英文:

Can't compare types of variables in C++

问题

我有以下在一个类中的函数，它获取一个复数的实部：

```cpp
double inputReal(){
        double real;
        cout << "Enter real part: ";
        cin >> real;
        bool compare = typeid(real) == typeid(double);
        while (compare != true){
            cout << "Incorrect input. Try another input: ";
            cin >> real;
        }
        return real;
    }

它应该检查实部是否实际上是一个数字，如果实部不是一个数字 - 它应该获取另一个输入。然而，如果我尝试输入类似于："aa" 或 "#@"，它会跳过 while 部分并继续输出，忽略获取虚部：

double inputImag(){
        double imaginary;
        cout << "Enter imaginary part: ";
        cin >> imaginary;
        bool compare = typeid(imaginary) == typeid(double);
        while (compare != true){
            cout << "Incorrect input. Try another input: ";
            cin >> imaginary;
        }
        return imaginary;
    }

如果我尝试将 "aa" 输入为实部，输出如下：

Enter real part: aa
Enter imaginary part: 复数的绝对值: 0
复数的幅角: 0
程序以退出代码结束: 0

如果我正确输入，代码运行得很好，所以我怀疑是这些函数内部类型比较出了问题。我需要知道我做错了什么以及如何修复它。我对 C++ 相对较新，我的大学教授要求检查用户输入是否正确。如果问题看起来不清楚，请随时询问。

<details>
<summary>英文:</summary>

I have following function within a class, which gets real part of a complex number:

double inputReal(){
double real;
cout << "Enter real part: ";
cin >> real;
bool compare = typeid(real) == typeid(double);
while (compare != true){
cout << "Incorrect input. Try another input: ";
cin >> real;
}
return real;
}

It should check if real is *actually a number* and if real isn't a number - it should get another input. However, if I try inputing something like: "aa" or "#@", it skips the *while* part and proceeds to output, ignoring getting the imaginary part: 

double inputImag(){
double imaginary;
cout << "Enter imaginary part: ";
cin >> imaginary;
bool compare = typeid(imaginary) == typeid(double);
while (compare != true){
cout << "Incorrect input. Try another input: ";
cin >> imaginary;
}
return imaginary;
}

Output, if I try inputing "aa" as a real part:

Enter real part: aa
Enter imaginary part: Absolute value of complex number: 0
Argument of complex value: 0
Program ended with exit code: 0

The code works just fine, if I input correctly, so I suspect it's the comparison of types within those functions. I need to know what did I do wrong and how I can fix it. I'm relatively new to C++ and my college professor requires checking, that user inputs correctly. Feel free to ask anything, if the question seems unclear.

</details>


# 答案1
**得分**: 0

```cpp
`typeid(real) == typeid(double)` 将始终为true，因为`real`是一个`double`。

如果您想检查您是否有有效的输入，您需要实际检查输入。以下是一种方法：

double inputReal()
{
std::size_t end_pos{};
std::string input;
std::cout << "输入实部：";
while (getline(std::cin, input)) // 获取完整的输入行
{
double real = std::stod(input, &end_pos); // 转换为double
if (end_pos == input.size()) // 检查是否所有输入都被转换
return real;
else
std::cout << "输入不正确。请尝试另一个输入：";
}
}

<details>
<summary>英文:</summary>

`typeid(real) == typeid(double)` will always be true since `real` is a `double`.  

If you want to check that you have valid input, you need to actually check the input.  Here is one way of doing that

double inputReal()
{
std::size_t end_pos{};
std::string input;
std::cout << "Enter real part: ";
while (getline(std::cin, input)) // get the full line of input
{
double real = std::stod(input, &end_pos); // convert to double
if (end_pos == input.size()) // check to see if all of the input was converted
return real;
else
std::cout << "Incorrect input. Try another input: ";
}
}