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我正在尝试编写一个程序，该程序从文本文件中读取信息并使用结构和联合以及 cstrings 将其复制到数组中。我的文本文件看起来像这样。

F South Korea
Male Psy Park Jae Sang
31 - 12 - 1977
3 CSCI114 55 CSCI103 44 GangNam 100

S Female Super Junior
5 - 8 - 1978 
2 CSCI114 60 CSCI103 80

F People Republic Of China
Unknown James Bond
11 - 12 - 1976
4 CSCI114 54 CSCI124 66 CSCI007 99 CSCI123 28

我的查询是，我编写了一个带有 for 循环的简单 switch 案例，该循环根据其条件读取信息。例如，每段前的第一个字母表示学生是外国人，F 还是新加坡人，S。根据他们的国籍，我选择将他们的信息复制到哪个结构/联合。为了给你一个更好的主意，这是我从原始文本文件中获取信息并处理它后的最终文本输出文件的样子。

http://i.stack.imgur.com/Bv2YS.jpg

以下是我遇到问题的代码部分。看来 char 变量是否读取“S”或“F”，我的 switch 语句仅将其读取为“F”。

        //file to array.


        char dateJunk;
        int numOfCourses;

        int k = 0;
        while (!afile.eof())
        {
            for (int k = 0; k < 3; k++)
            {
            afile >> locale;
                switch (locale)
                {
                    case 'F':
                        afile.getline(x[k].st.foreignStudent.nationality, MAX);
                        afile >> x[k].st.foreignStudent.gender;
                        afile.getline(x[k].st.foreignStudent.name, MAX);
                        afile >> x[k].st.foreignStudent.bd.day;
                        afile >> dateJunk;
                        afile >> x[k].st.foreignStudent.bd.month;
                        afile >> dateJunk;
                        afile >> x[k].st.foreignStudent.bd.year;
                        afile >> x[k].st.foreignStudent.numOfCourses;

                        for (int i = 0; i < x[k].st.foreignStudent.numOfCourses; i++)
                        {
                            afile >> x[i].st.foreignStudent.subjects[k];

                            afile >> x[i].st.foreignStudent.grades[k];
                        }
                        break;

                    case 'S':
                        afile >> x[k].st.localStudent.gender;
                        afile.getline(x[k].st.localStudent.name, MAX);
                        afile >> x[k].st.localStudent.bd.day;
                        afile >> dateJunk;
                        afile >> x[k].st.localStudent.bd.month;
                        afile >> dateJunk;
                        afile >> x[k].st.localStudent.bd.year;
                        afile >> x[k].st.localStudent.numOfCourses;;
                        for (int i = 0; i < x[k].st.localStudent.numOfCourses; i++)
                        {
                            afile >> x[i].st.localStudent.subjects[k];

                            afile >> x[i].st.localStudent.grades[k];
                        }
                }
                }
        }

        //Tests my cstring arrays to see everything is copied in correctly.
        for (int k = 0; k < 3; k++)
        {
        cout << locale << " " << x[k].st.foreignStudent.nationality;
        cout << endl;
        cout << x[k].st.foreignStudent.gender;
        cout << x[k].st.foreignStudent.name;
        cout << endl;
        cout << x[k].st.foreignStudent.bd.day << " - ";
        cout << x[k].st.foreignStudent.bd.month << " - ";
        cout << x[k].st.foreignStudent.bd.year;
        cout << endl;
        cout << x[k].st.foreignStudent.numOfCourses;
        cout << endl;
        for(int i = 0; i < x[k].st.foreignStudent.numOfCourses; i++)
        {
        cout << x[i].st.foreignStudent.subjects[k] << " ";
        cout << x[i].st.foreignStudent.grades[k] << " ";

        }
            cout << endl;
        }

        return 0;
    }

下面是完整的代码。

    #include <iostream>
    #include <fstream>
    #include <cstdlib>
    #include <ctime>
    using namespace std;

    const int MAX = 80;

    struct Birthday
    {
        char day[MAX];
        char month[MAX];
        char year[MAX];
    };

    struct Local
    {
        char name[MAX];
        char nationality[MAX];
        char gender[MAX];
        Birthday bd;
        char subjects [MAX][MAX];
        char grades [MAX][MAX];
        int numOfCourses;

    };

    struct Foreigner
    {
        char name[MAX];
        char nationality[MAX];
        char gender[MAX];
        Birthday bd;
        char subjects [MAX][MAX];
        char grades [MAX][MAX];
        int numOfCourses;
    };

    union Student
    {
        Local     localStudent;
        Foreigner foreignStudent;
    };

    enum CountryType {S, F};

    struct UowStudents
    {
        CountryType ct;
        Student st;
    };

    int fileToArray (fstream& afile, char fileName [], UowStudents* x, char& locale);

    int main ()
    {
        srand(time_t(NULL));

        fstream afile;
        UowStudents x [MAX];
        char fileName[MAX];
        char locale;
        cout << "Enter filename: ";
        cin >> fileName;
        int size = fileToArray (afile, fileName, x, locale);


    }

    int fileToArray (fstream& afile, char fileName [], UowStudents* x, char& locale)
    {
        afile.open(fileName, ios::in);

        if (!afile)
        {
            cout << fileName << "could not be opened for read" << endl;
            exit (-1);
        }
        //file to array.


        char dateJunk;
        int numOfCourses;

        int k = 0;
        while (!afile.eof())
        {
            for (int k = 0; k < 3; k++)
            {
            afile >> locale;
                switch (locale)
                {
                    case 'F':
                        afile.getline(x[k].st.foreignStudent.nationality, MAX);
                        afile >> x[k].st.foreignStudent.gender;
                        afile.getline(x[k].st.foreignStudent.name, MAX);
                        afile >> x[k].st.foreignStudent.bd.day;
                        afile >> dateJunk;
                        afile >> x[k].st.foreignStudent.bd.month;
                        afile >> dateJunk;
                        afile >> x[k].st.foreignStudent.bd.year;
                        afile >> x[k].st.foreignStudent.numOfCourses;

                        for (int i = 0; i < x[k].st.foreignStudent.numOfCourses; i++)
                        {
                            afile >> x[i].st.foreignStudent.subjects[k];

                            afile >> x[i].st.foreignStudent.grades[k];
                        }
                        break;

                    case 'S':
                        afile >> x[k].st.localStudent.gender;
                        afile.getline(x[k].st.localStudent.name, MAX);
                        afile >> x[k].st.localStudent.bd.day;
                        afile >> dateJunk;
                        afile >> x[k].st.localStudent.bd.month;
                        afile >> dateJunk;
                        afile >> x[k].st.localStudent.bd.year;
                        afile >> x[k].st.localStudent.numOfCourses;;
                        for (int i = 0; i < x[k].st.localStudent.numOfCourses; i++)
                        {
                            afile >> x[i].st.localStudent.subjects[k];

                            afile >> x[i].st.localStudent.grades[k];
                        }
                }
                }
        }

        //Tests my cstring arrays to see everything is copied in correctly.
        //The print for foreign student cstrings also has information for the
        //one Singaporean student "S" in the middle. Singaporean must go into
        //the local cstrings.
        for (int k = 0; k < 3; k++)
        {
        cout << locale << " " << x[k].st.foreignStudent.nationality;
        cout << endl;
        cout << x[k].st.foreignStudent.gender;
        cout << x[k].st.foreignStudent.name;
        cout << endl;
        cout << x[k].st.foreignStudent.bd.day << " - ";
        cout << x[k].st.foreignStudent.bd.month << " - ";
        cout << x[k].st.foreignStudent.bd.year;
        cout << endl;
        cout << x[k].st.foreignStudent.numOfCourses;
        cout << endl;
        for(int i = 0; i < x[k].st.foreignStudent.numOfCourses; i++)
        {
        cout << x[i].st.foreignStudent.subjects[k] << " ";
        cout << x[i].st.foreignStudent.grades[k] << " ";

        }
            cout << endl;
        }

//as you can see, everything gets copied into localStudent struct as well.
        for (int k = 0; k < 3; k++)
        {

            cout << x[k].st.localStudent.gender;
            cout << x[k].st.localStudent.name;
            cout << endl;
            cout << x[k].st.localStudent.bd.day << " - ";
            cout << x[k].st.localStudent.bd.month << " - ";
            cout << x[k].st.localStudent.bd.year;
            cout << endl;
            cout << x[k].st.localStudent.numOfCourses;
            cout << endl;
            for(int i = 0; i < x[k].st.localStudent.numOfCourses; i++)
            {
                cout << x[i].st.localStudent.subjects[k] << " ";
                cout << x[i].st.localStudent.grades[k] << " ";

            }
            cout << endl;
        }




        return 0;
    }
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1 回答 1

0

您正在看到union在 C++ 中使用 a 的效果。

联合的主要特征是所有成员共享同一块内存。并且由于您的成员结构Local和Foreigner布局完全相同，您可以使用其中任一来打印联合的内容。

如果您更改其中一种结构（例如，从 中删除nationality）Local，您将看到一些打印输出出现乱码（尤其是那些将外国学生打印为本地学生的条目，反之亦然）。这是因为用于学生的内存块对于本地和外国学生的解释不同。
注意：从技术上讲，这会导致未定义的行为，但在这种情况下，结果不太可能是灾难性的。正式地，您只能读取您上次写入联合的同一成员，或者（如果成员是结构）结构的初始成员与上次写入联合的结构中的初始成员相同。

于 2013-01-06T10:28:40.047 回答