlinevariables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
constexprandinlinevariables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
CONTENTS
12. AggregatesandDesignatedInitializersinC++20 . . . . . . . . . . . . . . . . . . 199
AggregatesinC++20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
ThebasicsofDesignatedInitializers . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
Advantagesofdesignatedinitialization . . . . . . . . . . . . . . . . . . . . . . . . . 204
Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
13. TechniquesandUseCases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Usingexplicitforstrongtypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Bestwaytoinitializestringdatamembers . . . . . . . . . . . . . . . . . . . . . . 212
Reducingextracopiesthroughemplaceandin_place . . . . . . . . . . . . . . . . 215
Thecopyandswapidiom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
CRTPclasscounter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
Severalinitializationtypesinoneclass . . . . . . . . . . . . . . . . . . . . . . . . . 226
Meyer’sSingletonandC++11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
Factorywithself-registeringtypesandstaticinitialization . . . . . . . . . . . . . . 231
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
14. TheFinalQuizAndExercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
AppendixA-RulesforSpecialMemberFunctionGeneration . . . . . . . . . . . . 248
Thediagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
AppendixB-QuizandExercisesAnswers . . . . . . . . . . . . . . . . . . . . . . . . 254
Thequizfromchapters1…6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
Thefinalquiz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254
Solutiontothefirstcodingproblem,NSDMI . . . . . . . . . . . . . . . . . . . . . . 255
Solutiontothesecondcodingproblem,NSDMI . . . . . . . . . . . . . . . . . . . . 255
Solutiontothethirdcodingproblem,inline . . . . . . . . . . . . . . . . . . . . . 257
Solutiontothefourthcodingproblem,fixcode . . . . . . . . . . . . . . . . . . . . 257
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259
About the Book
Initialization in C++ is a hot topic! The internet is full of discussions about best practices,
andthereareevenfunnymemesonthatsubject.Thesituationisnotsurprising,asthereare
morethanadozenwaystoinitializeasimpleintegervalue,complexrulesfortheauto-type
deduction,datamembers,andobjectlifetimenuances.
Andherecomesthebook.
Throughout this text, you will learn practical options to initialize various categories of
variables and data members in Modern C++. More specifically, this text teaches multiple
types of initialization, constructors, non-static data member initialization, inline variables,
designated initializers, and more. Additionally, you’ll see the changes and new techniques
fromC++11toC++20andlotsofexamplestoroundoutyourunderstanding.
The plan is to explain most (if not all) parts of initialization, learn lots of excellent C++
techniques,andseewhathappensunderthehood.
Why should you read this book?
With Modern C++ (since C++11), we have many new features to streamline work and
simplify our code. One area of improvement is the initialization. Modern C++ added new
initialization rules, trying to make it easy while keeping old behavior and compatibility
(mainly from the C language). Sometimes the rules might seem confusing and complex,
though,andeventheISOcommitteemightneedtocorrectsomethingsalongtheway.The
bookwillhelpyounavigatethroughthoseprinciplesandunderstandthistopicbetter.What’s
more,initializationisjustoneaspectofthistext.You’lllearnallrelatedtopicsaroundclasses,
constructors,destructors,objectlifetime,orevenhowthecompilerprocessesdataatstart-up.
Learning objectives
Thegoalistoequipyouwiththefollowingknowledge:
• Explain rules about object initialization, including regular variables, data members,
andnon-localobjects.
i
AbouttheBook ii
• How to implement special member functions (constructors, destructors, copy/move
operations)andwhentheyarehelpful.
• How to efficiently initialize non-static data members using C++11 features like non-
staticdatamemberinitialization,inheriting,anddelegatingconstructors.
• Howtostreamlineworkingwithstaticvariablesandstaticdatamemberswithinline
variablesfromC++17.
• How to work with container-like members, non-copyable data members (like const
datamembers)ormove-ableonlydatamembers,orevenlambdas.
• Whatisanaggregate,andhowtocreatesuchobjectswithdesignatedinitializersfrom
C++20.
The structure of the book
Thebookcontains14chaptersinthefollowingstructure:
• Chapters 1 to 5 create a foundation for the rest of the book. They cover basic
initializationrules,constructors,destructors,andthebasicsofdatamembers.
• Chapter6isashortquizonconstructors.Youcancheckyourknowledgefromthefirst
“part”ofthebook.
• Chapter7(inprogress):Typededuction.
• Chapter 8 describes Non-static Data Member Initialization (NSDMI), a powerful
feature from C++11 that improves how we work with data members. At the end of
thechapter,youcansolveafewexercises.
• Chapter9discusseshowtoinitializecontainer-likedatamembers.
• Chapter10containsinformationaboutnon-regulardatamembersandhowtohandle
theminaclass.You’lllearnaboutconstdatamembers,unique_ptrasadatamember,
andreferences.
• Chapter11describesstaticnon-localvariables,staticobjects,variousstorageduration
options,andinlinevariablesfromC++17andconstinitfromC++20.
• Chapter 12 moves to C++20 and describes Designated Initializers, a handy feature
basedonsimilarthingfromtheClanguage.
• Chapter 13 shows various techniques like passing strings into constructors, strong
typing,CRTPclasscounter,Copyandswapidiom,andmore.
• Chapter14isthefinalquizwithquestionsfromthewholebook.
AbouttheBook iii
Andtherearetwoappendices:
• Appendix A - a handy guide about rules for compiler-generated special member
functions.
• AppendixB-answerstoquizzesandexercises.
Who is this book for?
Thebookisintendedforbeginner/intermediateC++programmerswhowanttolearnvarious
aspectsofinitializationinModernC++(fromC++11toC++20).
Youshouldknowatleastsomeofthebasicsofcreatingandusingcustomclasses.
This text is also helpful for experienced programmers who know older C++ standards and
wanttomoveintoC++17/C++20.
Prerequisites
• YoushouldhavebasicknowledgeofC++expressionsandprimitivetypes.
• Youshouldbeabletoimplementanelementaryclasswithseveraldatamembers.Know
howtocreateandmanipulateobjectsofsuchaclassinabasicway.
Reader feedback & errata
Ifyouspotanerror,atypo,agrammarmistake,oranythingelse(especiallylogicalissues!)
thatshouldbecorrected,[email protected].
Here’stheerratawiththelistoffixes:
https://www.cppstories.com/p/cppinitbook/
Yourfeedbackmatters!Writinganhonestreviewcanhelpwiththebookpromotionandthe
qualityofmyfurtherwork.
What’smore,thebookhasadedicatedpageatGoodReads.Pleaseshareyourfeedbackat:
C++InitializationStorybyBartłomiejFilipek¹.
¹https://www.goodreads.com/book/show/62606823-c-initialization-story