c++: std::expected, std::unexpected

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std::expected

std::unexpected

operator bool

c++ Example

std::expected

c++ std::expected: The first template type is the expected value type, the second template type is the unexpected value type.

c++ std::unexpected: The template type is the unexpected value type.

Method .value(): The expected value if result is expected;

Method .error(): Set a value if result is unexpected;

Also: .value_or()

The class template std::expected provides a way to represent either of two values: an expected value of type T, or an unexpected value of type E. expected is never valueless.
(https://en.cppreference.com/cpp/utility/expected)

operator bool

In real practical programming, we'd like use if conditional statement, so operator bool and method .value() are better than method .value_or(...).

auto exp = the_expected_or_unexpected_expression;
if (exp)
	exp.value();
else
	exp.error();

c++ Example

c++ Example:

#include <expected>
#include <iostream>
#include <stdfloat>
#include <complex>	// std::abs, std::cos

namespace app
{
	class division_by_zero final
	{
	private:
		constexpr static std::string_view __value = "ERROR: Division by zero.";
	public:
		friend std::ostream & operator<<(std::ostream & out, const app::division_by_zero & self)
		{
			out << self.__value;
			return out;
		}
		constexpr std::string_view get() const
		{
			return __value;
		}
	};

	using expected = std::expected<std::float64_t, app::division_by_zero>;
	using unexpected = std::unexpected<app::division_by_zero>;

	class my_class
	{
	public:
		app::expected divide(std::float64_t a, std::float64_t b)
		{
			if (std::abs(b) < 0.00000001)
				return app::unexpected{std::in_place_t{}};
			// else
			return app::expected{a/b};
		}
	};
}

int main()
{
	for (std::float64_t a=-11; a<=11; a++)
	{
		auto result = app::my_class{}.divide(7, a);
		if (result)
			std::cout << "Result: " << result.value() << std::endl;
		else
			std::cout << result.error() << std::endl;
	}
}

Running output:

Result: -0.636364
Result: -0.7
Result: -0.777778
Result: -0.875
Result: -1
Result: -1.16667
Result: -1.4
Result: -1.75
Result: -2.33333
Result: -3.5
Result: -7
ERROR: Division by zero.
Result: 7
Result: 3.5
Result: 2.33333
Result: 1.75
Result: 1.4
Result: 1.16667
Result: 1
Result: 0.875
Result: 0.777778
Result: 0.7
Result: 0.636364

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