Read from file after end-of-file is reached

Question

I have a function named readNextString(ifstream &file , char* &pBuffer) which extracts the next string from a file, until ',' or '\n' is reached, removes whitespace at the beginning and at the end of the string, saves the rest in pBuffer and returns true if everything worked fine - false otherwise. Everything works until the end-of-file is reached. When the eof flag is set I can't move my get pointer. I've tried this:

if(file.eof())
{
   file.clear();
   file.seekg(0 , ios::end)
}

...and then remove whitespaces at the end of the string. This almost helped. The function extracts the string without the spaces but I get an infinite loop.

My actual question is: How can I check if the next char is EOF, and if I can't - is there any alternative way to do this?

Here is my actual function:

bool readNextString(ifstream &file , char* &pBuffer)
{
    if(file.eof()){
        return false;
    }
    for(; file.good() && isWhitespace(file.peek()) && !file.eof() ; file.seekg(1 , ios::cur))
        ;
    if(file.eof()){
        cout << "The file is empty.\n";
        return false;
    }else{
        streamoff startPos = file.tellg();
        cout << "startPos : " << startPos << endl;
        for(;file.good() && file.peek()!='\n' && file.peek()!=',' && file.peek()!= EOF; file.seekg(1 , ios::cur))
            ;
        streamoff A = file.tellg();
        cout << "A : " << A << endl;
        file.seekg(-1 , ios::cur);
        for(;file.good() && isWhitespace(file.peek()) ; file.seekg(-1 , ios::cur))
            ;
        file.seekg(2 , ios::cur);
        streamoff endPos = file.tellg();
        cout << "endPos : " << endPos << endl;
        pBuffer = new char[endPos-startPos];
        if(pBuffer)
        {
            file.seekg(startPos , ios::beg);
            file.get(pBuffer , endPos-startPos , ',' || '\n');
            for(;file.good() && file.peek()!='\n' && file.peek()!=',' && file.peek()!= EOF; file.seekg(1 , ios::cur))
                ;
            file.seekg(2 , ios::cur);
            streamoff temp = file.tellg();
            cout << "temp : " << temp << endl;
            return true;
        }else{
            cout << "Error! Not enough memory to complete the task.\nPlease close some applications and try again.\n";
            return false;
        }
    }
}

And that's one place where I call it:

void printCities()
{
    ifstream city ;
    city.open("cities.txt", fstream::in);
    if(city.is_open())
    {
        char *currCity;
        int counter = 1;
        while(readNextString(city , currCity))
        {
            cout << counter++ << ". " << currCity << endl;
            delete[] currCity;
            currCity = NULL;
        }
        if(city.eof())
            cout << "There are no cities added.\n";
        city.close();
    }else
        cout << "Error by opening 'cities.txt'.Make sure that the file exist and try again.\n";
}

Hope I was clear enough. If you find some other mistakes or possible errors I'll be glad to hear it and learn from it.

Answer 1

Ok.I did it otherwise! getline() for the WIN ! :D So this is my code (this time is much more readable) :

bool readNextString(ifstream &file , char pBuffer[] )
{
    while(isWhitespace(file.peek()) && !file.eof())
        file.ignore(1);
    if(!file.eof())
    {
        streamoff start = file.tellg();
        stringstream toComma;
        if(file.getline(pBuffer , 200 , ','))
        {
            toComma << pBuffer;
            toComma.getline(pBuffer ,200, '\n');
            int i=strlen(pBuffer)-1;
            for(; isWhitespace(pBuffer[i]) ;i--)
                ;
            pBuffer[i+1] = '\0';
            file.clear();
            file.seekg(start + strlen(pBuffer) , file.beg);
            return true;
        }else return false;
    }
    return false;
}

I've made some changes in my other function too:

void printCities()
{
    ifstream city ;
    city.open("cities.txt", fstream::in);
    if(city.is_open())
    {
        if(!isEmpty(city))
        {
            char currCity[200];
            int counter = 1;
            while(readNextString(city , currCity) && counter < 10)
                cout << counter++ << ". " << currCity << endl;
        }else
            cout << "There are no cities added.\n";
        city.close();
    }else
        cout << "Error by opening 'cities.txt'.Make sure that the file exist and try again.\n";
}

The function isEmpty(ifstream &file) returns true if the file is empty and false otherwise.

Thanks to all for help ! Best regards !

Answer 2

How can I check if the next char is EOF?

Like this

if (file.peek() == EOF)
{
    // next char is EOF
    ...
}

Answer 3

First, don't use seek to skip whitespace. Just get the character and be done with it.

Second, you seem to misunderstand the meaning of istream::good() and istream::eof(). There is never anytime where istream::good() is appropriate, and istream::eof() is usually only appropriate after input has failed. As for your loop skipping white space, the usual solution would be:

while ( isspace( file.peek() ) && file.peek() != EOF ) {
    file.get();     //  ignore read character...
}

Similar comments hold for the other loops as well, except that you won't want to ignore the read characters. To collect characters up to the next ',', for example:

std::string field;
while ( file.peek() != ',' && file.peek() != EOF ) {
    field.push_back( file.get() );
}

(And your file.get( pBuffer, endPos - startPos, ',' || '\n' ) certainly doesn't do what you expect it to; the expression ',' || '\n' will always evaluate to true, which, when converted to a char, is '\01.)

Finally, while the above strategy will work, it's far preferable to input larger units of text into an std::stream, and parse that. If the text is line oriented, use something like:

std::string line;
while ( std::getline( file, line ) ) {
    //  Parse line, using std::istringstream if appropriate,
    //  although this doesn't seem to be the case for your code.
}

This is orders of magnitude simpler than what you are doing.