写入/读取二进制/文本文件

Question

wFile.open(fileName, ios::out | ios::trunc);
if (!(wFile.is_open()))
{
    cout << "Error in opening the file" << endl;
    return false;
}
else
{
    for (unsigned int i = 0; i < Widgets.size(); i++)
    {
        Widgets.at(i)->copyToBinary(fileName);
    }
    wFile.close();
    return true;
}

I'm trying to copy different objects types from a vector. My problem is that when this code runs for the copy, it only copies the last object. It seems like the code just overrides the existing text.

Also, I have this code in each class (this is the copyToBinary function):

ofstream file(fName);
file << *this;
file << endl;

What am I missing here?

Answer 1

You should not reopen the file in copyToBinary . Pass wFile as argument not filename :

...copyToBinary(ostream &file) {
  file << *this;
  file << endl;
}

and call ...copyToBinary(wFile) .

Answer 2

Problem:

You pass the finename to copyToBinary() , reopening a stream and overwriting it.

Solution:

Pass the wFile stream as reference and write to it without reopening each time

void copyToBinary(ostream& file) {
   file << *this;
   file << endl;
}

Answer 3

First, when you are opening the file

wFile.open(fileName, ios::out | ios::trunc);

Everytime it is called the existing content get lost as you are using trunc mode. It will be better if you use ios::app mode. So that whenever the above code runs, it opens the file in append mode.

Second, You are passing the filename in the copyToBinary function. And By using the default constructor

ofstream file(fName);

your file is getting opened everytime in default ios::out mode but not in ios::app mode. It will be better either you pass the refernece of opened file or open the file in function as append mode.