std::monostate

 

C++

Language

Standard library headers

Concepts

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

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

optional, any and variant (C++17)

optional

any

variant

in_placein_place_typein_place_indexin_place_tin_place_type_tin_place_index_t

Pairs and tuples

pair

tuple (C++11)

apply (C++17)

make_from_tuple (C++17)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

integer_sequence (C++14)

Swap, forward and move

swap

exchange (C++14)

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

Elementary string conversions

to_chars (C++17)

from_chars (C++17)

chars_format (C++17)

Type operations

declval (C++11)

as_const (C++17)

 

std::variant

Member functions

variant::variant

variant::~variant

variant::operator=

Observers

variant::index

variant::valueless_by_exception

Modifiers

variant::emplace

variant::swap

Non-member functions

visit

holds_alternative

get

get_if

operator==operator!=operator<operator<=operator>operator>=

swap

Helper classes

monostate

bad_variant_access

variant_size

variant_alternative

hash

uses_allocator

Helper objects

variant_npos

 

Defined in header <variant>
struct monostate { }		(since C++17)

Unit type intended for use as a well-behaved empty alternative in std::variant. In particular, a variant of non-default-constructible types may list std::monostate as its first alternative: this makes the variant itself default-contructible.

Member functions

(constructor) (implicitly declared)	trivial implicit default/copy/move constructor (public member function)
(destructor) (implicitly declared)	trivial implicit destructor (public member function)
operator= (implicitly declared)	trivial implicit copy/move assignment (public member function)

Non-member functions

constexpr bool operator<(monostate, monostate) noexcept { return false; } constexpr bool operator>(monostate, monostate) noexcept { return false; } constexpr bool operator<=(monostate, monostate) noexcept { return true; } constexpr bool operator>=(monostate, monostate) noexcept { return true; } constexpr bool operator==(monostate, monostate) noexcept { return true; } constexpr bool operator!=(monostate, monostate) noexcept { return false; }

All instances of std::monostate compare equal.

Exceptions

noexcept specification:  

noexcept

  

Helper classes

std::hash<std::monostate>

template <> struct std::hash<monostate>;

Specializes the std::hash algorithm for std::monostate

Example

Run this code

#include <variant>
#include <iostream>
 
struct S
{
    S(int i) : i(i) {}
    int i;
};
 
int main() {
 
    // Without the monostate type this declaration will fail.
    // This is because S is not default-constructible.
 
    std::variant<std::monostate, S> var; 
 
    // var.index() is now 0 - the first element
    // std::get<S> will throw! We need to assign a value
 
    var = 12; 
 
    std::cout << std::get<S>(var).i << '\n';
}

Output:

12

See also

(constructor)

constructs the variant object (public member function)