std::optional<T>::begin

constexpr iterator begin() noexcept;

constexpr const_iterator begin() const noexcept;

若 *this 包含值，则返回指向所含值的迭代器。否则，返回尾后迭代器值。

返回值

若 has_value ( ) 为 true ，则返回指向所含值的迭代器。否则返回逾尾迭代器。

复杂度

常量。

注释

示例

#include <optional>
#include <print>
#include <vector>
int main()
{
    constexpr std::optional<int> none{std::nullopt};
    constexpr std::optional<int> some{42};
    static_assert(none.begin() == none.end());
    static_assert(some.begin() != some.end());
    // 范围for循环支持
    for (int i : none)
        std::println("'none' has a value of {}", i);
    for (int i : some)
        std::println("'some' has a value of {}", i);
    std::optional<std::vector<int>> many({0, 1, 2});
    for (const auto& v : many)
        std::println("'many' has a value of {}", v);
}

'some' has a value of 42
'many' has a value of [0, 1, 2]

参见