如何在C ++中读取带有逗号分隔值的流？

我主要集中在增加重载operator<<和operator>>因为你表现出这些早期的一些兴趣和描述他们在代码中的注释在做什么。

由于您要混合使用逗号分隔的流和其他流的输入和输出，因此我添加了用于CSV流传输的适配器以及用于向/从用户流传输的重载。

首先，创建一个class，将所有在一起的数据保留在一个数据记录中。我在struct这里做了一个简单的介绍，它默认情况下class可以public访问其成员。

#include <fstream>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>

// your student record
struct student {
    std::string name; // It's usually good to have larger types first so name goes first
    int roll;
    int math;
    int phy;
    int chem;
    int bio;
};

// read a student from an istream (like std::cin) - whitespace separated
std::istream& operator>>(std::istream& is, student& s) {
    return is >> s.roll >> s.name >> s.math >> s.phy >> s.chem >> s.bio;
}

// write a student to an ostream (like std::cout)
std::ostream& operator<<(std::ostream& os, const student& s) {
    return os << "Details of Roll " << s.roll << ":\n"
              << "Name: " << s.name << '\n'
              << "Maths: " << s.math << '\n'
              << "Physics: " << s.phy << '\n'
              << "Chemistry: " << s.chem << '\n'
              << "Biology: " << s.bio << '\n';
}
//--------------------------------------------------------------------------------------
// An adapter for comma separated streaming
struct CSVStudent {
    CSVStudent(student& s) : stud(s) {}
    CSVStudent(const CSVStudent&) = delete;

    // The CSVStudent holds a reference to a "student"
    student& stud;
};

// read a record from an istream - comma separated
std::istream& operator>>(std::istream& is, CSVStudent& csvstud) {
    std::string line;

    student& s = csvstud.stud; // an alias to the student to have to type less

    if(std::getline(is, line)) { // read a complete line
        // put the line in an istringstream for extraction:
        std::istringstream ss(line);

        char delim; // a dummy for reading commas

        // Extract the comma separated values. "delim" is not checked so it could be
        // any char breaking up the int:s.
        //
        // The below does things in the following order:
        // 1.  "ss >> s.roll >> delim"
        //     This extracts roll and a comma and returns
        //     a reference to ss, which is used in 2.
        // 2.  std::getline(ss, s.name, ',')
        //     Extracts a string until a comma is encountered.
        // 3.  Normal extraction for the rest of the int:s with the
        //     dummy variable "delim" where the commas are supposed to be.

        if(not(std::getline(ss >> s.roll >> delim, s.name, ',') >> s.math >> delim >>
               s.phy >> delim >> s.chem >> delim >> s.bio)) {
            // If we get here, the extraction from the istringstream failed, so set
            // the failstate on the istream too. Note the "not" on the line above.
            is.setstate(std::ios::failbit);
        }
    }
    return is;
}

// write a record to an ostream - comma separated
std::ostream& operator<<(std::ostream& os, const CSVStudent& csvstud) {
    const student& s = csvstud.stud;
    os << s.roll << ',' << s.name << ',' << s.math << ',' << s.phy << ',' << s.chem
       << ',' << s.bio << '\n';
    return os;
}
//--------------------------------------------------------------------------------------
// get all students in the file as a std::vector<student>
std::vector<student> read_student_file(const std::string& filename) {
    std::vector<student> retval;
    std::ifstream fin(filename);
    if(fin) { // file opened successfully
        student stud;
        CSVStudent csvstud{stud}; // holds a reference to stud

        // loop for as long as student records can be read successfully
        while(fin >> csvstud)       // use the csv sdapter
            retval.push_back(stud); // and put the stud in the vector
    }
    return retval;
}
//--------------------------------------------------------------------------------------
void create(const std::string& filename) {
    // open an existing csv file or creates a new file.
    std::ofstream fout(filename, std::ios::out | std::ios::app);
    if(fout) {
        std::cout << "Enter the details of 5 students:"
                     " roll name maths phy chem bio\n";

        // Read the input
        for(int i = 0; i < 5; i++) {
            student stud;
            std::cout << (i + 1) << ": ";
            if(std::cin >> stud) {
                // Insert the data to file if one was entered successfully
                fout << CSVStudent(stud); // uses the adapters operator<<
            } else {
                std::cerr << "You failed to enter data for student " << (i + 1) << '\n';
                break;
            }
        }
    }
}
//--------------------------------------------------------------------------------------
int main() {
    std::string filename = "reportcard.csv";

    std::vector<student> students = read_student_file(filename);

    std::cout << "There are " << students.size() << " students in the file.\n";
    if(not students.empty()) {
        // show the last record if there are any records in the file
        std::cout << "Record " << students.size() << " is:\n\n";
        std::cout << students.back() << '\n';
    }

    // create 5 new records
    create(filename);
}

如果reportcard.csv包含以下内容：

1,Ted,1,2,3,4
2,Foo,2,3,4,5
3,Bar,3,4,5,6
4,Baz,4,5,6,7
5,Bork,5,6,7,8

该程序应如下启动：

There are 5 students in the file.
Record 5 is:

Details of Roll 5:
Name: Bork
Maths: 5
Physics: 6
Chemistry: 7
Biology: 8

Enter the details of 5 students: roll name maths phy chem bio
1: <and here is where you're supposed to enter the first of 5 new students>