how to discard from streams? .ignore() doesnt work for this purpose, any other methods? - c++

Question

I have a lack of understanding about streams. The idea is, to read a file to the ifstream and then working with it. Extract Data from the stream to a string, and discard the part which is now in a string from the stream. Is that possible? Or how to handle those problems?
The following method, is for inserting a file which is properly read by the ifstream. (its a text file, containing informations about "Lost" episodes, its an episodeguide. It works fine, for one element of the class episodes. Every time i instantiate a episode file, i want to check the stream of that file, discard the informations about one episode (its indicated by "****", then the next episode starts) and process the informations discarded in a string. If I create a new object of Episode I want to discard the next informations about the episodes after "****" to the next "****" and so on.
void Episode::read(ifstream& in) {
string contents((istreambuf_iterator<char>(in)), istreambuf_iterator<char>());
size_t episodeEndPos = contents.find("****");
if ( episodeEndPos == -1) {
in.ignore(numeric_limits<char>::max());
in.clear(), in.sync();
fullContent = contents;
}
else { // empty stream for next episode
in.ignore(episodeEndPos + 4);
fullContent = contents.substr(0, episodeEndPos);
}
// fill attributes
setNrHelper();
setTitelHelper();
setFlashbackHelper();
setDescriptionHelper();
}
I tried it with inFile >> words (to read the words, this is a way to get the words out of the stream) another way i was thinking about is, to use .ignore (to ignore an amount of characters in the stream). But that doesnt work as intended. Sorry for my bad english, hopefully its clear what i want to do.

Answer 1

If your goal is at each call of Read() to read the next episode and advance in the file, then the trick is to to use tellg() and seekg() to bookmark the position and update it:
void Episode::Read(ifstream& in) {
streampos pos = in.tellg(); // backup current position
string fullContent;
string contents((istreambuf_iterator<char>(in)), istreambuf_iterator<char>());
size_t episodeEndPos = contents.find("****");
if (episodeEndPos == -1) {
in.ignore(numeric_limits<char>::max());
in.clear(), in.sync();
fullContent = contents;
}
else { // empty stream for next episode
fullContent = contents.substr(0, episodeEndPos);
in.seekg(pos + streamoff(episodeEndPos + 4)); // position file at next episode
}
}
In this way, you can call several time your function, every call reading the next episode.
However, please note that your approach is not optimised. When you construct your contents string from a stream iterator, you load the full rest of the file in the memory, starting at the current position in the stream. So here you keep on reading and reading again big subparts of the file.
Edit: streamlined version adapted to your format
You just need to read the line, check if it's not a separator line and concatenate...
void Episode::Read(ifstream& in) {
string line;
string fullContent;
while (getline(in, line) && line !="****") {
fullContent += line + "\n";
}
cout << "DATENSATZ: " << fullContent << endl; // just to verify content
// fill attributes
//...
}

Answer 2

The code you got reads the entire stream in one go just to use some part of the read text to initialize an object. Imagining a gigantic file that is almost certainly a bad idea. The easier approach is to just read until the end marker is found. In an ideal world, the end marker is easily found. Based on comments it seems to be on a line of its own which would make it quite easy:
void Episode::read(std::istream& in) {
std::string text;
for (std::string line; in >> line && line != "****"; ) {
text += line + "\n";
}
fullContent = text;
}
If the separate isn't on a line of its own, you could use code like this instead:
void Episode::read(std::istream& in) {
std::string text;
for (std::istreambuf_iterator<char> it(in), end; it != end; ++it) {
text.push_back(*it);
if (*it == '*' && 4u <= text.size() && text.substr(text.size() - 4) == "****") {
break;
}
if (4u <= text.size() && text.substr(text.size() - 4u) == "****") {
text.resize(text.size() - 4u);
}
fullContent = text;
}
Both of these approaches would simple read the file from start to end and consume the characters to be extracted in the process, stopping as soon as reading of one record is done.