C++学习笔记 40 C++17 Optional

Optional

Optional: 三种处理可能存在，可能不存在的数据的方式之一

#include<iostream>
#include<string>
#include<fstream>
#include<optional>

std::string ReadFileAsString(const std::string& filePath, bool& outSuccess) {
	std::ifstream stream(filePath);
	if (stream) {
		//read file
		std::string result;
		stream.close();
		return result;
	}
	return std::string();
}


std::optional<std::string> ReadFileAsStringOptional(const std::string& filePath) {
	std::ifstream stream(filePath);
	if (stream) {
		//read file
		std::string result;
		stream.close();
		return result;
	}
	return {};
}

int testReadFileAsStringOptional() {
	bool outSuccess;
	std::string data = ReadFileAsString("data.txt", outSuccess);
	//判断方式一，返回结果
	if (data != "") {}
	//判断方式二，外部值传递
	if (outSuccess) {}

	//判断方式三，Optional
	std::optional<std::string> data2 = ReadFileAsStringOptional("data.txt");

	if (data2.has_value()) {
		std::cout << "File read successfully. data: " << data2.value() << "\n";
		std::cout << "File read successfully. data: " << data2.value_or("hello world") << "\n";
	} else {
		std::cout << "File couldn't be opened.\n";
	}

	//因为data2对象有一个bool运算符，所以不用谢has_value
	if (data2) {
	}

	std::optional<int> count;
	int c = count.value_or(100);
}


int main() {
	testReadFileAsStringOptional();
	std::cin.get();
}