std::optional<T>::operator bool，std::optional<T>::has_value

constexpr explicit operator bool() const noexcept;

constexpr bool has_value() const noexcept;

检查*this是否包含值。

参数

(无)

返回值

若 * this 包含值则为 true ，若 * this 不包含值则为 false 。

示例

#include <iostream>
#include <optional>
int main()
{
    std::cout << std::boolalpha;
    std::optional<int> opt;
    std::cout << opt.has_value() << '\n';
    opt = 43;
    if (opt)
        std::cout << "value set to " << opt.value() << '\n';
    else
        std::cout << "value not set\n";
    opt.reset();
    if (opt.has_value())
        std::cout << "value still set to " << opt.value() << '\n';
    else
        std::cout << "value no longer set\n";
}

false
value set to 43
value no longer set