[concept.booleantestable]

# 18 Concepts library [[concepts]](./#concepts)

## 18.5 Comparison concepts [[concepts.compare]](concepts.compare#concept.booleantestable)

### 18.5.2 Boolean testability [concept.booleantestable]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L859)

The exposition-only [*boolean-testable*](#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]") concept
specifies the requirements on expressions
that are convertible to bool and
for which the logical operators ([[expr.log.and]](expr.log.and "7.6.14 Logical AND operator"), [[expr.log.or]](expr.log.or "7.6.15 Logical OR operator"), [[expr.unary.op]](expr.unary.op "7.6.2.2 Unary operators"))
have the conventional semantics[.](#1.sentence-1)

[🔗](#concept:boolean-testable-impl)

`template<class T>
  concept [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]") = [convertible_to](concept.convertible#concept:convertible_to "18.4.4 Concept convertible_­to [concept.convertible]")<T, bool>;  // exposition only
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L871)

Let e be an expression such thatdecltype((e)) is T[.](#2.sentence-1)

T models [*boolean-testable-impl*](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]") only if

- [(2.1)](#2.1)

either remove_cvref_t<T> is not a class type, or
a search for the names operator&& and operator|| in the scope of remove_cvref_t<T> finds nothing; and

- [(2.2)](#2.2)

argument-dependent lookup ([[basic.lookup.argdep]](basic.lookup.argdep "6.5.4 Argument-dependent name lookup"))
for the names operator&& and operator|| with T as the only argument type
finds no disqualifying declaration (defined below)[.](#2.sentence-2)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L889)

A [*disqualifying parameter*](#def:parameter,disqualifying "18.5.2 Boolean testability [concept.booleantestable]") is a function parameter whose declared type P

- [(3.1)](#3.1)

is not dependent on a template parameter, and
there exists an implicit conversion sequence ([[over.best.ics]](over.best.ics "12.2.4.2 Implicit conversion sequences"))
from e to P; or

- [(3.2)](#3.2)

is dependent on one or more template parameters, and either
  * [(3.2.1)](#3.2.1)

P contains no template parameter that
participates in template argument deduction ([[temp.deduct.type]](temp.deduct.type "13.10.3.6 Deducing template arguments from a type")), or

  * [(3.2.2)](#3.2.2)

template argument deduction
using the rules for deducing template arguments
in a function call ([[temp.deduct.call]](temp.deduct.call "13.10.3.2 Deducing template arguments from a function call")) ande as the argument succeeds[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L912)

A [*key parameter*](#def:parameter,key "18.5.2 Boolean testability [concept.booleantestable]") of a function template D is a function parameter of type cv X or reference thereto,
where X names a specialization of a class template that
has the same innermost enclosing non-inline namespace as D, andX contains at least one template parameter that
participates in template argument deduction[.](#4.sentence-1)

[*Example [1](#example-1)*:

Innamespace Z {template<class> struct C {}; template<class T>void operator&&(C<T> x, T y); template<class T>void operator||(C<type_identity_t<T>> x, T y);} the declaration of Z​::​operator&& contains one key parameter, C<T> x, and
the declaration of Z​::​operator|| contains no key parameters[.](#4.sentence-2)

— *end example*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L937)

A [*disqualifying declaration*](#def:declaration,disqualifying "18.5.2 Boolean testability [concept.booleantestable]") is

- [(5.1)](#5.1)

a (non-template) function declaration that
contains at least one disqualifying parameter; or

- [(5.2)](#5.2)

a function template declaration that
contains at least one disqualifying parameter, where
  * [(5.2.1)](#5.2.1)

at least one disqualifying parameter is a key parameter; or

  * [(5.2.2)](#5.2.2)

the declaration contains no key parameters; or

  * [(5.2.3)](#5.2.3)

the declaration declares a function template
to which no name is bound ([[dcl.meaning]](dcl.meaning "9.3.4 Meaning of declarators"))[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L955)

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

The intention is to ensure that
given two types T1 and T2 that each model [*boolean-testable-impl*](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]"),
the && and || operators within the expressionsdeclval<T1>() && declval<T2>() anddeclval<T1>() || declval<T2>() resolve to the corresponding built-in operators[.](#6.sentence-1)

— *end note*]

[🔗](#concept:boolean-testable)

`template<class T>
  concept [boolean-testable](#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]") =                // exposition only
    [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]")<T> && requires(T&& t) {
      { !std::forward<T>(t) } -> [boolean-testable-impl](#concept:boolean-testable-impl "18.5.2 Boolean testability [concept.booleantestable]");
    };
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L974)

Let e be an expression such thatdecltype((e)) is T[.](#7.sentence-1)

T models [*boolean-testable*](#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]") only ifbool(e) == !bool(!e)[.](#7.sentence-2)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/concepts.tex#L980)

[*Example [2](#example-2)*:

The typesbool,true_type ([[meta.type.synop]](meta.type.synop "21.3.3 Header <type_­traits> synopsis")),int*, andbitset<N>​::​reference ([[template.bitset]](template.bitset "22.9.2 Class template bitset"))
model [*boolean-testable*](#concept:boolean-testable "18.5.2 Boolean testability [concept.booleantestable]")[.](#8.sentence-1)

— *end example*]