how to update record file in c++?

Question

Here is my code:

void person :: edit_teacher(char *Id)
{
    int count = 0;
    fstream file; // as fstream pointer is on the last position so we have to mmove the pointer on the first poistion //
    
    file.open("TeachersRecord.txt", ios::in| ios::out| ios::ate);
    
    file.seekg(0); // it will move the pointer on the first location //
    if(!file)
    {
        cout << "File is not exist." <<endl;
    }
    else{
        
        file.read((char*)this, sizeof(*this));
        
        while(!file.eof()){
            if(!strcmp(Id,this->id)){
                
                cout << "\n\t\t\t*********Enter new record********" << endl << endl;
                setData();
                
                file.seekp((int)file.tellp()-sizeof(*this));
                
                file.write((char *)this, sizeof(*this));
            }
            file.read((char*)this, sizeof(*this));
        }
        if(count == 0)
        {
            cout << endl <<"Record is not found.";
            
        }
        else{
            cout << "\nRecord is Successfully Updated.";
        }
        file.close();
    }
}

The update file only edits the first two data types, others show as empty, and the last two give address?

Can anyone help me with this?

You should write all your changes and the old content to a new file, rather than trying to modify an existing file. — Thomas Matthews, May 06 '21 at 21:30
Also, see [Why is iostream::eof inside a loop condition (i.e. `while (!stream.eof())`) considered wrong?](https://stackoverflow.com/questions/5605125/) — Remy Lebeau, May 06 '21 at 22:10

Remy Lebeau · Answer 1 · 2021-05-06T22:35:29.220

Don't manipulate the input file while you are reading from it. Write your changes to a new file first, and then replace the original file with the new file only if everything is successful.

Also, you are reading each record in the file into this, which is just bad design, and will potentially corrupt this. More importantly, you did not show the declaration of person, but based on your description, it sounds like person contains data members that simply can't be read/written as-is and will need additional serialization. For instance, char*/std::string strings (flat char[] arrays are OK).

Also, you are not incrementing count if the requested Id is found, but you are checking if count has been incremented when the loop is finished.

Also, your code lacks adequate error handling.

Try something more like this instead:

#inlcude <fstream>
#include <cstdio>
...

bool readPerson(std::istream &in, person &p)
{
    // do whatever is needed to de-serialize a person record *properly*...
    return in.read(reinterpret_cast<char*>(&p), sizeof(p));
}

bool writePerson(std::ostream &out, const person &p)
{
    // do whatever is needed to serialize a person record *properly*...
    return out.write(reinterpret_cast<char*>(&p), sizeof(p));
}

void person::edit_teacher(char *Id)
{
    std::ifstream infile("TeachersRecord.txt");
    if (!infile)
    {
        std::cout << "Can't open File to read." << std::endl;
    }
    else
    {
        std::ofstream outfile("TeachersRecord_new.txt");
        if (!outfile)
        {
            std::cout << "Can't create File to write." << std::endl;
        }
        else
        {
            bool found = false;
            person p;

            while (readPerson(infile, p))
            {
                if ((!found) && (strcmp(Id, p.id) == 0))
                {
                    found = true;
                
                    std::cout << "\n\t\t\t*********Enter new record********" << std::endl << std::endl;

                    p.setData();
                    *this = p;
                }
                
                if (!writePerson(outfile, p))
                {
                    outfile.close();
                    std::remove("TeachersRecord_new.txt");
                    std::cout << "Can't write to File." << std::endl;
                    return;
                }
            }

            if (!infile.eof())
            {
                std::cout << "Can't read from File." << std::endl;
                return;
            }

            infile.close();
            outfile.close();

            if (!found)
            {
                std::remove("TeachersRecord_new.txt");
                std::cout << "Record is not found." << std::endl;
            }
            else if (std::rename("TeachersRecord.txt", "TeachersRecord_bak.txt") != 0)
            {
                std::remove("TeachersRecord_new.txt");
                std::cout << "Can't Backup File." << std::endl;
            }
            else if (std::rename("TeachersRecord_new.txt", "TeachersRecord.txt") != 0)
            {
                std::rename("TeachersRecord_bak.txt", "TeachersRecord.txt");
                std::remove("TeachersRecord_new.txt");
                std::cout << "Can't replace File." << std::endl;
            }
            else
            {
                std::remove("TeachersRecord_bak.txt");
                std::cout << "Record is Successfully Updated." << std::endl;
            }
        }
    }
}

how to update record file in c++?

1 Answers1