错误的字符串类型转换为unsigned int [英] Malfunctioning type-casting of string to unsigned int

问题描述

我写了一个C ++函数有一个恼人的问题，其目的是验证用户输入。该函数读取用户输入，验证它是否是一个数字，如果是，则是否在[min，max]范围内。

I'm having a annoying problem with a C++ function that I wrote and whose purpose is to validate de user input. The function reads the user input, verifies if it's a number and, if so, if it is in the range [min, max].

模板函数使用无符号类型，如 size_t ，输入是负数。字符串流将字符串转换为类似4294967291.我可以看到，程序正在将数据转换为接近无符号数据类型的最大值（在numeric_limits头中定义）的值，但我的问题是为什么，因为 if 语句应停止在 sstream>> value ？

The problem occurs when I invoke the template function with a unsigned type, like size_t, and the input is a negative number. The string stream converts the string to something like 4294967291. I can see that the program is converting the data to a value near de maximum value of the unsigned data type (defined in the numeric_limits header) but my question is why, since the if statement should stop at sstream >> value?

我的代码：

template <class T>
T getNumberInput(std::string prompt, T min, T max) {
    std::string input;
    T value;

    while (true) {
        try {
            std::cout << prompt;
            std::cin.clear();
            std::getline(std::cin, input);
            std::stringstream sstream(input);

            if (input.empty()) {
                throw EmptyInput<std::string>(input);
            } else if (sstream >> value && value >= min && value <= max) {
                std::cout << std::endl;
                return value;
            } else {
                throw InvalidInput<std::string>(input);
            }
        } catch (EmptyInput<std::string> & emptyInput) {
            std::cout << "O campo não pode ser vazio!\n" << std::endl;
        } catch (InvalidInput<std::string> & invalidInput){
            std::cout << "Tipo de dados inválido!\n" << std::endl;
        }
    }
}

谢谢您的时间！

推荐答案

在包含无符号 值位，保证为模2 ^ n 。这意味着通过加上或减去2 ^ em em n的适当的倍数，任何结果被回绕到范围0到2 ^ n em -1中。这也是在C。

In C++ arithmetic involving an unsigned type with n value bits, is guaranteed to be modulo 2^n. That means any result is wrapped back into the range 0 through 2^n-1, by adding or subtracting a suitable multiple of 2^n. This is so also in C.

所以你需要检查输入的减号，或添加一些其他检查。

So you need to check the input for minus sign, or add some other check.

顺便说一句，如果与>> 和& & 对我的坏码仪产生了一些影响。我永远不会记得>> 与&& 的运算符优先级。我想如果它编译它一定是OK，但是，因为>> 不能取右侧的值。检查...确定，但我会使用括号来澄清这一点。

By the way, your if with >> and && produced some effect on my bad-code-meter. I can never remember the operator precedences for >> versus &&. I guess if it compiled it must be OK, though, since >> can't take a value right-hand-side. Checking... OK, but I'd use parentheses to clarify that.

此外，在代码结构上，从检查输入。例如，您可以使用编辑字段中的输入在GUI程序中使用任何该代码吗？不，不是因为...

Also, on the code structure, it would be a good idea to separate the interactive input thing from the checking of the input. E.g., can you use any of that code in a GUI program, with input from an edit field? No, not as it is...

干杯& hth。，

Cheers & hth.,

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