std::expected

#include <cmath>
#include <expected>
#include <iomanip>
#include <iostream>
#include <string_view>
 
enum class parse_error
{
 invalid_input,
 overflow
};
 
auto parse_number(std::string_view & str) -> std::expected<double, parse_error>
{
 const char* begin = str.data();
 char* end;
 double retval = std::strtod (begin, &end);
 
 if (begin == end)
 return std::unexpected (parse_error::invalid_input);
 else if (std::isinf (retval))
 return std::unexpected (parse_error::overflow);
 
 str.remove_prefix(end - begin);
 return retval;
}
 
int main()
{
 auto process = [](std::string_view str)
 {
 std::cout << "str: " << std::quoted (str) << ", ";
 if (const auto num = parse_number(str); num.has_value())
 std::cout << "value: " << *num << '\n';
 // If num did not have a value, dereferencing num
 // would cause an undefined behavior, and
 // num.value() would throw std::bad_expected_access.
 // num.value_or(123) uses specified default value 123.
 else if (num.error() == parse_error::invalid_input)
 std::cout << "error: invalid input\n";
 else if (num.error() == parse_error::overflow)
 std::cout << "error: overflow\n";
 else
 std::cout << "unexpected!\n"; // or invoke std::unreachable();
 };
 
 for (auto src : {"42", "42abc", "meow", "inf"})
 process(src);
}

str: "42", value: 42
str: "42abc", value: 42
str: "meow", error: invalid input
str: "inf", error: overflow