std::optional

Predefinição:cpp/utility/optional/navbar

A classe genérica std::optional que gerencia o optional contendo o valor, sendo válido ou não.

O uso comum envolvendo o optional é retornar o valor de uma função que provavelmente falhe. Ao contrário de outras abordagens, como std::pair<T,bool>, optional lida bem com objetos de alto custo para construir e ainda é mais legível, pois a intenção é expressa explicitamente.

Qualquer instância do optional<T> a qualquer momento, seja contém um valor ou nã contém valor algum.

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:

• O objeto é inicializável with/assigned from a value of type T or another optional that contains a value.

O objeto não contém o valor perante as devidas condções:

• 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.
• The member function reset() is called.

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 (possibly cv-qualified) tag types std::nullopt_t or std::in_place_t.

[editar] Parâmetros de template

[editar] Tipos dos membros

[editar] Funções membros

[editar] Funções não-membros

[editar] Helper classes

[editar] Helpers

[editar] Deduction guides

[editar] Exemplo

#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';
    }
}

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

[editar] See also