non-volatile non-template non-inline const-qualified variables (including constexpr) that aren't declared extern and aren't previously declared to have external Properties of entities with static and thread storage duration in C++17 # Property local const std::string s4 = "4"; // If not a literal type; implicitly static // Anywhere
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[PDF] Constexpr Lambda - Standard C++
28 avr 2015 · literal type if the type of each of its data-members is a literal type; In C++14, a lambda-expression is prohibited from appearing within a are verboten inside constant expressions; either because those functions can not be specified I'm confident we'll have a good answer for lambdas + constexpr in
[PDF] constexpr Introduction - Scott Schurr - CppCon 2015key
constexpr objects can't change at runtime Constant 9 • May be any literal type including: • Floating point types • Character literals • Pointer calculations might not have the same error: constexpr variable 'mask' must be initialized by a
[PDF] Generalized Constant Expressions - Bjarne Stroustrups Homepage
26 fév 2006 · the proposal for Literals for user-defined types [Str03], Generalized initializer In current C++, a variable or static data member declared const can be used in an integral A constant-expression function cannot be called before it is As for other const variables, storage need not be allocated for a constant-
[PDF] Permitting trivial default initialization in constexpr contexts - Open-std
15 juil 2019 · default initialization does not compile in constexpr As of C++17, this fails to compile meaning `Example1::f(const T&)` is not the need for this proposed change As always, undefined behavior cannot be invoked in constexpr (3 4 4) - a definition of a variable of non-literal type or of static or thread
[PDF] Exploring constexpr at Runtime - Open-std
13 mar 2013 · 9 1 How Does constexpr Participate in Overloading in C++11 Today? While declaring each variable constexpr is not onerous, it is error prone It's very easy to forget object shall have literal type and shall be initialized constexpr cannot be used to reliably perform such error checks during compilation
[PDF] Extern template constexpr - Squarespace
Today I have Daniele Parastrelli's guest post about extern templates for you It was thought to start in (C++20 or later) If the constexpr variable is not the following requirements: the return type of c++20 (up to C++20) must be literal in its return Destructors cannot be constexpr, but constant expressions can implicitly call
[PDF] Михаил Матросов
non-volatile non-template non-inline const-qualified variables (including constexpr) that aren't declared extern and aren't previously declared to have external Properties of entities with static and thread storage duration in C++17 # Property local const std::string s4 = "4"; // If not a literal type; implicitly static // Anywhere
[PDF] z/OS XL C/C++ Language Reference - IBM
14 déc 2020 · The decltype(expression) type specifier (C++11) The constexpr specifier The aligned variable attribute If you do not have the Adobe Reader, you can download it You cannot use a name with no linkage to declare an entity with Adjacent identifiers, keywords, and literals must be separated with
[PDF] c++ fopen exclusive
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Спецификаторы, квалификаторы и шаблоныМихаил МатросовAlign Technology
2Building a C/C++ program3a.cpp#include "a.hpp"#include "b.hpp"a.hppb.hppb.cpp#include "a.hpp"#include "b.hpp"a.ia.hppb.hppa.cppb.ia.hppb.hppb.cpppreprocesstranslation unitssource filesa.obj/.ob.obj/.ocompileobject filesresult.lib/.dll/.exe/.a/.so/.outlinklibrary / executable
Linkage5intsqr(intx){returnx*x;}intsqr(intx);boolcheck(inta,intb,intc){returnsqr(a)+sqr(b)==sqr(c);}a.cppb.cppexternal linkage⇒link time error: sqrnot found in b.objstaticintsqr(intx){returnx*x;}a'.cppinternal linkage
Linkage6can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scope
Storage duration7boolbeetlejuice(){intcounter=0;return++counter>=3;}automatic storage durationboolbeetlejuice(){static intcounter=0;return++counter>=3;}static storage duration
Storage duration•Applicable only to objects•Time of initialization/destruction is more complicated•Storage class specifiers: static, extern, thread_local, mutable8storage allocated/deallocatedautomaticWhen entering/leaving the scope or when creating/destroying an object of a classstaticWhen the program begins/endsthreadWhen the thread begins/endsdynamicWhen new/deleteis called
10Any of the following names declared at namespace scope have internal linkage:non-volatile non-template non-inline const-qualified variables (including constexpr) that aren't declared extern and aren't previously declared to have external linkage;11constdoublethickness=0.65;constchar*name="tooth";common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cpp⇒link time error: multiple definitions of symbol "name"internal linkageexternal linkage
Any of the following names declared at namespace scope have internal linkage:non-volatile non-template non-inline const-qualified variables (including constexpr) that aren't declared extern and aren't previously declared to have external linkage;12constexprdoublethickness=0.65;conststd::stringname="tooth";common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppAconstexprspecifier used in an object declarationimpliesconst.internal linkageinternal linkage
13#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 14inline conststd::stringname="tooth";constchar*getName();#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 15intsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal linkage⇒link time error: multiple definitions of symbol "sqr"
16constexprintsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal (weak) linkageA constexprspecifier used in a functionor static member variable declaration implies inline.
17voidother();structLocal{staticvoidfoo(){std::cout<<"main ";}};intmain(){Local::foo();other();}main.cppexternallinkagestructLocal{staticvoidfoo(){std::cout<<"other ";}};voidother(){Local::foo();}other.cpp⇒main mainexternallinkageEvery program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (9.4.1); no diagnostic required.
18voidother();namespace {structLocal{staticvoidfoo(){std::cout<<"main ";}};}intmain(){Local::foo();other();}main.cppinternallinkagenamespace {structLocal{staticvoidfoo(){std::cout<<"other ";}};}voidother(){Local::foo();}other.cpp⇒main otherinternallinkageIn addition, all names declared in unnamed namespace or a namespace within an unnamed namespace, even ones explicitly declared extern, have internal linkage.
Собираем в кучу19 Цвета имеют значение•statickeyword•static storage duration•internal linkage20
Допустимые комбинацииstorage duration и linkage21Storage durationno linkageinternal linkageexternal linkageautomaticlocal variablestaticstaticlocal variablestaticglobal variableglobal variablethreadthread_locallocal variablethread_localstatic global variablethread_localglobal variabledynamicLinkage
How to identifystorage duration of an object?22scope?staticnamespace (global)block (local) or class (member)yesnostatic?automaticthread_local?threadyesno
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8anonymous (unnamed) namespace⇒internal linkage23* if not volatileand not templateApply in orderEntity
24
26#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 27#include"common.hpp"#includeconststd::stringname="tooth";booldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage29* if not volatile,not template, and not previously declared externApply in orderEntity
Practice time!30
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage32* if not volatile,not template, and not previously declared externApply in orderEntity
34compile timerun time* when used to modify storage duration
Шаблоны36
38
39
inline никогда не подведёт40templateinlineconstexprboolis_const_v=is_const::value;
Long road to const43
44// header.hpp#definen42//Cannotcontainexpressions;simplyevilconstintn=42;//Duplicatedineachtranslationunitexternconstintn;//Initializerisnotvisible; need manual definitioninlineintn(){//Needsparenstobeused;returnsrvaluereturn42;}enum{n=42//Worksonlyforintegers};How to declare a constant before C++17?
45//header.hppinlineconstexprintn1=1;//Defaultchoiceinlineconststd::strings2="2";//Ifnotaliteraltype//source.cppormodule.ixxconstexprintn3=3;//Defaultchoice;implicitlystaticconststd::strings4="4";//Ifnotaliteraltype;implicitlystatic//AnywherestructA{staticconstexprintn=5;//Defaultchoice;implicitlyinlinestaticinlineconststd::strings="6";//Ifnotaliteraltype};voidf(){staticconstexprintn=7;//Defaultchoicestaticconststd::strings="8";//Ifnotaliteraltype}How to declare a constant with an initializer?
46//headers are no longer used//whatever possible is constexpr//module.ixxconstexprintn3=3;//AnywherestructA{staticconstexprintn=5;};voidf(){staticconstexprintn=7;}How to declare a constant with an initializer in an ideal world?
How to declare a constant with an initializer?47global?+staticyesnoliteral?global && header?constmember?+constexpr+const+inline+type name = init;noyesnoyesyesno1.If not global add static2.If possible add constexpr, otherwise add const3.If global in a header, or constmember, add inline
Пример48
templatestaticinlinethread_localconstexprconstvolatileTx={};Implied by constexpr templatestaticinlinethread_localconstexprvolatileTx={};Overridden by static constexprvolatile variable template with thread storage duration and internal linkagetemplatestaticthread_localconstexprvolatileTx={};thread storage durationinternal linkage
C++2052
Modules53can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scopecan be referred to fromexternal linkageAll modulesmodule linkageAll translation units within this moduleinternal linkageCurrent translation unitno linkageCurrent scopeC++17C++20
58
Properties of entities with static and thread storage duration in C++20**#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3consteval⇒constexprN/AN/AN/A4constexpr⇒const&& constinit5constexpr⇒inline6const⇒* internal linkageN/A7inline⇒external (weak) linkageN/A8static ⇒internal linkageRequiredRequiredRequired9extern ⇒external linkage (declaration)N/AN/ARedundantN/A10anonymous (unnamed) namespace⇒internal linkage59* if not volatile,not template, and not previously declared externApply in orderEntity** ignoring modules
constexprconst60 constexprconst61constexprconstchar*foo() {return"cppconf";} constexprconst62structParams{doubleparam1;doubleparam2;};constexprParamsGlobalParamsWithDescriptiveName={3.14,2.72};voidfoo(){constexprParams¶ms=GlobalParamsWithDescriptiveName;}⇒error: binding reference of type 'Params&' to 'constParams' discards qualifiers
constexprreferences64Paramsp;//Value;constParams*pcp=&p;//PointertoconstParams*constcpp=&p;//ConstpointerconstParams&rcp=p;//ReferencetoconstParams&constcrp=p;//Constreference(doesnotcompile)static_assert(!std::is_const_v);voidfoo(){constexprconstParams¶ms=GlobalParamsWithDescriptiveName;}Applies to objectApplies to reference itself
68
Подвал69
constexprtrap•constexprimpliesinline for a function•constexprfunctions with same signature and different bodies might interfere!-Note that return type is not part of a mangled name!-Different behavior depending on optimization!•If the functions weren't inline, the linker would warn you70
Random•An inline functionor variablewithexternal linkage must be declaredinlinein every translation unit-However, works for gcc/clang (clang -c; nm, weak symbol)-Indeed does not work for VC++ (dumpbin/symbols)74
Things to clarify•https://github.com/ericniebler/range-v3/blob/9930c89e005e203ff8294f461e5ac99277d3b088/include/range/v3/iterator/diffmax_t.hpp#L430-why need inline definitions for integral static members?•Does inline slow down linker?•linkage of template instantiations•inline and extern, https://stackoverflow.com/a/14017155/26121775
Переводы•implies -влечёт•linking -компоновка76
quotesdbs_dbs22.pdfusesText_28
14inline conststd::stringname="tooth";constchar*getName();#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 15intsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal linkage⇒link time error: multiple definitions of symbol "sqr"
16constexprintsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal (weak) linkageA constexprspecifier used in a functionor static member variable declaration implies inline.
17voidother();structLocal{staticvoidfoo(){std::cout<<"main ";}};intmain(){Local::foo();other();}main.cppexternallinkagestructLocal{staticvoidfoo(){std::cout<<"other ";}};voidother(){Local::foo();}other.cpp⇒main mainexternallinkageEvery program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (9.4.1); no diagnostic required.
18voidother();namespace {structLocal{staticvoidfoo(){std::cout<<"main ";}};}intmain(){Local::foo();other();}main.cppinternallinkagenamespace {structLocal{staticvoidfoo(){std::cout<<"other ";}};}voidother(){Local::foo();}other.cpp⇒main otherinternallinkageIn addition, all names declared in unnamed namespace or a namespace within an unnamed namespace, even ones explicitly declared extern, have internal linkage.
Собираем в кучу19 Цвета имеют значение•statickeyword•static storage duration•internal linkage20
Допустимые комбинацииstorage duration и linkage21Storage durationno linkageinternal linkageexternal linkageautomaticlocal variablestaticstaticlocal variablestaticglobal variableglobal variablethreadthread_locallocal variablethread_localstatic global variablethread_localglobal variabledynamicLinkage
How to identifystorage duration of an object?22scope?staticnamespace (global)block (local) or class (member)yesnostatic?automaticthread_local?threadyesno
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8anonymous (unnamed) namespace⇒internal linkage23* if not volatileand not templateApply in orderEntity
24
26#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 27#include"common.hpp"#includeconststd::stringname="tooth";booldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage29* if not volatile,not template, and not previously declared externApply in orderEntity
Practice time!30
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage32* if not volatile,not template, and not previously declared externApply in orderEntity
34compile timerun time* when used to modify storage duration
Шаблоны36
38
39
inline никогда не подведёт40templateinlineconstexprboolis_const_v=is_const::value;
Long road to const43
44// header.hpp#definen42//Cannotcontainexpressions;simplyevilconstintn=42;//Duplicatedineachtranslationunitexternconstintn;//Initializerisnotvisible; need manual definitioninlineintn(){//Needsparenstobeused;returnsrvaluereturn42;}enum{n=42//Worksonlyforintegers};How to declare a constant before C++17?
45//header.hppinlineconstexprintn1=1;//Defaultchoiceinlineconststd::strings2="2";//Ifnotaliteraltype//source.cppormodule.ixxconstexprintn3=3;//Defaultchoice;implicitlystaticconststd::strings4="4";//Ifnotaliteraltype;implicitlystatic//AnywherestructA{staticconstexprintn=5;//Defaultchoice;implicitlyinlinestaticinlineconststd::strings="6";//Ifnotaliteraltype};voidf(){staticconstexprintn=7;//Defaultchoicestaticconststd::strings="8";//Ifnotaliteraltype}How to declare a constant with an initializer?
46//headers are no longer used//whatever possible is constexpr//module.ixxconstexprintn3=3;//AnywherestructA{staticconstexprintn=5;};voidf(){staticconstexprintn=7;}How to declare a constant with an initializer in an ideal world?
How to declare a constant with an initializer?47global?+staticyesnoliteral?global && header?constmember?+constexpr+const+inline+type name = init;noyesnoyesyesno1.If not global add static2.If possible add constexpr, otherwise add const3.If global in a header, or constmember, add inline
Пример48
templatestaticinlinethread_localconstexprconstvolatileTx={};Implied by constexpr templatestaticinlinethread_localconstexprvolatileTx={};Overridden by static constexprvolatile variable template with thread storage duration and internal linkagetemplatestaticthread_localconstexprvolatileTx={};thread storage durationinternal linkage
C++2052
Modules53can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scopecan be referred to fromexternal linkageAll modulesmodule linkageAll translation units within this moduleinternal linkageCurrent translation unitno linkageCurrent scopeC++17C++20
58
Properties of entities with static and thread storage duration in C++20**#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3consteval⇒constexprN/AN/AN/A4constexpr⇒const&& constinit5constexpr⇒inline6const⇒* internal linkageN/A7inline⇒external (weak) linkageN/A8static ⇒internal linkageRequiredRequiredRequired9extern ⇒external linkage (declaration)N/AN/ARedundantN/A10anonymous (unnamed) namespace⇒internal linkage59* if not volatile,not template, and not previously declared externApply in orderEntity** ignoring modules
constexprconst60 constexprconst61constexprconstchar*foo() {return"cppconf";} constexprconst62structParams{doubleparam1;doubleparam2;};constexprParamsGlobalParamsWithDescriptiveName={3.14,2.72};voidfoo(){constexprParams¶ms=GlobalParamsWithDescriptiveName;}⇒error: binding reference of type 'Params&' to 'constParams' discards qualifiers
constexprreferences64Paramsp;//Value;constParams*pcp=&p;//PointertoconstParams*constcpp=&p;//ConstpointerconstParams&rcp=p;//ReferencetoconstParams&constcrp=p;//Constreference(doesnotcompile)static_assert(!std::is_const_v);voidfoo(){constexprconstParams¶ms=GlobalParamsWithDescriptiveName;}Applies to objectApplies to reference itself
68
Подвал69
constexprtrap•constexprimpliesinline for a function•constexprfunctions with same signature and different bodies might interfere!-Note that return type is not part of a mangled name!-Different behavior depending on optimization!•If the functions weren't inline, the linker would warn you70
Random•An inline functionor variablewithexternal linkage must be declaredinlinein every translation unit-However, works for gcc/clang (clang -c; nm, weak symbol)-Indeed does not work for VC++ (dumpbin/symbols)74
Things to clarify•https://github.com/ericniebler/range-v3/blob/9930c89e005e203ff8294f461e5ac99277d3b088/include/range/v3/iterator/diffmax_t.hpp#L430-why need inline definitions for integral static members?•Does inline slow down linker?•linkage of template instantiations•inline and extern, https://stackoverflow.com/a/14017155/26121775
Переводы•implies -влечёт•linking -компоновка76
quotesdbs_dbs22.pdfusesText_28
15intsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal linkage⇒link time error: multiple definitions of symbol "sqr"
16constexprintsqr(intx){returnx*x;}common.hpp#include"common.hpp"a.cpp#include"common.hpp"b.cppexternal (weak) linkageA constexprspecifier used in a functionor static member variable declaration implies inline.
17voidother();structLocal{staticvoidfoo(){std::cout<<"main ";}};intmain(){Local::foo();other();}main.cppexternallinkagestructLocal{staticvoidfoo(){std::cout<<"other ";}};voidother(){Local::foo();}other.cpp⇒main mainexternallinkageEvery program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (9.4.1); no diagnostic required.
18voidother();namespace {structLocal{staticvoidfoo(){std::cout<<"main ";}};}intmain(){Local::foo();other();}main.cppinternallinkagenamespace {structLocal{staticvoidfoo(){std::cout<<"other ";}};}voidother(){Local::foo();}other.cpp⇒main otherinternallinkageIn addition, all names declared in unnamed namespace or a namespace within an unnamed namespace, even ones explicitly declared extern, have internal linkage.
Собираем в кучу19Цвета имеют значение•statickeyword•static storage duration•internal linkage20
Допустимые комбинацииstorage duration и linkage21Storage durationno linkageinternal linkageexternal linkageautomaticlocal variablestaticstaticlocal variablestaticglobal variableglobal variablethreadthread_locallocal variablethread_localstatic global variablethread_localglobal variabledynamicLinkage
How to identifystorage duration of an object?22scope?staticnamespace (global)block (local) or class (member)yesnostatic?automaticthread_local?threadyesno
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8anonymous (unnamed) namespace⇒internal linkage23* if not volatileand not templateApply in orderEntity
2426#include"common.hpp"#includebooldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< 27#include"common.hpp"#includeconststd::stringname="tooth";booldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage29* if not volatile,not template, and not previously declared externApply in orderEntity
Practice time!30
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage32* if not volatile,not template, and not previously declared externApply in orderEntity
34compile timerun time* when used to modify storage duration
Шаблоны36
38
39
inline никогда не подведёт40templateinlineconstexprboolis_const_v=is_const::value;
Long road to const43
44// header.hpp#definen42//Cannotcontainexpressions;simplyevilconstintn=42;//Duplicatedineachtranslationunitexternconstintn;//Initializerisnotvisible; need manual definitioninlineintn(){//Needsparenstobeused;returnsrvaluereturn42;}enum{n=42//Worksonlyforintegers};How to declare a constant before C++17?
45//header.hppinlineconstexprintn1=1;//Defaultchoiceinlineconststd::strings2="2";//Ifnotaliteraltype//source.cppormodule.ixxconstexprintn3=3;//Defaultchoice;implicitlystaticconststd::strings4="4";//Ifnotaliteraltype;implicitlystatic//AnywherestructA{staticconstexprintn=5;//Defaultchoice;implicitlyinlinestaticinlineconststd::strings="6";//Ifnotaliteraltype};voidf(){staticconstexprintn=7;//Defaultchoicestaticconststd::strings="8";//Ifnotaliteraltype}How to declare a constant with an initializer?
46//headers are no longer used//whatever possible is constexpr//module.ixxconstexprintn3=3;//AnywherestructA{staticconstexprintn=5;};voidf(){staticconstexprintn=7;}How to declare a constant with an initializer in an ideal world?
How to declare a constant with an initializer?47global?+staticyesnoliteral?global && header?constmember?+constexpr+const+inline+type name = init;noyesnoyesyesno1.If not global add static2.If possible add constexpr, otherwise add const3.If global in a header, or constmember, add inline
Пример48
templatestaticinlinethread_localconstexprconstvolatileTx={};Implied by constexpr templatestaticinlinethread_localconstexprvolatileTx={};Overridden by static constexprvolatile variable template with thread storage duration and internal linkagetemplatestaticthread_localconstexprvolatileTx={};thread storage durationinternal linkage
C++2052
Modules53can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scopecan be referred to fromexternal linkageAll modulesmodule linkageAll translation units within this moduleinternal linkageCurrent translation unitno linkageCurrent scopeC++17C++20
58
Properties of entities with static and thread storage duration in C++20**#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3consteval⇒constexprN/AN/AN/A4constexpr⇒const&& constinit5constexpr⇒inline6const⇒* internal linkageN/A7inline⇒external (weak) linkageN/A8static ⇒internal linkageRequiredRequiredRequired9extern ⇒external linkage (declaration)N/AN/ARedundantN/A10anonymous (unnamed) namespace⇒internal linkage59* if not volatile,not template, and not previously declared externApply in orderEntity** ignoring modules
constexprconst60 constexprconst61constexprconstchar*foo() {return"cppconf";} constexprconst62structParams{doubleparam1;doubleparam2;};constexprParamsGlobalParamsWithDescriptiveName={3.14,2.72};voidfoo(){constexprParams¶ms=GlobalParamsWithDescriptiveName;}⇒error: binding reference of type 'Params&' to 'constParams' discards qualifiers
constexprreferences64Paramsp;//Value;constParams*pcp=&p;//PointertoconstParams*constcpp=&p;//ConstpointerconstParams&rcp=p;//ReferencetoconstParams&constcrp=p;//Constreference(doesnotcompile)static_assert(!std::is_const_v);voidfoo(){constexprconstParams¶ms=GlobalParamsWithDescriptiveName;}Applies to objectApplies to reference itself
68
Подвал69
constexprtrap•constexprimpliesinline for a function•constexprfunctions with same signature and different bodies might interfere!-Note that return type is not part of a mangled name!-Different behavior depending on optimization!•If the functions weren't inline, the linker would warn you70
Random•An inline functionor variablewithexternal linkage must be declaredinlinein every translation unit-However, works for gcc/clang (clang -c; nm, weak symbol)-Indeed does not work for VC++ (dumpbin/symbols)74
Things to clarify•https://github.com/ericniebler/range-v3/blob/9930c89e005e203ff8294f461e5ac99277d3b088/include/range/v3/iterator/diffmax_t.hpp#L430-why need inline definitions for integral static members?•Does inline slow down linker?•linkage of template instantiations•inline and extern, https://stackoverflow.com/a/14017155/26121775
Переводы•implies -влечёт•linking -компоновка76
quotesdbs_dbs22.pdfusesText_28
27#include"common.hpp"#includeconststd::stringname="tooth";booldumbCmp(constchar*s1,constchar*s2){returns1==s2;}intmain(){std::cout< Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage29* if not volatile,not template, and not previously declared externApply in orderEntity
Practice time!30
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage32* if not volatile,not template, and not previously declared externApply in orderEntity
34compile timerun time* when used to modify storage duration
Шаблоны36
38
39
inline никогда не подведёт40templateinlineconstexprboolis_const_v=is_const::value;
Long road to const43
44// header.hpp#definen42//Cannotcontainexpressions;simplyevilconstintn=42;//Duplicatedineachtranslationunitexternconstintn;//Initializerisnotvisible; need manual definitioninlineintn(){//Needsparenstobeused;returnsrvaluereturn42;}enum{n=42//Worksonlyforintegers};How to declare a constant before C++17?
45//header.hppinlineconstexprintn1=1;//Defaultchoiceinlineconststd::strings2="2";//Ifnotaliteraltype//source.cppormodule.ixxconstexprintn3=3;//Defaultchoice;implicitlystaticconststd::strings4="4";//Ifnotaliteraltype;implicitlystatic//AnywherestructA{staticconstexprintn=5;//Defaultchoice;implicitlyinlinestaticinlineconststd::strings="6";//Ifnotaliteraltype};voidf(){staticconstexprintn=7;//Defaultchoicestaticconststd::strings="8";//Ifnotaliteraltype}How to declare a constant with an initializer?
46//headers are no longer used//whatever possible is constexpr//module.ixxconstexprintn3=3;//AnywherestructA{staticconstexprintn=5;};voidf(){staticconstexprintn=7;}How to declare a constant with an initializer in an ideal world?
How to declare a constant with an initializer?47global?+staticyesnoliteral?global && header?constmember?+constexpr+const+inline+type name = init;noyesnoyesyesno1.If not global add static2.If possible add constexpr, otherwise add const3.If global in a header, or constmember, add inline
Пример48
templatestaticinlinethread_localconstexprconstvolatileTx={};Implied by constexpr templatestaticinlinethread_localconstexprvolatileTx={};Overridden by static constexprvolatile variable template with thread storage duration and internal linkagetemplatestaticthread_localconstexprvolatileTx={};thread storage durationinternal linkage
C++2052
Modules53can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scopecan be referred to fromexternal linkageAll modulesmodule linkageAll translation units within this moduleinternal linkageCurrent translation unitno linkageCurrent scopeC++17C++20
58
Properties of entities with static and thread storage duration in C++20**#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3consteval⇒constexprN/AN/AN/A4constexpr⇒const&& constinit5constexpr⇒inline6const⇒* internal linkageN/A7inline⇒external (weak) linkageN/A8static ⇒internal linkageRequiredRequiredRequired9extern ⇒external linkage (declaration)N/AN/ARedundantN/A10anonymous (unnamed) namespace⇒internal linkage59* if not volatile,not template, and not previously declared externApply in orderEntity** ignoring modules
constexprconst60 constexprconst61constexprconstchar*foo() {return"cppconf";} constexprconst62structParams{doubleparam1;doubleparam2;};constexprParamsGlobalParamsWithDescriptiveName={3.14,2.72};voidfoo(){constexprParams¶ms=GlobalParamsWithDescriptiveName;}⇒error: binding reference of type 'Params&' to 'constParams' discards qualifiers
constexprreferences64Paramsp;//Value;constParams*pcp=&p;//PointertoconstParams*constcpp=&p;//ConstpointerconstParams&rcp=p;//ReferencetoconstParams&constcrp=p;//Constreference(doesnotcompile)static_assert(!std::is_const_v);voidfoo(){constexprconstParams¶ms=GlobalParamsWithDescriptiveName;}Applies to objectApplies to reference itself
68
Подвал69
constexprtrap•constexprimpliesinline for a function•constexprfunctions with same signature and different bodies might interfere!-Note that return type is not part of a mangled name!-Different behavior depending on optimization!•If the functions weren't inline, the linker would warn you70
Random•An inline functionor variablewithexternal linkage must be declaredinlinein every translation unit-However, works for gcc/clang (clang -c; nm, weak symbol)-Indeed does not work for VC++ (dumpbin/symbols)74
Things to clarify•https://github.com/ericniebler/range-v3/blob/9930c89e005e203ff8294f461e5ac99277d3b088/include/range/v3/iterator/diffmax_t.hpp#L430-why need inline definitions for integral static members?•Does inline slow down linker?•linkage of template instantiations•inline and extern, https://stackoverflow.com/a/14017155/26121775
Переводы•implies -влечёт•linking -компоновка76
quotesdbs_dbs22.pdfusesText_28
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage29* if not volatile,not template, and not previously declared externApply in orderEntity
Practice time!30
Properties of entities with static and thread storage duration in C++17#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3constexpr⇒const4constexpr⇒inline5const⇒* internal linkageN/A6inline⇒external (weak) linkageN/A7static ⇒internal linkageRequiredRequiredRequired8extern ⇒external linkage (declaration)N/AN/ARedundantN/A9anonymous (unnamed) namespace⇒internal linkage32* if not volatile,not template, and not previously declared externApply in orderEntity
34compile timerun time* when used to modify storage duration
Шаблоны36
3839
inline никогда не подведёт40template
Long road to const43
44// header.hpp#definen42//Cannotcontainexpressions;simplyevilconstintn=42;//Duplicatedineachtranslationunitexternconstintn;//Initializerisnotvisible; need manual definitioninlineintn(){//Needsparenstobeused;returnsrvaluereturn42;}enum{n=42//Worksonlyforintegers};How to declare a constant before C++17?
45//header.hppinlineconstexprintn1=1;//Defaultchoiceinlineconststd::strings2="2";//Ifnotaliteraltype//source.cppormodule.ixxconstexprintn3=3;//Defaultchoice;implicitlystaticconststd::strings4="4";//Ifnotaliteraltype;implicitlystatic//AnywherestructA{staticconstexprintn=5;//Defaultchoice;implicitlyinlinestaticinlineconststd::strings="6";//Ifnotaliteraltype};voidf(){staticconstexprintn=7;//Defaultchoicestaticconststd::strings="8";//Ifnotaliteraltype}How to declare a constant with an initializer?
46//headers are no longer used//whatever possible is constexpr//module.ixxconstexprintn3=3;//AnywherestructA{staticconstexprintn=5;};voidf(){staticconstexprintn=7;}How to declare a constant with an initializer in an ideal world?
How to declare a constant with an initializer?47global?+staticyesnoliteral?global && header?constmember?+constexpr+const+inline+type name = init;noyesnoyesyesno1.If not global add static2.If possible add constexpr, otherwise add const3.If global in a header, or constmember, add inline
Пример48
templateconstexprvolatile variable template with thread storage duration and internal linkagetemplate
C++2052
Modules53can be referred to fromexternal linkageAll translation unitsinternal linkageCurrent translation unitno linkageCurrent scopecan be referred to fromexternal linkageAll modulesmodule linkageAll translation units within this moduleinternal linkageCurrent translation unitno linkageCurrent scopeC++17C++20
58Properties of entities with static and thread storage duration in C++20**#Propertylocalvariableglobal variablemember variableglobal functionmember function1no linkage2external linkage3consteval⇒constexprN/AN/AN/A4constexpr⇒const&& constinit5constexpr⇒inline6const⇒* internal linkageN/A7inline⇒external (weak) linkageN/A8static ⇒internal linkageRequiredRequiredRequired9extern ⇒external linkage (declaration)N/AN/ARedundantN/A10anonymous (unnamed) namespace⇒internal linkage59* if not volatile,not template, and not previously declared externApply in orderEntity** ignoring modules
constexprconst60 constexprconst61constexprconstchar*foo() {return"cppconf";}constexprconst62structParams{doubleparam1;doubleparam2;};constexprParamsGlobalParamsWithDescriptiveName={3.14,2.72};voidfoo(){constexprParams¶ms=GlobalParamsWithDescriptiveName;}⇒error: binding reference of type 'Params&' to 'constParams' discards qualifiers
constexprreferences64Paramsp;//Value;constParams*pcp=&p;//PointertoconstParams*constcpp=&p;//ConstpointerconstParams&rcp=p;//ReferencetoconstParams&constcrp=p;//Constreference(doesnotcompile)static_assert(!std::is_const_v
Подвал69
constexprtrap•constexprimpliesinline for a function•constexprfunctions with same signature and different bodies might interfere!-Note that return type is not part of a mangled name!-Different behavior depending on optimization!•If the functions weren't inline, the linker would warn you70
Random•An inline functionor variablewithexternal linkage must be declaredinlinein every translation unit-However, works for gcc/clang (clang -c; nm, weak symbol)-Indeed does not work for VC++ (dumpbin/symbols)74
Things to clarify•https://github.com/ericniebler/range-v3/blob/9930c89e005e203ff8294f461e5ac99277d3b088/include/range/v3/iterator/diffmax_t.hpp#L430-why need inline definitions for integral static members?•Does inline slow down linker?•linkage of template instantiations•inline and extern, https://stackoverflow.com/a/14017155/26121775
Переводы•implies -влечёт•linking -компоновка76
quotesdbs_dbs22.pdfusesText_28