Cpp Weekly Lambdas As State Machines

使用Lambads实现状态机

Code

#include <format>
#include <exception>
#include <variant>
#include <iostream>

auto int_parser() {
	enum class ParseState {
		StartUp,
        Numbers,
        Invalid,
    };
	return [value = 0, state = ParseState::StartUp, is_negative = false](char input)mutable ->std::variant<int,char>  {
		switch (state)
		{
		case ParseState::StartUp:
			if (input == '-') {
                is_negative = true;
				state = ParseState::Numbers;
				return value;
            }
			[[fallthrough]];
		case ParseState::Numbers:
			if (input >= '0' && input <= '9') {
                value = value * 10 + (input - '0');
                return (is_negative)? -1*value:value;
            }
			state = ParseState::Invalid;
			return input;
		case ParseState::Invalid:
			return input;
		default:
			return input;
		}
	};
}

int main() {

	auto parser = int_parser();

	parser('-');
	parser('3');
	parser('0');

	auto res = parser('1');

	std::cout << std::get<int>(res) << std::endl;// -301

	return 0;
}