[over.literal]

# 12 Overloading [[over]](./#over)

## 12.6 User-defined literals [over.literal]

[literal-operator-id:](#nt:literal-operator-id "12.6 User-defined literals [over.literal]")  
operator [*unevaluated-string*](lex.string.uneval#nt:unevaluated-string "5.13.6 Unevaluated strings [lex.string.uneval]") [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]")  
operator [*user-defined-string-literal*](lex.ext#nt:user-defined-string-literal "5.13.9 User-defined literals [lex.ext]")

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4119)

The [*user-defined-string-literal*](lex.ext#nt:user-defined-string-literal "5.13.9 User-defined literals [lex.ext]") in a [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") shall have no [*encoding-prefix*](lex.ccon#nt:encoding-prefix "5.13.3 Character literals [lex.ccon]")[.](#1.sentence-1)

The [*unevaluated-string*](lex.string.uneval#nt:unevaluated-string "5.13.6 Unevaluated strings [lex.string.uneval]") or[*user-defined-string-literal*](lex.ext#nt:user-defined-string-literal "5.13.9 User-defined literals [lex.ext]") shall be empty[.](#1.sentence-2)

The [*ud-suffix*](lex.ext#nt:ud-suffix "5.13.9 User-defined literals [lex.ext]") of the [*user-defined-string-literal*](lex.ext#nt:user-defined-string-literal "5.13.9 User-defined literals [lex.ext]") or
the [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") in a [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") is called a[*literal suffix identifier*](#def:literal,suffix_identifier "12.6 User-defined literals [over.literal]")[.](#1.sentence-3)

The first form of [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") is deprecated ([[depr.lit]](depr.lit "D.8 Literal operator function declarations using an identifier"))[.](#1.sentence-4)

Some literal suffix identifiers are reserved for future standardization;
see [[usrlit.suffix]](usrlit.suffix "16.4.5.3.6 User-defined literal suffixes")[.](#1.sentence-5)

A declaration whose [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") uses
such a literal suffix identifier is ill-formed, no diagnostic required[.](#1.sentence-6)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4133)

A declaration whose [*declarator-id*](dcl.decl.general#nt:declarator-id "9.3.1 General [dcl.decl.general]") is a[*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") shall declare a function or function template
that belongs to a namespace (it could be a friend function ([[class.friend]](class.friend "11.8.4 Friends"))) or
an explicit instantiation or specialization of a function template[.](#2.sentence-1)

A function declared with a [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") is a [*literal
operator*](#def:literal,operator "12.6 User-defined literals [over.literal]")[.](#2.sentence-2)

A function template declared with a [*literal-operator-id*](#nt:literal-operator-id "12.6 User-defined literals [over.literal]") is a [*literal operator template*](#def:literal,operator,template "12.6 User-defined literals [over.literal]")[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4142)

The declaration of a literal operator shall have a[*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") equivalent to one of the following:const char*unsigned long long intlong doublecharwchar_tchar8_tchar16_tchar32_tconst char*, std::size_tconst wchar_t*, std::size_tconst char8_t*, std::size_tconst char16_t*, std::size_tconst char32_t*, std::size_t

If a parameter has a default argument ([[dcl.fct.default]](dcl.fct.default "9.3.4.7 Default arguments")), the program is
ill-formed[.](#3.sentence-2)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4164)

A [*raw literal operator*](#def:literal,operator,raw "12.6 User-defined literals [over.literal]") is a literal operator with a single parameter
whose type is const char*[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4168)

A [*numeric literal operator template*](#def:literal,operator,template_numeric "12.6 User-defined literals [over.literal]") is a literal operator template whose [*template-parameter-list*](temp.pre#nt:template-parameter-list "13.1 Preamble [temp.pre]") has a single [*template-parameter*](temp.param#nt:template-parameter "13.2 Template parameters [temp.param]") that is a constant template parameter pack ([[temp.variadic]](temp.variadic "13.7.4 Variadic templates"))
with element type char[.](#5.sentence-1)

A [*string literal operator template*](#def:literal,operator,template_string "12.6 User-defined literals [over.literal]") is a literal operator template whose [*template-parameter-list*](temp.pre#nt:template-parameter-list "13.1 Preamble [temp.pre]") comprises
a single [*parameter-declaration*](dcl.fct#nt:parameter-declaration "9.3.4.6 Functions [dcl.fct]") that declares a
constant template parameter of class type[.](#5.sentence-2)

The declaration of a literal operator template
shall have an empty [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]") and shall declare either a numeric literal operator template
or a string literal operator template[.](#5.sentence-3)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4184)

Literal operators and literal operator templates shall not have C language linkage[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4187)

[*Note [1](#note-1)*:

Literal operators and literal operator templates are usually invoked
implicitly through user-defined literals ([[lex.ext]](lex.ext "5.13.9 User-defined literals"))[.](#7.sentence-1)

However, except for
the constraints described above, they are ordinary namespace-scope functions and
function templates[.](#7.sentence-2)

In particular, they are looked up like ordinary functions
and function templates and they follow the same overload resolution rules[.](#7.sentence-3)

Also,
they can be declared inline or constexpr,
they can have internal, module, or external linkage,
they can be called explicitly, their addresses can be
taken, etc[.](#7.sentence-4)

— *end note*]

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/overloading.tex#L4200)

[*Example [1](#example-1)*: void operator ""_km(long double); // OK string operator "" _i18n(const char*, std::size_t); // OK, deprecatedtemplate <char...> double operator ""_\u03C0(); // OK, UCN for lowercase pifloat operator ""_e(const char*); // OKfloat operator ""E(const char*); // ill-formed, no diagnostic required:// reserved literal suffix ([[usrlit.suffix]](usrlit.suffix "16.4.5.3.6 User-defined literal suffixes"), [[lex.ext]](lex.ext "5.13.9 User-defined literals"))double operator""_Bq(long double); // OK, does not use the reserved [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") _Bq ([[lex.name]](lex.name "5.11 Identifiers"))double operator"" _Bq(long double); // ill-formed, no diagnostic required:// uses the reserved [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") _Bq ([[lex.name]](lex.name "5.11 Identifiers"))float operator " "B(const char*); // error: non-empty [*string-literal*](lex.string#nt:string-literal "5.13.5 String literals [lex.string]") string operator ""5X(const char*, std::size_t); // error: invalid literal suffix identifierdouble operator ""_miles(double); // error: invalid [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]")template <char...> int operator ""_j(const char*); // error: invalid [*parameter-declaration-clause*](dcl.fct#nt:parameter-declaration-clause "9.3.4.6 Functions [dcl.fct]")extern "C" void operator ""_m(long double); // error: C language linkage — *end example*]