3 Terms and definitions [intro.defs]

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For the purposes of this document, the terms and definitions given in ISO/IEC 2382, the terms, definitions, and symbols given in ISO 80000-2:2009, and the following apply.
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ISO and IEC maintain terminology databases for use in standardization at the following addresses:

3.1[defns.access]access

⟨execution-time action⟩ read or modify the value of an object
[Note 1: 
Only glvalues of scalar type can be used to access objects.
Reads of scalar objects are described in [conv.lval] and modifications of scalar objects are described in [expr.ass], [expr.post.incr], and [expr.pre.incr].
Attempts to read or modify an object of class type typically invoke a constructor or assignment operator; such invocations do not themselves constitute accesses, although they may involve accesses of scalar subobjects.
— end note]

3.2[defns.argument]argument

⟨function call expression⟩ expression in the comma-separated list bounded by the parentheses

3.3[defns.argument.macro]argument

⟨function-like macro⟩ sequence of preprocessing tokens in the comma-separated list bounded by the parentheses

3.4[defns.argument.throw]argument

⟨throw expression⟩ operand of throw

3.5[defns.argument.templ]argument

⟨template instantiation⟩ constant-expression, type-id, or id-expression in the comma-separated list bounded by the angle brackets

3.6[defns.block]block

⟨execution⟩ wait for some condition (other than for the implementation to execute the execution steps of the thread of execution) to be satisfied before continuing execution past the blocking operation

3.7[defns.block.stmt]block

⟨statement⟩ compound statement

3.8[defns.c.lib]C standard library

library described in ISO/IEC 9899:2018, Clause 7
[Note 1: 
With the qualifications noted in [support] through [thread] and in [diff.library], the C standard library is a subset of the C++ standard library.
— end note]

3.9[defns.character]character

⟨library⟩ object which, when treated sequentially, can represent text
[Note 1: 
The term does not mean only char, char8_t, char16_t, char32_t, and wchar_t objects ([basic.fundamental]), but any value that can be represented by a type that provides the definitions specified in [strings], [localization], [input.output], or [re].
— end note]

3.10[defns.character.container]character container type

⟨library⟩ class or a type used to represent a character ([defns.character])
[Note 1: 
It is used for one of the template parameters of the string, iostream, and regular expression class templates.
— end note]

3.11[defns.regex.collating.element]collating element

sequence of one or more characters ([defns.character]) within the current locale that collate as if they were a single character

3.12[defns.component]component

⟨library⟩ group of library entities directly related as members, parameters ([defns.parameter]), or return types
[Note 1: 
For example, the class template basic_string and the non-member function templates that operate on strings are referred to as the string component.
— end note]

3.13[defns.cond.supp]conditionally-supported

program construct that an implementation is not required to support
[Note 1: 
Each implementation documents all conditionally-supported constructs that it does not support.
— end note]

3.14[defns.const.subexpr]constant subexpression

expression whose evaluation as subexpression of a conditional-expression CE would not prevent CE from being a core constant expression

3.15[defns.deadlock]deadlock

⟨library⟩ situation wherein one or more threads are unable to continue execution because each is blocked ([defns.block]) waiting for one or more of the others to satisfy some condition

3.16[defns.default.behavior.impl]default behavior

⟨library implementation⟩ specific behavior provided by the implementation, within the scope of the required behavior ([defns.required.behavior])

3.17[defns.diagnostic]diagnostic message

message belonging to an implementation-defined subset of the implementation's output messages

3.18[defns.dynamic.type]dynamic type

⟨glvalue⟩ type of the most derived object to which the glvalue refers
[Example 1: 
If a pointer p whose static type is “pointer to class B” is pointing to an object of class D, derived from B, the dynamic type of the expression *p is “D”.
References are treated similarly.
— end example]

3.19[defns.dynamic.type.prvalue]dynamic type

⟨prvalue⟩ static type ([defns.static.type]) of the prvalue expression

3.20[defns.expression.equivalent]expression-equivalent

⟨library⟩ expressions that all have the same effects, either are all potentially-throwing or are all not potentially-throwing, and either are all constant subexpressions ([defns.const.subexpr]) or are all not constant subexpressions
[Example 1: 
For a value x of type int and a function f that accepts integer arguments, the expressions f(x + 2), f(2 + x), and f(1 + x + 1) are expression-equivalent.
— end example]

3.21[defns.regex.finite.state.machine]finite state machine

⟨regular expression⟩ unspecified data structure that is used to represent a regular expression ([defns.regex.regular.expression]), and which permits efficient matches against the regular expression to be obtained

3.22[defns.regex.format.specifier]format specifier

⟨regular expression⟩ sequence of one or more characters ([defns.character]) that is to be replaced with some part of a regular expression ([defns.regex.regular.expression]) match

3.23[defns.handler]handler function

⟨library⟩ non-reserved function whose definition may be provided by a C++ program
[Note 1: 
A C++ program may designate a handler function at various points in its execution by supplying a pointer to the function when calling any of the library functions that install handler functions ([support]).
— end note]

3.24[defns.ill.formed]ill-formed program

program that is not well-formed

3.25[defns.impl.defined]implementation-defined behavior

behavior, for a well-formed program ([defns.well.formed]) construct and correct data, that depends on the implementation and that each implementation documents

3.26[defns.order.ptr]implementation-defined strict total order over pointers

⟨library⟩ implementation-defined strict total ordering over all pointer values such that the ordering is consistent with the partial order imposed by the builtin operators <, >, <=, >=, and <=>

3.27[defns.impl.limits]implementation limit

restriction imposed upon programs by the implementation

3.28[defns.locale.specific]locale-specific behavior

behavior that depends on local conventions of nationality, culture, and language that each implementation documents

3.29[defns.regex.matched]matched

⟨regular expression⟩ condition when a sequence of zero or more characters ([defns.character]) correspond to a sequence of characters defined by the pattern

3.30[defns.modifier]modifier function

⟨library⟩ class member function other than a constructor, assignment operator, or destructor that alters the state of an object of the class

3.31[defns.move.assign]move assignment

⟨library⟩ assignment of an rvalue of some object type to a modifiable lvalue of the same type

3.32[defns.move.constr]move construction

⟨library⟩ direct-initialization of an object of some type with an rvalue of the same type

3.33[defns.nonconst.libcall]non-constant library call

invocation of a library function that, as part of evaluating any expression E, prevents E from being a core constant expression

3.34[defns.ntcts]NTCTS

⟨library⟩ sequence of values that have character ([defns.character]) type that precede the terminating null character type value charT()

3.35[defns.observer]observer function

⟨library⟩ class member function that accesses the state of an object of the class but does not alter that state
[Note 1: 
Observer functions are specified as const member functions.
— end note]

3.36[defns.parameter]parameter

⟨function or catch clause⟩ object or reference declared as part of a function declaration or definition or in the catch clause of an exception handler that acquires a value on entry to the function or handler

3.37[defns.parameter.macro]parameter

⟨function-like macro⟩ identifier from the comma-separated list bounded by the parentheses immediately following the macro name

3.38[defns.parameter.templ]parameter

⟨template⟩ member of a template-parameter-list

3.39[defns.regex.primary.equivalence.class]primary equivalence class

⟨regular expression⟩ set of one or more characters ([defns.character]) which share the same primary sort key: that is the sort key weighting that depends only upon character shape, and not accents, case, or locale specific tailorings

3.40[defns.prog.def.spec]program-defined specialization

⟨library⟩ explicit template specialization or partial specialization that is not part of the C++ standard library and not defined by the implementation

3.41[defns.prog.def.type]program-defined type

⟨library⟩ non-closure class type or enumeration type that is not part of the C++ standard library and not defined by the implementation, or a closure type of a non-implementation-provided lambda expression, or an instantiation of a program-defined specialization ([defns.prog.def.spec])
[Note 1: 
Types defined by the implementation include extensions ([intro.compliance]) and internal types used by the library.
— end note]

3.42[defns.projection]projection

⟨library⟩ transformation that an algorithm applies before inspecting the values of elements
[Example 1: 
std::pair<int, std::string_view> pairs[] = {{2, "foo"}, {1, "bar"}, {0, "baz"}}; std::ranges::sort(pairs, std::ranges::less{}, [](auto const& p) { return p.first; }); sorts the pairs in increasing order of their first members: {{0, "baz"}, {1, "bar"}, {2, "foo"}}
— end example]

3.43[defns.referenceable]referenceable type

type that is either an object type, a function type that does not have cv-qualifiers or a ref-qualifier, or a reference type
[Note 1: 
The term describes a type to which a reference can be created, including reference types.
— end note]

3.44[defns.regex.regular.expression]regular expression

pattern that selects specific strings from a set of character ([defns.character]) strings

3.45[defns.replacement]replacement function

⟨library⟩ non-reserved function whose definition is provided by a C++ program
[Note 1: 
Only one definition for such a function is in effect for the duration of the program's execution, as the result of creating the program ([lex.phases]) and resolving the definitions of all translation units ([basic.link]).
— end note]

3.46[defns.required.behavior]required behavior

⟨library⟩ description of replacement function ([defns.replacement]) and handler function ([defns.handler]) semantics applicable to both the behavior provided by the implementation and the behavior of any such function definition in the program
[Note 1: 
If such a function defined in a C++ program fails to meet the required behavior when it executes, the behavior is undefined.
— end note]

3.47[defns.reserved.function]reserved function

⟨library⟩ function, specified as part of the C++ standard library, that is defined by the implementation
[Note 1: 
If a C++ program provides a definition for any reserved function, the results are undefined.
— end note]

3.48[defns.signature]signature

⟨function⟩ name, parameter-type-list, and enclosing namespace
[Note 1: 
Signatures are used as a basis for name mangling and linking.
— end note]

3.49[defns.signature.friend]signature

⟨non-template friend function with trailing requires-clause⟩ name, parameter-type-list, enclosing class, and trailing requires-clause

3.50[defns.signature.templ]signature

⟨function template⟩ name, parameter-type-list, enclosing namespace, return type, signature ([defns.signature.template.head]) of the template-head, and trailing requires-clause (if any)

3.51[defns.signature.templ.friend]signature

⟨friend function template with constraint involving enclosing template parameters⟩ name, parameter-type-list, return type, enclosing class, signature ([defns.signature.template.head]) of the template-head, and trailing requires-clause (if any)

3.52[defns.signature.spec]signature

⟨function template specialization⟩ signature ([defns.signature.templ]) of the template of which it is a specialization and its template arguments ([defns.argument.templ]) (whether explicitly specified or deduced)

3.53[defns.signature.member]signature

⟨class member function⟩ name, parameter-type-list, class of which the function is a member, cv-qualifiers (if any), ref-qualifier (if any), and trailing requires-clause (if any)

3.54[defns.signature.member.templ]signature

⟨class member function template⟩ name, parameter-type-list, class of which the function is a member, cv-qualifiers (if any), ref-qualifier (if any), return type (if any), signature ([defns.signature.template.head]) of the template-head, and trailing requires-clause (if any)

3.55[defns.signature.member.spec]signature

⟨class member function template specialization⟩ signature ([defns.signature.member.templ]) of the member function template of which it is a specialization and its template arguments (whether explicitly specified or deduced)

3.56[defns.signature.template.head]signature

⟨template-head⟩ template parameter ([defns.parameter.templ]) list, excluding template parameter names and default arguments ([defns.argument.templ]), and requires-clause (if any)

3.57[defns.stable]stable algorithm

⟨library⟩ algorithm that preserves, as appropriate to the particular algorithm, the order of elements
[Note 1: 
Requirements for stable algorithms are given in [algorithm.stable].
— end note]

3.58[defns.static.type]static type

type of an expression resulting from analysis of the program without considering execution semantics
[Note 1: 
The static type of an expression depends only on the form of the program in which the expression appears, and does not change while the program is executing.
— end note]

3.59[defns.regex.subexpression]sub-expression

⟨regular expression⟩ subset of a regular expression ([defns.regex.regular.expression]) that has been marked by parentheses

3.60[defns.traits]traits class

⟨library⟩ class that encapsulates a set of types and functions necessary for class templates and function templates to manipulate objects of types for which they are instantiated

3.61[defns.unblock]unblock

satisfy a condition that one or more blocked ([defns.block]) threads of execution are waiting for

3.62[defns.undefined]undefined behavior

behavior for which this document imposes no requirements
[Note 1: 
Undefined behavior may be expected when this document omits any explicit definition of behavior or when a program uses an erroneous construct or erroneous data.
Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message ([defns.diagnostic])), to terminating a translation or execution (with the issuance of a diagnostic message).
Many erroneous program constructs do not engender undefined behavior; they are required to be diagnosed.
Evaluation of a constant expression ([expr.const]) never exhibits behavior explicitly specified as undefined in [intro] through [cpp].
— end note]

3.63[defns.unspecified]unspecified behavior

behavior, for a well-formed program ([defns.well.formed]) construct and correct data, that depends on the implementation
[Note 1: 
The implementation is not required to document which behavior occurs.
The range of possible behaviors is usually delineated by this document.
— end note]

3.64[defns.valid]valid but unspecified state

⟨library⟩ value of an object that is not specified except that the object's invariants are met and operations on the object behave as specified for its type
[Example 1: 
If an object x of type std​::​vector<int> is in a valid but unspecified state, x.empty() can be called unconditionally, and x.front() can be called only if x.empty() returns false.
— end example]

3.65[defns.well.formed]well-formed program

C++ program constructed according to the syntax and semantic rules