Standard library header <expected> (C++23)

 

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Standard Library headers

Note: a slash '/' in a revision mark means that the header was deprecated and/or removed.

Language Support
<cstddef> <cstdlib> <version> (C++20) <limits> <climits>	<cfloat> <cstdint> (C++11) <new> <typeinfo>	<source_location> (C++20) <exception> <initializer_list> (C++11)   <compare> (C++20)	<coroutine> (C++20) <csignal> <csetjmp> <cstdarg>
Concepts
<concepts> (C++20)
Diagnostics
<stdexcept>	<cassert>	<cerrno>	<system_error> (C++11)
General utilities
<utility> <memory> <memory_resource> (C++17) <scoped_allocator> (C++11)   <expected> (C++23)	<bitset> <tuple> (C++11) <optional> (C++17) <any> (C++17)	<variant> (C++17) <type_traits> (C++11) <ratio> (C++11) <chrono> (C++11)	<typeindex> (C++11) <functional> <stacktrace> (C++23) <ctime>
Strings
<string> <string_view> (C++17) <cstring>	<charconv> (C++17) <format> (C++20)	<cctype> <cwctype>	<cwchar> <cuchar> (C++11)
Localization
<locale> <clocale>	<codecvt> (C++11/17)
Containers
<span> (C++20) <array> (C++11) <vector>	<deque> <forward_list> (C++11) <list>	<map> <set> <queue>	<unordered_map> (C++11) <unordered_set> (C++11) <stack>
Iterators
<iterator>
Ranges
<ranges> (C++20)
Algorithms
<algorithm>	<execution> (C++17)
Numerics
<complex> <random> (C++11)	<valarray> <numeric>	<bit> (C++20) <numbers> (C++20)	<cfenv> (C++11) <cmath>
Input/Output
<iosfwd> <ios> <iomanip> <streambuf>	<istream> <ostream> <iostream>	<syncstream> (C++20) <spanstream> (C++23) <sstream> <fstream>	<cstdio> <cinttypes> (C++11) <strstream> (C++98/)
Regular expressions
<regex> (C++11)
Filesystem support
<filesystem> (C++17)
Thread support
<thread> (C++11) <atomic> (C++11) <mutex> (C++11)	<shared_mutex> (C++14) <condition_variable> (C++11)   <future> (C++11)	<stop_token> (C++20) <semaphore> (C++20)	<latch> (C++20) <barrier> (C++20)
C compatibility
<cstdbool> (C++11/17/20) <ccomplex> (C++11/17/20)	<ctgmath> (C++11/17/20) <cstdalign> (C++11/17/20)	<ciso646> (until C++20) <stdatomic.h>

 

This header is part of the general utility library.

Classes
expected (C++23)	a wrapper that contains either an expected or error value (class template)
unexpected (C++23)	represented as an unexpected value in expected (class template)
bad_expected_access (C++23)	exception indicating checked access to an expected that contains an unexpected value (class template)
unexpect_t (C++23)	in-place construction tag for unexpected value in expected (class)
Constants
unexpect (C++23)	object of type unexpect_t (constant)

Synopsis

namespace std {
  // class template unexpected
  template<class E> class unexpected;
 
  // class template bad_expected_access
  template<class E> class bad_expected_access
 
  // specialization of bad_expected_access for void
  template<> class bad_expected_access<void>;
 
  // in-place construction of unexpected values
  struct unexpect_t {
    explicit unexpect_t() = default;
  };
  inline constexpr unexpect_t unexpect{};
 
  // class template expected
  template<class T, class E> class expected;
 
  // partial specialization of expected for void types
  template<class T, class E> requires is_void_v<T> class expected<T, E>;
}

Class template std::unexpected

namespace std {
  template<class E>
  class unexpected {
  public:
    // constructors
    constexpr unexpected(const unexpected&) = default;
    constexpr unexpected(unexpected&&) = default;
    template<class... Args>
      constexpr explicit unexpected(in_place_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit unexpected(in_place_t, initializer_list<U>, Args&&...);
    template<class Err = E>
      constexpr explicit unexpected(Err&&);
 
    // assignment
    constexpr unexpected& operator=(const unexpected&) = default;
    constexpr unexpected& operator=(unexpected&&) = default;
 
    // observer
    constexpr const E& error() const& noexcept;
    constexpr E& error() & noexcept;
    constexpr const E&& error() const&& noexcept;
    constexpr E&& error() && noexcept;
 
    // swap
    constexpr void swap(unexpected& other) noexcept(/* see description */);
 
    friend constexpr void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y)));
 
    // equality operator
    template<class E2>
      friend constexpr bool operator==(const unexpected&, const unexpected<E2>&);
 
  private:
    E unex;    // exposition only
  };
 
  template<class E> 
    unexpected(E) -> unexpected<E>;
}

Class template std::bad_expected_access

namespace std {
  template<class E>
  class bad_expected_access : public bad_expected_access<void> {
  public:
 
    // explicit constructor
    explicit bad_expected_access(E);
 
    // observers
    const char* what() const noexcept override;
    E& error() & noexcept;
    const E& error() const& noexcept;
    E&& error() && noexcept;
    const E&&  error() const&& noexcept;
 
  private:
    E unex;              // exposition only
  };
}

Class template specialization std::bad_expected_access<void>

namespace std {
  template<>
  class bad_expected_access<void> : public exception {
  protected:
    // constructors
    bad_expected_access() noexcept;
    bad_expected_access(const bad_expected_access&);
    bad_expected_access(bad_expected_access&&);
    bad_expected_access& operator=(const bad_expected_access&);
    bad_expected_access& operator=(bad_expected_access&&);
 
    ~bad_expected_access();
 
  public:
    const char* what() const noexcept override;
  };
}

Class template std::expected

namespace std {
  template<class T, class E>
  class expected {
  public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;
 
    template<class U>
    using rebind = expected<U, error_type>;
 
    // constructors
    constexpr expected();
    constexpr explicit(/* see description */) 
      expected(const expected&);
    constexpr explicit(/* see description */) 
      expected(expected&&) noexcept(/* see description */);
    template<class U, class G>
      constexpr explicit(/* see description */) expected(const expected<U, G>&);
    template<class U, class G>
      constexpr explicit(/* see description */) expected(expected<U, G>&&);
 
    template<class U = T>
      constexpr explicit(/* see description */) expected(U&& v);
 
    template<class G>
      constexpr expected(const unexpected<G>&);
    template<class G>
      constexpr expected(unexpected<G>&&);
 
    template<class... Args>
      constexpr explicit expected(in_place_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(in_place_t, initializer_list<U>, Args&&...);
    template<class... Args>
      constexpr explicit expected(unexpect_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);
 
    // destructor
    constexpr ~expected();
 
    // assignment
    constexpr expected& operator=(const expected&);
    constexpr expected& operator=(expected&&) noexcept(/* see description */);
    template<class U = T> constexpr expected& operator=(U&&);
    template<class G>
      constexpr expected& operator=(const unexpected<G>&);
    template<class G>
      constexpr expected& operator=(unexpected<G>&&);
 
    template<class... Args>
      constexpr T& emplace(Args&&...) noexcept;
    template<class U, class... Args>
      constexpr T& emplace(initializer_list<U>, Args&&...) noexcept;
 
    // swap
    constexpr void swap(expected&) noexcept(/* see description */);
    friend constexpr void swap(expected&, expected&) noexcept(/* see description */);
 
    // observers
    constexpr const T* operator->() const noexcept;
    constexpr T* operator->() noexcept;
    constexpr const T& operator*() const& noexcept;
    constexpr T& operator*() & noexcept;
    constexpr const T&& operator*() const&& noexcept;
    constexpr T&& operator*() && noexcept;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr const T&& value() const&&;
    constexpr T&& value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const&&;
    constexpr E&& error() &&;
    template<class U> constexpr T value_or(U&&) const&;
    template<class U> constexpr T value_or(U&&) &&;
 
    // equality operators
    template<class T2, class E2>
      friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
    template<class T2>
      friend constexpr bool operator==(const expected&, const T2&);
    template<class E2>
      friend constexpr bool operator==(const expected&, const unexpected<E2>&);
 
  private:
    bool has_val;       // exposition only
    union {
      T val;            // exposition only
      E unex;           // exposition only
    };
  };
}

Partial specialization of std::expected for void types

namespace std {
  template<class T, class E> requires is_void_v<T>
  class expected<T, E> {
  public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;
 
    template<class U>
    using rebind = expected<U, error_type>;
 
    // constructors
    constexpr expected() noexcept;
    constexpr explicit(/* see description */)
      expected(const expected&);
    constexpr explicit(/* see description */)
      expected(expected&&) noexcept(/* see description */);
    template<class U, class G>
      constexpr explicit(/* see description */) expected(const expected<U, G>&);
    template<class U, class G>
      constexpr explicit(/* see description */) expected(expected<U, G>&&);
 
    template<class G>
      constexpr expected(const unexpected<G>&);
    template<class G>
      constexpr expected(unexpected<G>&&);
 
    constexpr explicit expected(in_place_t) noexcept;
    template<class... Args>
      constexpr explicit expected(unexpect_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);
 
    // destructor
    constexpr ~expected();
 
    // assignment
    constexpr expected& operator=(const expected&);
    constexpr expected& operator=(expected&&) noexcept(/* see description */);
    template<class G>
      constexpr expected& operator=(const unexpected<G>&);
    template<class G>
      constexpr expected& operator=(unexpected<G>&&);
    constexpr void emplace() noexcept;
 
    // swap
    constexpr void swap(expected&) noexcept(/* see description */);
    friend constexpr void swap(expected&, expected&) noexcept(/* see description */);
 
    // observers
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr void operator*() const noexcept;
    constexpr void value() const&;
    constexpr void value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const&&;
    constexpr E&& error() &&;
 
    // equality operators
    template<class T2, class E2> requires is_void_v<T2>
      friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
    template<class E2>
      friend constexpr bool operator==(const expected&, const unexpected<E2>&);
 
  private:
    bool has_val;        // exposition only
    union {
      E unex;            // exposition only
    };
  };
}