C ++在一项任务中读/写txt文件->交换字符

Question

I try to open a txt File for reading / writing in one task. My prior target is to exchange some characters by the saved ones in an array:

   void Inputfile::decryptFile(string filename)
{
    for(int i=0;i<15;i++)
    {
        fstream filedest(filename.c_str(), ios::in | ios::out);
        if(!filedest)
            cerr << "Konnte Zieldatei nicht oeffnen!\n";
        else 
            cout << endl << filename << " geoeffnet zum entschluesseln!\n";

        while(!filedest.eof())
        {
            filedest.get(ch);
            if(ch == char(this->mostcharsencrypted[i]))
            {
                filedest.put(char(this->mostchars[i]));
            }

        }
        filedest.close();
        cout << "Fertig!";
    }
}

mostcharsencrypted[] and mostchars[] are integer Arrays that hold the characters. I am sure that there is just 8 Bit Ansii Value in, and I check that even before this method gets called.

So if the currently read character is the one at the current array position (i: 0 - 14) then i want to exchange the character in the txt-file with the one from mostchars[].

Currently I can see that I get exactly the type of matches, but my txt file still shows the same content.

Answer 1

When replacing a file, usually two separate streams are used for input and output individually. If you need to use one stream for both input and output, the while loop will be like the following:

while(filedest)
{
    streampos p = filedest.tellg();
    if(!filedest.get(ch)) break;
    filedest.seekp(p);
    char ch2 = (ch == char(this->mostcharsencrypted[i])) ?
        char(this->mostchars[i]) : ch;
    filedest.put(ch2);
    filedest.sync();
}