std::optional

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std::optional
 
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 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.

Template parameters

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

Member types

Member type Definition
value_type T

Member functions

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

Non-member functions

(C++17)
 compares optional objects
(function template)
(C++17)
 creates an optional object
(function template)
(C++17)
 specializes the std::swap algorithm
(function)

Helper classes

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

Helpers

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

Deduction guides

Example

Run this code
#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

See also

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