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Defined in header <expected> | ||
template<class E> class unexpected; | (since C++23) | |
The class templatestd::unexpected represents an unexpected value stored instd::expected. In particular,std::expected has constructors withstd::unexpected as a single argument, which creates anexpected object that contains an unexpected value.
A program is ill-formed if it instantiates anunexpected with a non-object type, an array type, a specialization ofstd::unexpected, or a cv-qualified type.
Contents |
| E | - | the type of the unexpected value. The type must not be an array type, a non-object type, a specialization ofstd::unexpected, or a cv-qualified type. |
constructs theunexpected object(public member function) | |
(destructor) (implicitly declared) | destroys theunexpected object, along with the stored value(public member function) |
operator= (implicitly declared) | assigns the stored value (public member function) |
| accesses the stored value (public member function) | |
| swaps the stored value (public member function) |
(C++23) | compares the stored value (function template) |
(C++23) | specializes thestd::swap algorithm (function template) |
constexpr unexpected(const unexpected&)=default; | (1) | |
constexpr unexpected( unexpected&&)=default; | (2) | |
template<class Err= E> constexprexplicit unexpected( Err&& e); | (3) | |
template<class...Args> constexprexplicit unexpected(std::in_place_t, Args&&...args); | (4) | |
template<class U,class...Args> constexprexplicit unexpected(std::in_place_t, | (5) | |
Constructs astd::unexpected object.
E fromstd::forward<Err>(e).E from the argumentsstd::forward<Args>(args)....E from the argumentsil,std::forward<Args>(args)....| e | - | value with which to initialize the contained value |
| args... | - | arguments with which to initialize the contained value |
| il | - | initializer list with which to initialize the contained value |
Throws any exception thrown by the constructor ofE.
constexprconst E& error()const&noexcept; constexpr E& error()&noexcept; | ||
Returns a reference to the stored value.
constexprvoid swap( unexpected& other)noexcept(std::is_nothrow_swappable_v<E>); | ||
Swaps the stored values, as if byusingstd::swap; swap(error(), other.error());.
The program is ill-formed ifstd::is_swappable_v<E> is false.
template<class E2> friendconstexprbool operator==( unexpected& x, std::unexpected<E2>& y); | ||
Compares the stored values, as if byreturn x.error()== y.error().
If the expressionx.error()== e.error() is not well-formed, or if its result is not convertible tobool, the program is ill-formed.
This function is not visible to ordinaryunqualified orqualified lookup, and can only be found byargument-dependent lookup whenstd::unexpected<E> is an associated class of the arguments.
friendconstexprvoid swap( unexpected& x, unexpected& y)noexcept(noexcept(x.swap(y))); | ||
Equivalent tox.swap(y).
This overload participates in overload resolution only ifstd::is_swappable_v<E> is true.
This function is not visible to ordinaryunqualified orqualified lookup, and can only be found byargument-dependent lookup whenstd::unexpected<E> is an associated class of the arguments.
template<class E> unexpected(E)-> unexpected<E>; | (since C++23) | |
Thededuction guide is provided forunexpected to allow deduction from the constructor argument.
Prior to C++17, the namestd::unexpected denoted the function called by the C++ runtime when a dynamic exception specification was violated.
#include <expected>#include <iostream> enumclass error{ compile_time_error, runtime_error}; [[nodiscard]]auto unexpected_runtime_error()->std::expected<int, error>{return std::unexpected(error::runtime_error);} int main(){std::expected<double,int> ex= std::unexpected(3); if(!ex)std::cout<<"ex contains an error value\n"; if(ex== std::unexpected(3))std::cout<<"The error value is equal to 3\n"; constauto e= unexpected_runtime_error(); e.and_then([](constauto& e)->std::expected<int, error>{std::cout<<"and_then: "<<int(e);// not printedreturn{};}) .or_else([](constauto& e)->std::expected<int, error>{std::cout<<"or_else: "<<int(e);// prints this linereturn{};});}
Output:
ex contains an error valueThe error value is equal to 3or_else: 1
constructs theexpected object(public member function)[edit] | |
| returns the expected value (public member function)[edit] | |
| exchanges the contents (public member function)[edit] | |
(C++23) | specializes thestd::swap algorithm (function)[edit] |
(C++23) | comparesexpected objects(function template)[edit] |
