cpp/language/default initialization - Revision history

D41D8CD98F: /* Special cases */ reword

2026-04-26T10:06:16Z

Special cases: reword

← Older revision		Revision as of 10:06, 26 April 2026
Line 92:		Line 92:
	}}		}}

	If an evaluation ~~{{c\|eval}}~~ produces an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}} of an uninitialized-friendly type, the behavior is well-defined in the ~~following cases~~:		If an evaluation produces an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}} of an uninitialized-friendly type, the behavior is well-defined if the evaluation is:

	* ~~{{c\|eval}} is~~ the evaluation of one of the following expressions and operands:		* the evaluation of one of the following expressions and operands:
	:* The second or third operand of a {{rlp\|operator other#Conditional operator\|conditional expression}}.		:* The second or third operand of a {{rlp\|operator other#Conditional operator\|conditional expression}}.
	:* The right operand of a {{rlp\|operator other#Built-in comma operator\|comma expression}}.		:* The right operand of a {{rlp\|operator other#Built-in comma operator\|comma expression}}.
Line 101:		Line 101:
	: In this case, the result of the operation is the uninitialized result value of {{c\|value}}.		: In this case, the result of the operation is the uninitialized result value of {{c\|value}}.

	* ~~{{c\|eval}} is~~ an evaluation of the right operand of a {{rlp\|operator assignment#Built-in simple assignment operator\|simple assignment operator}} whose left operand is an lvalue of an uninitialized-friendly type.		* an evaluation of the right operand of a {{rlp\|operator assignment#Built-in simple assignment operator\|simple assignment operator}} whose left operand is an lvalue of an uninitialized-friendly type.
	: In this case, the value of the object referred to by the left operand is replaced by the uninitialized result value of {{c\|value}}.		: In this case, the value of the object referred to by the left operand is replaced by the uninitialized result value of {{c\|value}}.

	* ~~{{c\|eval}} is~~ the evaluation of the initialization expression when initializing an object of an uninitialized-friendly type.		* the evaluation of the initialization expression when initializing an object of an uninitialized-friendly type.
	~~{{rrev\|since=c++17\|~~
	~~:* {{c\|value}} cannot be of type {{ltt std\|cpp/types/byte}} if the object being initialized is not of type {{ltt std\|cpp/types/byte}}.~~
	}}
	: In this case, that object is initialized to the uninitialized result value of {{c\|value}}.		: In this case, that object is initialized to the uninitialized result value of {{c\|value}}.

D41D8CD98F: + std::meta::info, CWG 3089

2026-04-26T09:39:17Z

+ std::meta::info, CWG 3089

← Older revision		Revision as of 09:39, 26 April 2026
Line 29:		Line 29:

	===Default-initialization of a const object===		===Default-initialization of a const object===
	If a program calls for the default-initialization of an object of a {{rlp\|cv\|const}}-qualified type {{tt\|T}}, T ~~shall~~ be a ''const-default-constructible'' ~~class~~ type ~~or array thereof~~.		If a program calls for the default-initialization of an object of a {{rlp\|cv\|const}}-qualified type {{tt\|T}}, T must be a ''const-default-constructible'' type.

	A ~~class~~ type {{tt\|T}} is const-default-constructible if ~~default~~-initialization of {{tt\|T}} would invoke a user-provided constructor of {{tt\|T}} {{rev inl\|since=c++11\|(not inherited from a base class)}} ~~or if~~		A type {{tt\|T}} is const-default-constructible if
			{{rev begin}}
			{{rev\|since=c++26\|
			* {{tt\|T}} is {{ltt\|cpp/meta/info\|std::meta::info}}.
			}}
			{{rev\|since=c++11\|
			* {{tt\|T}} is {{lc\|std::nullptr_t}}.
			}}
			{{rev end}}
			* Default-initialization of {{tt\|T}} would invoke a user-provided constructor of {{tt\|T}} {{rev inl\|since=c++11\|(not inherited from a base class)}}.
			* {{tt\|T}} is a class type such that:
	{{rrev multi\|until1=c++11\|rev1=		{{rrev multi\|until1=c++11\|rev1=
	* each direct non-static data member ~~{{tt\|M}}~~ of {{tt\|T}} is of class type {{tt\|X}} (or array thereof), {{tt\|X}} is const-default-constructible, and		:* each direct non-static data member of {{tt\|T}} is of class type {{tt\|X}} (or array thereof), {{tt\|X}} is const-default-constructible, and
	* {{tt\|T}} has no direct {{rlp\|union#Union-like classes\|variant members}}, and		:* {{tt\|T}} has no direct {{rlp\|union#Union-like classes\|variant members}}, and
			:* each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.
	\|rev2=		\|rev2=
	* each direct non-variant non-static data member ~~{{tt\|M}}~~ of {{tt\|T}} has a {{rlp\|data members#Member initialization\|default member initializer}} or~~, if {{tt\|M}}~~ is of ~~class type {{tt\|X}} (or array thereof), {{tt\|X}} is~~ const-default-constructible,		:* each direct non-variant non-static data member of {{tt\|T}} has a {{rlp\|data members#Member initialization\|default member initializer}} or is of const-default-constructible type,
	* if {{tt\|T}} is a union with at least one non-static data member, exactly one {{rlp\|union#Union-like classes\|variant member}} has a default member initializer,		:* if {{tt\|T}} is a union with at least one non-static data member, exactly one {{rlp\|union#Union-like classes\|variant member}} has a default member initializer,
	* if {{tt\|T}} is not a union, ~~for~~ each anonymous union member ~~with at least one non~~-~~static data member (if any)~~, ~~exactly one non~~-~~static data member has a~~ default ~~member initializer, and~~		:* if {{tt\|T}} is not a union, each anonymous union member is const-default-constructible, and
			:* each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.
	}}		}}
	~~each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of~~ {{tt\|T}} is const-default-~~constructible~~.		* {{tt\|T}} is an array of const-default-initializable type.

	===Indeterminate and erroneous values===		===Indeterminate and erroneous values===
Line 198:		Line 210:
	{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}		{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}
	{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}		{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}
			{{dr list item\|wg=cwg\|dr=3089\|std=C++11\|before={{tt\|std::nullptr_t}} is not const-default-initializable\|after=made const-default-initializable}}
	{{dr list end}}		{{dr list end}}

D41D8CD98F: fix layout

2026-04-26T09:19:21Z

fix layout

← Older revision		Revision as of 09:19, 26 April 2026
Line 24:		Line 24:
	* otherwise, no initialization is performed (see [[#Notes\|notes]]).		* otherwise, no initialization is performed (see [[#Notes\|notes]]).

	{{~~rev begin}}~~		{{rrev\|until=c++11\|
	~~{{rev~~\|until=c++11\|
	Only (possibly cv-qualified) non-POD class types (or arrays thereof) with automatic storage duration were considered to be default-initialized when no initializer is used. Scalars and POD types with dynamic storage duration were considered to be not initialized (since C++11, this situation was reclassified as a form of default-initialization).		Only (possibly cv-qualified) non-POD class types (or arrays thereof) with automatic storage duration were considered to be default-initialized when no initializer is used. Scalars and POD types with dynamic storage duration were considered to be not initialized (since C++11, this situation was reclassified as a form of default-initialization).
	}}		}}

D41D8CD98F: /* Explanation */ + std::meta::info

2026-04-26T09:16:59Z

Explanation: + std::meta::info

← Older revision		Revision as of 09:16, 26 April 2026
Line 19:		Line 19:
	* if {{tt\|T}} is a (possibly cv-qualified) {{rev inl\|until=c++11\|non-POD}} class type, the constructors are considered and subjected to {{rlp\|overload resolution}} against the empty argument list. The constructor selected (which is one of the {{rlp\|default constructor}}s) is called to provide the initial value for the new object;		* if {{tt\|T}} is a (possibly cv-qualified) {{rev inl\|until=c++11\|non-POD}} class type, the constructors are considered and subjected to {{rlp\|overload resolution}} against the empty argument list. The constructor selected (which is one of the {{rlp\|default constructor}}s) is called to provide the initial value for the new object;
	* if {{tt\|T}} is an array type, every element of the array is default-initialized;		* if {{tt\|T}} is an array type, every element of the array is default-initialized;
			{{rrev\|since=c++26\|
			* if {{tt\|T}} is {{ltt\|cpp/meta/info\|std::meta::info}}, the object is initialized to the null reflection value;
			}}
	* otherwise, no initialization is performed (see [[#Notes\|notes]]).		* otherwise, no initialization is performed (see [[#Notes\|notes]]).

Xmcgcg: signed char -> unsigned char.

2024-04-23T07:42:41Z

signed char -> unsigned char.

← Older revision		Revision as of 07:42, 23 April 2024
Line 69:		Line 69:
	* {{ltt std\|cpp/types/byte}}		* {{ltt std\|cpp/types/byte}}
	}}		}}
	* {{c/core\|~~signed~~ char}}		* {{c/core\|unsigned char}}
	* {{c/core\|char}}, if its underlying type is {{c/core\|~~signed~~ char}}		* {{c/core\|char}}, if its underlying type is {{c/core\|unsigned char}}

	Given an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}}, the ''uninitialized result value'' of {{c\|value}} is:<!-- for exposition only, not defined in the standard -->		Given an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}}, the ''uninitialized result value'' of {{c\|value}} is:<!-- for exposition only, not defined in the standard -->

Xmcgcg: Applied P2795R5 (Erroneous behaviour for uninitialized reads).

2024-04-23T05:53:33Z

Applied P2795R5 (Erroneous behaviour for uninitialized reads).

Show changes

D41D8CD98F: /* Notes */

2024-04-20T03:20:17Z

Notes

← Older revision		Revision as of 03:20, 20 April 2024
Line 63:		Line 63:

	===Notes===		===Notes===
	Default-initialization of non-class variables with automatic and dynamic storage duration produces objects with indeterminate values (static and thread-local objects get {{rlp\|zero initialization\|zero initialized}}).		Default-initialization of non-class variables and data members with automatic and dynamic storage duration produces objects with indeterminate values (static and thread-local objects get {{rlp\|zero initialization\|zero initialized}}).

	References and const scalar objects cannot be default-initialized.		References and const scalar objects cannot be default-initialized.

2A02:587:7E12:3800:EDD7:94:E609:C379: Undo revision 160305 by WhatRhymesWithBest? (talk) Unwarranted undo, clearly vandalism

2023-10-08T11:47:28Z

Undo revision 160305 by WhatRhymesWithBest? (talk) Unwarranted undo, clearly vandalism

← Older revision		Revision as of 11:47, 8 October 2023
Line 5:		Line 5:
	===Syntax===		===Syntax===
	{{sdsc begin}}		{{sdsc begin}}
	{{sdsc\|num=1\|{{spar\|T~~}} {{spar\|~~object}} {{ttb\|;}}}}		{{sdsc\|num=1\|{{spar sep\|T object}}{{ttb\|;}}}}
	{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}		{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}
	}}		}}
Line 38:		Line 38:
	* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and		* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and
	}}		}}
	each potentially constructed base class of {{tt\|T}} is const-default-constructible.		each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.

	====Read from an indeterminate byte====		====Read from an indeterminate byte====
	Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:		Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from
	:* the second or third operand of a conditional expression,		:* the second or third operand of a conditional expression,
	:* the right operand of the comma operator,		:* the right operand of the comma operator,
	:* the operand of a cast or conversion to (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},		:* the operand of a cast or conversion to (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},
	:* a discarded-value expression.		:* a discarded-value expression.

Line 79:		Line 79:
	{		{
	int mem;		int mem;
	T2() { } // "mem" is not in the initializer list		T2() {} // "mem" is not in the initializer list
	};		};

Line 105:		Line 105:
	===Defect reports===		===Defect reports===
	{{dr list begin}}		{{dr list begin}}
	{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there's no value-initialization; empty initializer ~~invoke~~ default-~~init~~<br>(though {{tt\|new T()}} also performs zero-~~init~~)\|after=empty initializer ~~invoke~~ value-~~init~~}}		{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there was no value-initialization;<br>empty initializer invoked default-initialization<br>(though {{c\|new T()}} also performs zero-initialization)\|after=empty initializer invokes<br>value-initialization}}
	{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}		{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}
	{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any uninitialized object was always UB\|after=indeterminate {{c\|unsigned char}} is allowed}}		{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}
	{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c\|unsigned char}} cached in a register was UB\|after=made well-defined}}		{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}
	{{dr list end}}		{{dr list end}}

WhatRhymesWithBest?: Undo revision 160301 by Xmcgcg (talk)

2023-10-08T10:55:40Z

Undo revision 160301 by Xmcgcg (talk)

← Older revision		Revision as of 10:55, 8 October 2023
Line 5:		Line 5:
	===Syntax===		===Syntax===
	{{sdsc begin}}		{{sdsc begin}}
	{{sdsc\|num=1\|{{spar ~~sep~~\|T object}}{{ttb\|;}}}}		{{sdsc\|num=1\|{{spar\|T}} {{spar\|object}} {{ttb\|;}}}}
	{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}		{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}
	}}		}}
Line 38:		Line 38:
	* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and		* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and
	}}		}}
	each ~~{{rlp\|object#Potentially constructed subobjects\|~~potentially constructed}} base class of {{tt\|T}} is const-default-constructible.		each potentially constructed base class of {{tt\|T}} is const-default-constructible.

	====Read from an indeterminate byte====		====Read from an indeterminate byte====
	Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:		Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:
	* if an indeterminate value of type {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);		* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);
	* if an indeterminate value of type {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};		* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};
	* if an indeterminate value of type {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from		* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from
	:* the second or third operand of a conditional expression,		:* the second or third operand of a conditional expression,
	:* the right operand of the comma operator,		:* the right operand of the comma operator,
	:* the operand of a cast or conversion to (possibly cv-qualified) {{c~~/core~~\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},		:* the operand of a cast or conversion to (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},
	:* a discarded-value expression.		:* a discarded-value expression.

Line 79:		Line 79:
	{		{
	int mem;		int mem;
	T2() {} // "mem" is not in the initializer list		T2() { } // "mem" is not in the initializer list
	};		};

Line 105:		Line 105:
	===Defect reports===		===Defect reports===
	{{dr list begin}}		{{dr list begin}}
	{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there ~~was~~ no value-initialization;~~<br>~~empty initializer ~~invoked~~ default-~~initialization~~<br>(though {{c\|new T()}} also performs zero-~~initialization~~)\|after=empty initializer ~~invokes<br>~~value-~~initialization~~}}		{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there's no value-initialization; empty initializer invoke default-init<br>(though {{tt\|new T()}} also performs zero-init)\|after=empty initializer invoke value-init}}
	{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}		{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}
	{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any~~<br>~~uninitialized object was always UB\|after=indeterminate {{c~~/core~~\|unsigned char}} is allowed}}		{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any uninitialized object was always UB\|after=indeterminate {{c\|unsigned char}} is allowed}}
	{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c~~/core~~\|unsigned char}}~~<br>~~cached in a register was UB\|after=made well-defined}}		{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c\|unsigned char}} cached in a register was UB\|after=made well-defined}}
	{{dr list end}}		{{dr list end}}

Xmcgcg: Link to the definition of 'potentially constructed subobject'.

2023-10-08T03:34:59Z

Link to the definition of 'potentially constructed subobject'.

← Older revision		Revision as of 03:34, 8 October 2023
Line 5:		Line 5:
	===Syntax===		===Syntax===
	{{sdsc begin}}		{{sdsc begin}}
	{{sdsc\|num=1\|{{spar\|T~~}} {{spar\|~~object}} {{ttb\|;}}}}		{{sdsc\|num=1\|{{spar sep\|T object}}{{ttb\|;}}}}
	{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}		{{sdsc\|num=2\|{{ttb\|new}} {{spar\|T}}
	}}		}}
Line 38:		Line 38:
	* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and		* if {{tt\|T}} is not a union, for each anonymous union member with at least one non-static data member (if any), exactly one non-static data member has a default member initializer, and
	}}		}}
	each potentially constructed base class of {{tt\|T}} is const-default-constructible.		each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.

	====Read from an indeterminate byte====		====Read from an indeterminate byte====
	Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:		Use of an indeterminate value obtained by default-initializing a non-class variable of any type is {{rlp\|ub\|undefined behavior}} (in particular, it may be a {{rlp\|object#Object_representation_and_value_representation\|trap representation}}), except in the following cases:
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is assigned to another variable of type (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} (the value of the variable becomes indeterminate, but the behavior is not undefined);
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} is used to initialize another variable of type (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}};
	* if an indeterminate value of type {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from		* if an indeterminate value of type {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}} results from
	:* the second or third operand of a conditional expression,		:* the second or third operand of a conditional expression,
	:* the right operand of the comma operator,		:* the right operand of the comma operator,
	:* the operand of a cast or conversion to (possibly cv-qualified) {{c\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},		:* the operand of a cast or conversion to (possibly cv-qualified) {{c/core\|unsigned char}} {{rev inl\|since=c++17\|or {{ltt\|cpp/types/byte\|std::byte}}}},
	:* a discarded-value expression.		:* a discarded-value expression.

Line 79:		Line 79:
	{		{
	int mem;		int mem;
	T2() { } // "mem" is not in the initializer list		T2() {} // "mem" is not in the initializer list
	};		};

Line 105:		Line 105:
	===Defect reports===		===Defect reports===
	{{dr list begin}}		{{dr list begin}}
	{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there's no value-initialization; empty initializer ~~invoke~~ default-~~init~~<br>(though {{tt\|new T()}} also performs zero-~~init~~)\|after=empty initializer ~~invoke~~ value-~~init~~}}		{{dr list item\|wg=cwg\|dr=178\|std=C++98\|before=there was no value-initialization;<br>empty initializer invoked default-initialization<br>(though {{c\|new T()}} also performs zero-initialization)\|after=empty initializer invokes<br>value-initialization}}
	{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}		{{dr list item\|wg=cwg\|dr=253\|std=C++98\|before=default-initialization of a const object could not<br>call an implicitly declared default constructor\|after=allowed if all subobjects are initialized}}
	{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any uninitialized object was always UB\|after=indeterminate {{c\|unsigned char}} is allowed}}		{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}
	{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c\|unsigned char}} cached in a register was UB\|after=made well-defined}}		{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}
	{{dr list end}}		{{dr list end}}

← Older revision		Revision as of 10:06, 26 April 2026
Line 92:		Line 92:
	}}		}}

	If an evaluation ~~{{c\|eval}}~~ produces an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}} of an uninitialized-friendly type, the behavior is well-defined in the ~~following cases~~:		If an evaluation produces an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}} of an uninitialized-friendly type, the behavior is well-defined if the evaluation is:

	* ~~{{c\|eval}} is~~ the evaluation of one of the following expressions and operands:		* the evaluation of one of the following expressions and operands:
	:* The second or third operand of a {{rlp\|operator other#Conditional operator\|conditional expression}}.		:* The second or third operand of a {{rlp\|operator other#Conditional operator\|conditional expression}}.
	:* The right operand of a {{rlp\|operator other#Built-in comma operator\|comma expression}}.		:* The right operand of a {{rlp\|operator other#Built-in comma operator\|comma expression}}.
Line 101:		Line 101:
	: In this case, the result of the operation is the uninitialized result value of {{c\|value}}.		: In this case, the result of the operation is the uninitialized result value of {{c\|value}}.

	* ~~{{c\|eval}} is~~ an evaluation of the right operand of a {{rlp\|operator assignment#Built-in simple assignment operator\|simple assignment operator}} whose left operand is an lvalue of an uninitialized-friendly type.		* an evaluation of the right operand of a {{rlp\|operator assignment#Built-in simple assignment operator\|simple assignment operator}} whose left operand is an lvalue of an uninitialized-friendly type.
	: In this case, the value of the object referred to by the left operand is replaced by the uninitialized result value of {{c\|value}}.		: In this case, the value of the object referred to by the left operand is replaced by the uninitialized result value of {{c\|value}}.

	* ~~{{c\|eval}} is~~ the evaluation of the initialization expression when initializing an object of an uninitialized-friendly type.		* the evaluation of the initialization expression when initializing an object of an uninitialized-friendly type.
	~~{{rrev\|since=c++17\|~~
	~~:* {{c\|value}} cannot be of type {{ltt std\|cpp/types/byte}} if the object being initialized is not of type {{ltt std\|cpp/types/byte}}.~~
	}}
	: In this case, that object is initialized to the uninitialized result value of {{c\|value}}.		: In this case, that object is initialized to the uninitialized result value of {{c\|value}}.

← Older revision		Revision as of 09:39, 26 April 2026
Line 29:		Line 29:

	===Default-initialization of a const object===		===Default-initialization of a const object===
	If a program calls for the default-initialization of an object of a {{rlp\|cv\|const}}-qualified type {{tt\|T}}, T ~~shall~~ be a ''const-default-constructible'' ~~class~~ type ~~or array thereof~~.		If a program calls for the default-initialization of an object of a {{rlp\|cv\|const}}-qualified type {{tt\|T}}, T must be a ''const-default-constructible'' type.

	A ~~class~~ type {{tt\|T}} is const-default-constructible if ~~default~~-initialization of {{tt\|T}} would invoke a user-provided constructor of {{tt\|T}} {{rev inl\|since=c++11\|(not inherited from a base class)}} ~~or if~~		A type {{tt\|T}} is const-default-constructible if
			{{rev begin}}
			{{rev\|since=c++26\|
			* {{tt\|T}} is {{ltt\|cpp/meta/info\|std::meta::info}}.
			}}
			{{rev\|since=c++11\|
			* {{tt\|T}} is {{lc\|std::nullptr_t}}.
			}}
			{{rev end}}
			* Default-initialization of {{tt\|T}} would invoke a user-provided constructor of {{tt\|T}} {{rev inl\|since=c++11\|(not inherited from a base class)}}.
			* {{tt\|T}} is a class type such that:
	{{rrev multi\|until1=c++11\|rev1=		{{rrev multi\|until1=c++11\|rev1=
	* each direct non-static data member ~~{{tt\|M}}~~ of {{tt\|T}} is of class type {{tt\|X}} (or array thereof), {{tt\|X}} is const-default-constructible, and		:* each direct non-static data member of {{tt\|T}} is of class type {{tt\|X}} (or array thereof), {{tt\|X}} is const-default-constructible, and
	* {{tt\|T}} has no direct {{rlp\|union#Union-like classes\|variant members}}, and		:* {{tt\|T}} has no direct {{rlp\|union#Union-like classes\|variant members}}, and
			:* each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.
	\|rev2=		\|rev2=
	* each direct non-variant non-static data member ~~{{tt\|M}}~~ of {{tt\|T}} has a {{rlp\|data members#Member initialization\|default member initializer}} or~~, if {{tt\|M}}~~ is of ~~class type {{tt\|X}} (or array thereof), {{tt\|X}} is~~ const-default-constructible,		:* each direct non-variant non-static data member of {{tt\|T}} has a {{rlp\|data members#Member initialization\|default member initializer}} or is of const-default-constructible type,
	* if {{tt\|T}} is a union with at least one non-static data member, exactly one {{rlp\|union#Union-like classes\|variant member}} has a default member initializer,		:* if {{tt\|T}} is a union with at least one non-static data member, exactly one {{rlp\|union#Union-like classes\|variant member}} has a default member initializer,
	* if {{tt\|T}} is not a union, ~~for~~ each anonymous union member ~~with at least one non~~-~~static data member (if any)~~, ~~exactly one non~~-~~static data member has a~~ default ~~member initializer, and~~		:* if {{tt\|T}} is not a union, each anonymous union member is const-default-constructible, and
			:* each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of {{tt\|T}} is const-default-constructible.
	}}		}}
	~~each {{rlp\|object#Potentially constructed subobjects\|potentially constructed}} base class of~~ {{tt\|T}} is const-default-~~constructible~~.		* {{tt\|T}} is an array of const-default-initializable type.

	===Indeterminate and erroneous values===		===Indeterminate and erroneous values===
Line 198:		Line 210:
	{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}		{{dr list item\|wg=cwg\|dr=616\|std=C++98\|before=lvalue to rvalue conversion of any<br>uninitialized object was always UB\|after=indeterminate {{c/core\|unsigned char}} is allowed}}
	{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}		{{dr list item\|wg=cwg\|dr=1787\|std=C++98\|before=read from an indeterminate {{c/core\|unsigned char}}<br>cached in a register was UB\|after=made well-defined}}
			{{dr list item\|wg=cwg\|dr=3089\|std=C++11\|before={{tt\|std::nullptr_t}} is not const-default-initializable\|after=made const-default-initializable}}
	{{dr list end}}		{{dr list end}}

← Older revision		Revision as of 09:16, 26 April 2026
Line 19:		Line 19:
	* if {{tt\|T}} is a (possibly cv-qualified) {{rev inl\|until=c++11\|non-POD}} class type, the constructors are considered and subjected to {{rlp\|overload resolution}} against the empty argument list. The constructor selected (which is one of the {{rlp\|default constructor}}s) is called to provide the initial value for the new object;		* if {{tt\|T}} is a (possibly cv-qualified) {{rev inl\|until=c++11\|non-POD}} class type, the constructors are considered and subjected to {{rlp\|overload resolution}} against the empty argument list. The constructor selected (which is one of the {{rlp\|default constructor}}s) is called to provide the initial value for the new object;
	* if {{tt\|T}} is an array type, every element of the array is default-initialized;		* if {{tt\|T}} is an array type, every element of the array is default-initialized;
			{{rrev\|since=c++26\|
			* if {{tt\|T}} is {{ltt\|cpp/meta/info\|std::meta::info}}, the object is initialized to the null reflection value;
			}}
	* otherwise, no initialization is performed (see [[#Notes\|notes]]).		* otherwise, no initialization is performed (see [[#Notes\|notes]]).

← Older revision		Revision as of 07:42, 23 April 2024
Line 69:		Line 69:
	* {{ltt std\|cpp/types/byte}}		* {{ltt std\|cpp/types/byte}}
	}}		}}
	* {{c/core\|~~signed~~ char}}		* {{c/core\|unsigned char}}
	* {{c/core\|char}}, if its underlying type is {{c/core\|~~signed~~ char}}		* {{c/core\|char}}, if its underlying type is {{c/core\|unsigned char}}

	Given an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}}, the ''uninitialized result value'' of {{c\|value}} is:<!-- for exposition only, not defined in the standard -->		Given an indeterminate{{rev inl\|since=c++26\| or erroneous}} value {{c\|value}}, the ''uninitialized result value'' of {{c\|value}} is:<!-- for exposition only, not defined in the standard -->