Identifiers

An identifier is an arbitrarily long sequence of digits, underscores, lowercase and uppercase Latin letters, and most Unicode characters (see below for details). A valid identifier must begin with a non-digit character (Latin letter, underscore, or Unicode non-digit character). Identifiers are case-sensitive (lowercase and uppercase letters are distinct), and every character is significant.

Note: C++ grammar formally requires Unicode characters to be escaped with \u or \U, but due to translation phase 1, that is exactly how raw unicode characters from the source code are presented to the compiler. Also note that support of this feature may be limited, e.g. gcc

In declarations

An identifier can be used to name objects, references, functions, enumerators, types, class members, namespaces, templates, template specializations, parameter packs, goto labels, and other entities, with the following exceptions:

the identifiers that are keywords cannot be used for other purposes;
the identifiers with a double underscore anywhere are reserved;
the identifiers that begin with an underscore followed by an uppercase letter are reserved;
the identifiers that begin with an underscore are reserved in the global namespace.

"Reserved" here means that the standard library headers #define or declare such identifiers for their internal needs, the compiler may predefine non-standard identifiers of that kind, and that name mangling algorithm may assume that some of these identifiers are not in use. If the programmer uses such identifiers, the behavior is undefined.

In addition, it's undefined behavior to #define or #undef names identical to keywords. If at least one standard library header is included, it's undefined behavior to #define or #undef identifiers identical to names declared in any standard library header.

In expressions

An identifier that names a variable, a function, specialization of a concept, (since C++20) or an enumerator can be used as an expression. The result of an expression consisting of just the identifier is the entity named by the identifier. The value category of the expression is lvalue if the identifier names a function, a variable, a template parameter object (since C++20), or a data member, and prvalue otherwise (e.g. an enumerator is a prvalue expression, a specialization of a concept is a bool prvalue (since C++20)). The type of the expression is determined as follows:

If the entity named by the (unqualified) identifier is a local entity, and would result in an intervening lambda expression capturing it by copy if it were named outside of an unevaluated operand in the declarative region in which the identifier appears, then the type of the expression is the type of a class member access expression naming the non-static data member that would be declared for such a capture in the closure object of the innermost such intervening lambda expression.

void f() {
  float x, &r = x;
  [=] {
    decltype(x) y1;             // y1 has type float
    decltype((x)) y2 = y1;      // y2 has type float const& because this lambda
                                // is not mutable and x is an lvalue
    decltype(r) r1 = y1;        // r1 has type float&
    decltype((r)) r2 = y2;      // r2 has type float const&
  };
}

(since C++11)

If the entity named is a template parameter object for a template parameter of type T, the type of the expression is const T.

(since C++20)

Otherwise, the type of the expression is the same as the type of the entity named.

Within the body of a non-static member function, each identifier that names a non-static member is implicitly transformed to a class member access expression this->member.

Unqualified identifiers

Besides suitably declared identifiers, the following can be used in expressions in the same role:

an overloaded operator name in function notation, such as operator+ or operator new;
a user-defined conversion function name, such as operator bool;
a user-defined literal operator name, such as operator "" _km;
a template name followed by its argument list, such as MyTemplate<int>;
the character ~ followed by a class name, such as ~MyClass;
the character ~ followed by a decltype specifier, such as ~decltype(str).

Together with identifiers they are known as unqualified id-expressions.

Qualified identifiers

A qualified id-expression is an unqualified id-expression prepended by a scope resolution operator ::, and optionally, a sequence of enumeration, (since C++11)class or namespace names or decltype expressions (since C++11) separated by scope resolution operators. For example, the expression std::string::npos is an expression that names the static member npos in the class string in namespace std. The expression ::tolower names the function tolower in the global namespace. The expression ::std::cout names the global variable cout in namespace std, which is a top-level namespace. The expression boost::signals2::connection names the type connection declared in namespace signals2, which is declared in namespace boost.

The keyword template may appear in qualified identifiers as necessary to disambiguate dependent template names.

See qualified lookup for the details of the name lookup for qualified identifiers.

Names

A name is the use of one of the following to refer to an entity or to a label:

an identifier;
an overloaded operator name in function notation (operator+, operator new);
a user-defined conversion function name (operator bool);
a user-defined literal operator name (operator "" _km);
a template name followed by its argument list (MyTemplate<int>).

Every name that denotes an entity is introduced into the program by a declaration. Every name that denotes a label is introduced into the program either by a goto statement or by a labeled statement. A name used in more than one translation unit may refer to the same or different entities, depending on linkage.

When the compiler encounters an unknown name in a program, it associates it with the declaration that introduced the name by means of name lookup, except for the dependent names in template declarations and definitions (for those names, the compiler determines whether they name a type, a template, or some other entity, which may require explicit disambiguation).

Unicode characters in identifiers

The following Unicode character ranges are allowed in identifiers:

Code points	Description	Characters
U+00A8	DIARESIS
U+00AA	FEMININE ORDINAL INDICATOR
U+00AD	SOFT HYPHEN
U+00AF	MACRON
U+00B2 - U+00B5	SUPERSCRIPT TWO - MICRO SIGN
U+00B7 - U+00BA	MIDDLE DOT - MASCULINE ORDINAL INDICATOR
U+00BC - U+00BE	VULGAR FRACTION ONE QUARTER - VULGAR FRACTION THREE QUARTERS
U+00C0 - U+00D6	LATIN CAPITAL LETTER A WITH GRAVE - LATIN CAPITAL LETTER O WITH DIAERESIS	`...`
U+00D8 - U+00F6	LATIN CAPITAL LETTER O WITH STROKE - LATIN SMALL LETTER O WITH DIAERESIS	`...`
U+00F8 - U+167F	LATIN SMALL LETTER O WITH STROKE - CANADIAN SYLLABICS BLACKFOOT W	`...`
U+1681 - U+180D	OGHAM LETTER BEITH - MONGOLIAN FREE VARIATION SELECTOR THREE	`...`
U+180F - U+1FFF	SYRIAC LETTER BETH - GREEK DASIA	`...`
U+200B - U+200D	ZERO WIDTH SPACE - ZERO WIDTH JOINER
U+202A - U+202E	LEFT-TO-RIGHT EMBEDDING - RIGHT-TO-LEFT OVERRIDE
U+203F - U+2040	UNDERTIE - CHARACTER TIE
U+2054	INVERTED UNDERTIE
U+2060 - U+218F	WORD JOINER - TURNED DIGIT THREE	`...`
U+2460 - U+24FF	CIRCLED DIGIT ONE - NEGATIVE CIRCLED DIGIT ZERO	`...`
U+2776 - U+2793	DINGBAT NEGATIVE CIRCLED DIGIT ONE - DINGBAT NEGATIVE CIRCLED SANS-SERIF NUMBER TEN	`...`
U+2C00 - U+2DFF	GLAGOLITIC CAPITAL LETTER AZU - COMBINING CYRILLIC LETTER IOTIFIED BIG YUS	`...`
U+2E80 - U+2FFF	CJK RADICAL REPEAT - IDEOGRAPHIC DESCRIPTION CHARACTER OVERLAID	`...`
U+3004 - U+3007	JAPANESE INDUSTRIAL STANDARD SYMBOL - IDEOGRAPHIC NUMBER ZERO
U+3021 - U+302F	HANGZHOU NUMERAL ONE - HANGUL DOUBLE DOT TONE MARK	`...`
U+3031 - U+D7FF	VERTICAL KANA REPEAT MARK - HANGUL JONGSEONG PHIEUPH-THIEUTH	`...`
U+F900 - U+FD3D	CJK COMPATIBILITY IDEOGRAPH-F900 - ARABIC LIGATURE ALEF WITH FATHATAN ISOLATED FORM	`...`
U+FD40 - U+FDCF	ARABIC LIGATURE TEH WITH JEEM WITH MEEM INITIAL FORM - ARABIC LIGATURE NOON WITH JEEM WITH YEH FINAL FORM
U+FDF0 - U+FE44	ARABIC LIGATURE SALLA USED AS KORANIC STOP SIGN ISOLATED FORM - PRESENTATION FORM FOR VERTICAL RIGHT WHITE CORNER BRACKET	`...`
U+FE47 - U+FFFD	PRESENTATION FORM FOR VERTICAL LEFT SQUARE BRACKET - REPLACEMENT CHARACTER	`...`
U+10000 - U+1FFFD	LINEAR B SYLLABLE B008 A - CHEESE WEDGE (U+1F9C0)
U+20000 - U+2FFFD	<CJK Ideograph Extension B, First> - CJK COMPATIBILITY IDEOGRAPH-2FA1D (U+2FA1D)
U+30000 - U+3FFFD
U+40000 - U+4FFFD
U+50000 - U+5FFFD
U+60000 - U+6FFFD
U+70000 - U+7FFFD
U+80000 - U+8FFFD
U+90000 - U+9FFFD
U+A0000 - U+AFFFD
U+B0000 - U+BFFFD
U+C0000 - U+CFFFD
U+D0000 - U+DFFFD
U+E0000 - U+EFFFD	LANGUAGE TAG (U+E0001) - VARIATION SELECTOR-256 (U+E01EF)

The following Unicode character ranges are not allowed to begin an identifier:

Code points	Description	Characters
U+0300 - U+036F	COMBINING GRAVE ACCENT - COMBINING LATIN SMALL LETTER X
U+1DC0 - U+1DFF	COMBINING DOTTED GRAVE ACCENT - COMBINING RIGHT ARROWHEAD AND DOWN ARROWHEAD BELOW
U+20D0 - U+20FF	COMBINING LEFT HARPOON ABOVE - COMBINING ASTERISK ABOVE
U+FE20 - U+FE2F	COMBINING LIGATURE LEFT HALF - COMBINING CYRILLIC TITLO RIGHT HALF

Language
Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

language keywords
phases of translation
comments
the main() function
names and identifiers
types
fundamental types
objects
scope
object lifetime
storage duration and linkage
definitions and ODR
name lookup
qualified name lookup
unqualified name lookup
the as-if rule
undefined behavior
memory model