[dcl.fct.spec]

# 9 Declarations [[dcl]](./#dcl)

## 9.2 Specifiers [[dcl.spec]](dcl.spec#dcl.fct.spec)

### 9.2.3 Function specifiers [dcl.fct.spec]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L689)

A[*function-specifier*](#nt:function-specifier "9.2.3 Function specifiers [dcl.fct.spec]") can be used only in a function declaration[.](#1.sentence-1)

At most one [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") and
at most one virtual keyword shall appear in
a [*decl-specifier-seq*](dcl.spec.general#nt:decl-specifier-seq "9.2.1 General [dcl.spec.general]")[.](#1.sentence-2)

[function-specifier:](#nt:function-specifier "9.2.3 Function specifiers [dcl.fct.spec]")  
virtual  
[*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")

[explicit-specifier:](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")  
explicit ( [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") )  
explicit

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L709)

The virtual specifier shall be used only in the initial
declaration of a non-static member function; see [[class.virtual]](class.virtual "11.7.3 Virtual functions")[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L714)

An [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") shall be used only in the declaration of
a constructor or conversion function within its class definition;
see [[class.conv.ctor]](class.conv.ctor "11.4.8.2 Conversion by constructor") and [[class.conv.fct]](class.conv.fct "11.4.8.3 Conversion functions")[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L720)

In an [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]"),
the [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]"), if supplied, shall be a
contextually converted constant expression of type bool ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#4.sentence-1)

The [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") explicit without a [*constant-expression*](expr.const#nt:constant-expression "7.7 Constant expressions [expr.const]") is equivalent to
the [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]") explicit(true)[.](#4.sentence-2)

If the constant expression evaluates to true,
the function is explicit[.](#4.sentence-3)

Otherwise, the function is not explicit[.](#4.sentence-4)

A ( token that follows explicit is parsed as
part of the [*explicit-specifier*](#nt:explicit-specifier "9.2.3 Function specifiers [dcl.fct.spec]")[.](#4.sentence-5)

[*Example [1](#example-1)*: struct S {explicit(sizeof(char[2])) S(char); // error: narrowing conversion of value 2 to type boolexplicit(sizeof(char)) S(bool); // OK, conversion of value 1 to type bool is non-narrowing}; — *end example*]