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std::optional

From cppreference.com
< cpp‎ | utility
 
 
 
 
Defined in header <optional>
template< class T >
class optional;
(since C++17)

The class template std::optional manages an optional contained value, i.e. a value that may or may not be present.

A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T,bool>, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly.

Any instance of optional<T> at any given point in time either contains a value or does not contain a value.

If an optional<T> contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place. Thus, an optional object models an object, not a pointer, even though the operator*() and operator->() are defined.

When an object of type optional<T> is contextually converted to bool, the conversion returns true if the object contains a value and false if it does not contain a value.

The optional object contains a value in the following conditions:

  • The object is initialized with/assigned from a value of type T or another optional that contains a value.

The object does not contain a value in the following conditions:

  • The object is default-initialized.
  • The object is initialized with/assigned from a value of type std::nullopt_t or an optional object that does not contain a value.

There are no optional references; a program is ill-formed if it instantiates an optional with a reference type. Alternatively, an optional of a std::reference_wrapper of type T may be used to hold a reference. In addition, a program is ill-formed if it instantiates an optional with the tag types std::nullopt_t or std::in_place_t.

Contents

[edit] Template parameters

T - the type of the value to manage initialization state for. The type must meet the requirements of Destructible

[edit] Member types

Member type Definition
value_type T

[edit] Member functions

constructs the optional object
(public member function) [edit]
destroys the contained value, if there is one
(public member function) [edit]
assigns contents
(public member function) [edit]
Observers
accesses the contained value
(public member function) [edit]
checks whether the object contains a value
(public member function) [edit]
returns the contained value
(public member function) [edit]
returns the contained value if available, another value otherwise
(public member function) [edit]
Modifiers
exchanges the contents
(public member function) [edit]
destroys any contained value
(public member function) [edit]
constructs the contained value in-place
(public member function) [edit]

[edit] Non-member functions

compares optional objects
(function template) [edit]
creates an optional object
(function template) [edit]
specializes the std::swap algorithm
(function) [edit]

[edit] Helper classes

specializes the std::hash algorithm
(class template specialization) [edit]
(C++17)
indicator of optional type with uninitialized state
(class) [edit]
exception indicating checked access to an optional that doesn't contain a value
(class) [edit]

[edit] Helpers

(C++17)
an object of type nullopt_t
(constant) [edit]
in-place construction tag
(class template) [edit]

[edit] Deduction guides

[edit] Example

#include <string>
#include <functional>
#include <iostream>
#include <optional>
 
// optional can be used as the return type of a factory that may fail
std::optional<std::string> create(bool b) {
    if (b)
        return "Godzilla";
    return {};
}
 
// std::nullopt can be used to create any (empty) std::optional
auto create2(bool b) {
    return b ? std::optional<std::string>{"Godzilla"} : std::nullopt;
}
 
// std::reference_wrapper may be used to return a reference
auto create_ref(bool b) {
    static std::string value = "Godzilla";
    return b ? std::optional<std::reference_wrapper<std::string>>{value}
             : std::nullopt;
}
 
int main()
{
    std::cout << "create(false) returned "
              << create(false).value_or("empty") << '\n';
 
    // optional-returning factory functions are usable as conditions of while and if
    if (auto str = create2(true)) {
        std::cout << "create2(true) returned " << *str << '\n';
    }
 
    if (auto str = create_ref(true)) {
        // using get() to access the reference_wrapper's value
        std::cout << "create_ref(true) returned " << str->get() << '\n';
        str->get() = "Mothra";
        std::cout << "modifying it changed it to " << str->get() << '\n';
    }
}

Output:

create(false) returned empty
create2(true) returned Godzilla
create_ref(true) returned Godzilla
modifying it changed it to Mothra

[edit] See also

(C++17)
a type-safe discriminated union
(class template) [edit]
(C++17)
Objects that hold instances of any CopyConstructible type.
(class) [edit]