问题

我想从一个std::string创建一个std::array。

为此，我想要重载operator>>。

我有以下测试案例：

std::istream& operator>>(std::istream& is, const std::array<double, 3>& a)
{
    char p1, p2;

    is >> p1;
    // 如果失败，则警告用户

    for (unsigned int i = 1; i < a.size(); ++i) 
    {
        // 一些用于忽略/检查是否为数字的内容
    }

    is >> p2;
    // 如果失败，则警告用户
    
    return is;
}

int main()
{
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    iss >> arr;
    
    return 0;
}

我希望该操作符检查字符[和]是否在正确的位置，并使用括号内的元素构建数组。

如何检查提取是否成功？如何检查括号内的字符串是否为数字，如果是的话如何从中构建我的数组？

Kind regards

std::istream& operator>>(std::istream& is, const std::array<double, 3>& a)
{
    char p1, p2;

    is >> p1;
    // if fail warn the user

    for (unsigned int i = 1; i < a.size(); ++i) 
    {
        // something to ignore/ check if numeric
    }

    is >> p2;
    // if fail warn the user
    
    return is;
}

int main()
{
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    iss >> arr;
    
    return 0;
}

答案1

得分: 2

我会翻译代码部分：

我会翻译代码部分：

I'd add a helper class that checks for a certain character in the stream and removes it if it's there. If it's not, sets the failbit.

我会添加一个辅助类，该类在流中检查特定字符是否存在，并在存在时将其移除。如果不存在，将设置 failbit。

Example:
示例：

```c++
#include <cctype>
#include <istream>

template <char Ch, bool SkipWhitespace = false>
struct eater {
    friend std::istream& operator>>(std::istream& is, eater) {
        if /*constexpr*/ (SkipWhitespace) { // constexpr since C++17
            is >> std::ws;
        }
        if (is.peek() == Ch) // if the expected char is there, remove it
            is.ignore();
        else                 // else set the failbit
            is.setstate(std::ios::failbit);
        return is;
    }
};

And it could then be used like this:
然后可以像这样使用它：

std::istream& operator>>(std::istream& is, std::array<double, 3>& a) {
    // use the `eater` class template with '[' and ']' as template parameters:
    return is >> eater<'['>{} >> a[0] >> a[1] >> a[2] >> eater<']'>{};
}

int main() {
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    // iss.exceptions(std::ios::failbit); // if you want exceptions on failure

    if(iss >> arr) {
        std::cout << "success\n";
    } else {
        std::cout << "fail\n";
    }
}

Demo where ',' is used as separator in the input.
演示 中在输入中使用了 ',' 作为分隔符。

<details>
<summary>英文:</summary>

I&#39;d add a helper class that checks for a certain character in the stream and removes it if it&#39;s there. If it&#39;s not, sets the failbit.

Example:
```c++
#include &lt;cctype&gt;
#include &lt;istream&gt;

template &lt;char Ch, bool SkipWhitespace = false&gt;
struct eater {
    friend std::istream&amp; operator&gt;&gt;(std::istream&amp; is, eater) {
        if /*constexpr*/ (SkipWhitespace) { // constexpr since C++17
            is &gt;&gt; std::ws;
        }
        if (is.peek() == Ch) // if the expected char is there, remove it
            is.ignore();
        else                 // else set the failbit
            is.setstate(std::ios::failbit);
        return is;
    }
};

And it could then be used like this:

std::istream& operator>>(std::istream& is, std::array<double, 3>& a) {
    // use the `eater` class template with `[` and `]` as template parameters:
    return is >> eater<'['>{} >> a[0] >> a[1] >> a[2] >> eater<']'>{};
}

int main() {
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    // iss.exceptions(std::ios::failbit); // if you want exceptions on failure

    if(iss >> arr) {
        std::cout << "success\n";
    } else {
        std::cout << "fail\n";
    }
}

Demo where , is used as separator in the input.