std::expected<T,E>::operator->, std::expected<T,E>::operator*

 

C++

編譯器支援

自由（freestanding）與宿主（hosted）

語言

標準庫

標準庫標頭檔案

具名要求

特性測試宏 (C++20)

語言支援庫

概念庫 (C++20)

診斷庫

記憶體管理庫

超程式設計庫 (C++11)

通用工具庫

容器庫

迭代器庫

範圍庫 (C++20)

演算法庫

字串庫

文字處理庫

數值庫

日期和時間庫

輸入/輸出庫

檔案系統庫 (C++17)

併發支援庫 (C++11)

執行控制庫 (C++26)

技術規範

符號索引

外部庫

 

工具庫

語言支援 型別支援（基本型別、RTTI） 庫特性測試宏 (C++20) 程式工具 變參函式 initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) 協程支援 (C++20) 契約支援 (C++26) 三路比較 three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

通用工具 函式物件 位操作 (C++20) bitset hash (C++11) 關係運算符 (C++20 中棄用) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= 整數比較函式 cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) 交換與型別操作 swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) 常用詞彙型別 pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::expected

成員函式

expected::expected

expected::~expected

expected::operator=

觀察器

expected::operator->expected::operator*

expected::operator boolexpected::has_value

expected::value

expected::error

expected::value_or

expected::error_or

Monadic operations

expected::and_then

expected::or_else

expected::transform

expected::transform_error

修改器

expected::emplace

expected::swap

非成員函式

operator==(std::expected)

swap(std::expected)

輔助類

unexpected

bad_expected_access

unexpect_tunexpect

 

主模板
constexpr const T* operator->() const noexcept;	(1)	(C++23 起)
constexpr T* operator->() noexcept;	(2)	(C++23 起)
constexpr const T& operator*() const& noexcept;	(3)	(C++23 起)
constexpr T& operator*() & noexcept;	(4)	(C++23 起)
constexpr const T&& operator*() const&& noexcept;	(5)	(C++23 起)
constexpr T&& operator*() && noexcept;	(6)	(C++23 起)
void 偏特化
constexpr void operator*() const noexcept;	(7)	(C++23 起)

訪問 *this 中包含的預期值。

1,2) 返回指向預期值的指標。

3-6) 返回對預期值的引用。

7) 不返回任何內容。

如果 has_value() 為 false，則行為未定義。

返回值

1,2) std::addressof(val)

3,4) val

5,6) std::move(val)

注意

這些運算子不檢查 optional 是否表示預期值！您可以使用 has_value() 或簡單的 operator bool() 手動進行檢查。另外，如果需要檢查訪問，可以使用 value() 或 value_or()。

示例

執行此程式碼

#include <cassert>
#include <expected>
#include <iomanip>
#include <iostream>
#include <string>
 
int main()
{
    using namespace std::string_literals;
 
    std::expected<int, std::string> ex1 = 6;
    assert(*ex1 == 6);
 
    *ex1 = 9;
    assert(*ex1 == 9);
 
    // *ex1 = "error"s; // error, ex1 contains an expected value of type int
    ex1 = std::unexpected("error"s);
    // *ex1 = 13; // UB, ex1 contains an unexpected value
    assert(ex1.value_or(42) == 42);
 
    std::expected<std::string, bool> ex2 = "Moon"s;
    std::cout << "ex2: " << std::quoted(*ex2) << ", size: " << ex2->size() << '\n';
 
    // You can "take" the expected value by calling operator* on an std::expected rvalue
 
    auto taken = *std::move(ex2);
    std::cout << "taken " << std::quoted(taken) << "\n"
                 "ex2: " << std::quoted(*ex2) << ", size: " << ex2->size() << '\n';
}

可能的輸出

ex2: "Moon", size: 4
taken "Moon"
ex2: "", size: 0

參閱

value	返回預期值 (public member function)
value_or	如果存在，返回預期值；否則返回另一個值 (public member function)
operator boolhas_value	檢查物件是否包含預期值 (public member function)
error	返回非預期值 (public member function)