С++ 读/写文件

Question

Using C++, gcc.9x, Linux. I try to open and read file and then keep it opened for further operation - rewrite it for each iteration. But each time, after I do open this file - it gets wiped out. Is it possible to keep file content until I rewrite it? And I want to keep file opened for writing all the time.

constructor()
{
    {
        ifstream tmp("file.db");
        int date;
        tmp >> date;
    }

    // it gets wiped out here, but I don't want it. 
    // And I want to keep ofstream opened all the time.
    fileStreamMember_.open("file.db");  // std::ofstream
}


writeMethod()
{
    fileStreamMember_.seekp(0, ios::beg);
    fileStreamMember_<< date_ << endl;
}

Answer 1

1-st: File objects have state. State is updated after each operation. You can use code like:

if (!fileStreamMember_) {
  // There is an error after operation
}

You can/should check state of file object during manipulations.

2-nd: You should close file object (or destroy it) to write all changes.

Answer 2

std::ofstream open() function (and constructor) has a default flag of trunc . You should specify the flags/mode parameter explicitly or use std::fstream instead.