如何使用C ++将向量中的数据写入特定位置的文本文件？

Question

If my text file looks so..

usrname@gmail.com 3223434324 data1 data2

name@gamil.com 2343454454 data3 data4

I have read these values using vector. I need to add a string at the end of first line of the file. What should I do? My code is as follows:

system("cls");
cout << "\nView proposal\n";
fstream file1;
string ap, aname, aphno;
file1.open("C:\\Users\\Nandhu\\Desktop\\glosys\\joinpolicy.txt",
    ios::in|ios::app);
while(!file1.eof())
{
    while(getline(file1, text))
    {
        istringstream xstream(text);
        vector<string> vcol;
        while(getline(xstream, word, ','))
        {
            vcol.push_back(word);
        }
        vec.push_back(vcol);
    }
    file1.close();
}
for(vector<vector<string> >::iterator itr = vec.begin();
    itr != vec.end(); itr++)
{
    for(vector<string>::iterator itr2 = itr->begin();
        itr2 != itr->end(); itr2++)
    {
        string str;
        str = *itr2;
        cout << str << " ";
    }
    cout << "\n";
}
cout << "\n1.Approve";
cout << "\n2.Reject";
cout << "\nDo you wish to approve?";
cin >> ans;     
if(ans == 'y')
{
    file1.open("C:\\Users\\Nandhu\\Desktop\\glosys\\joinpolicy.txt",
        ios::in|ios::app);
    file1.seekg(0);
    cout << "\nEnter the customer id you wish to approve for:";
    cin >> aname;
    vec.clear();
    if(file1.is_open())
    {
        while(getline(file1, text))
        {
            istringstream xstream(text);
            vector<string> vcol;
            while(getline(xstream, word, ','))
            {
                vcol.push_back(word);
            }
            vec.push_back(vcol);
        }
        file1.close();
    }

    for(vector<vector<string> >::iterator itr = vec.begin();
        itr != vec.end(); itr++)
    {   
        vector<vector<string> > vec;
        vector<string> vcol;

        for(vector<string>::iterator itr2 = itr->begin();
            itr2 != itr->end(); itr2++)
        {
            string str;
            str=*itr2;
            if(str==aname)
            {
                itr + 3;    
                file1.open("C:\\Users\\Nandhu\\Desktop\\glosys\\joinpolicy.txt",
                    ios::out|ios::app);
                vector<string>.insert(itr->end(), "approved");
                file1 << "approved";
                cout << "approved";
                file1.close();
            }   
        }
    }
}       
break;

Answer 1

You cannot directly insert data in the middle of a file. You can only re-write the entire file or append to it at the end.

To insert data in the middle you have a few options:

1) Read the file into a memory buffer, transform it as needed and then truncate the file to zero length and rewrite it from the buffer.

2) As 1, but instead of truncating the original file, write a new (temporary) file and then rename it over the original.

3) Identify the point where you need to make changes, read everything after that point to a memory buffer and make your changes, then truncate the file to the size previously identified and then write the memory buffer to the file in append mode.

4) Memory map the file, make the changes needed and flush them to the file.