为什么从 std::istream 读取记录结构字段失败，我该如何解决? [英] Why does reading a record struct fields from std::istream fail, and how can I fix it?

问题描述

假设我们有以下情况:

• 一个记录结构体声明如下

• A record struct is declared as follows

struct Person {
    unsigned int id;
    std::string name;
    uint8_t age;
    // ...
};

记录使用以下格式存储在文件中:

Records are stored in a file using the following format:

ID      Forename Lastname Age
------------------------------
1267867 John     Smith    32
67545   Jane     Doe      36
8677453 Gwyneth  Miller   56
75543   J. Ross  Unusual  23
...

应该读入文件以收集任意数量的上述Person记录:

The file should be read in to collect an arbitrary number of the Person records mentioned above:

std::istream& ifs = std::ifstream("SampleInput.txt");
std::vector<Person> persons;

Person actRecord;
while(ifs >> actRecord.id >> actRecord.name >> actRecord.age) {
    persons.push_back(actRecord);
}

if(!ifs) {
    std::err << "Input format error!" << std::endl;
} 

问题:
我该怎么做才能读入将它们的值存储到一个 actRecord 变量字段中的单独值?

Question:
What can I do to read in the separate values storing their values into the one actRecord variables' fields?

上面的代码示例以运行时错误告终:

The above code sample ends up with run time errors:

Runtime error    time: 0 memory: 3476 signal:-1
stderr: Input format error!

推荐答案

这是我想出的一个操纵器的实现，它通过每个提取的字符计算分隔符.使用您指定的分隔符数量，它将从输入流中提取单词.这是一个工作演示.

Here's an implementation of a manipulator I came up with that counts the delimiter through each extracted character. Using the number of delimiters you specify, it will extract words from the input stream. Here's a working demo.

template<class charT>
struct word_inserter_impl {
    word_inserter_impl(std::size_t words, std::basic_string<charT>& str, charT delim)
        : str_(str)
        , delim_(delim)
        , words_(words)
    { }

    friend std::basic_istream<charT>&
    operator>>(std::basic_istream<charT>& is, const word_inserter_impl<charT>& wi) {
        typename std::basic_istream<charT>::sentry ok(is);

        if (ok) {
            std::istreambuf_iterator<charT> it(is), end;
            std::back_insert_iterator<std::string> dest(wi.str_);

            while (it != end && wi.words_) {
                if (*it == wi.delim_ && --wi.words_ == 0) {
                    break;
                }
                dest++ = *it++;
            }
        }
        return is;
    }
private:
    std::basic_string<charT>& str_;
    charT delim_;
    mutable std::size_t words_;
};

template<class charT=char>
word_inserter_impl<charT> word_inserter(std::size_t words, std::basic_string<charT>& str, charT delim = charT(' ')) {
    return word_inserter_impl<charT>(words, str, delim);
}

现在你可以这样做:

while (ifs >> actRecord.id >> word_inserter(2, actRecord.name) >> actRecord.age) {
    std::cout << actRecord.id << " " << actRecord.name << " " << actRecord.age << '
';
}

现场演示

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