Make getline() read the right line

Question

I have 3 functions that use getline. The first function checks which of the 2 functions to call depending on the first character of a line.

iMessage* getMessage(std::ifstream& ifs, char n) {

    iMessage* msg;
    std::string line;

    std::getline(ifs, line, n);

    if (line[0] == 'T') {

        msg = new Twitter(ifs, n);
    }

    else if (line[0] == 'e') {

        msg = new eMail(ifs, n);
    }

    else msg = nullptr;

    return msg;
}

And in the twitter / email constructors:

eMail::eMail(std::ifstream& ifs, char n) {


    std::string a;

    std::getline(ifs, a, n);

    //do stuff
}

Twitter::Twitter(std::ifstream& ifs, char n) {

    std::string a;

    std::getline(ifs, a, n);
    //do stuff
}

The problem is that the constructors are always reading the first line of the file. How do I make it so the constructors are reading the same line as the getMessage() function?

Answer 1

getMessage doesn't need to read an entire line to do its job. It can look at just the first character.

In fact, there's a function to look at the first character without removing it from the stream: peek() .

So the whole thing is as simple as

iMessage* getMessage(std::ifstream& ifs, char n)
{
    switch (ifs.peek())
    {
        case 'T': return new Twitter(ifs, n);
        case 'e': return new eMail(ifs, n);
        default:  return nullptr;
    }
}

but the return type should be std::unique_ptr<iMessage> . std::unique_ptr is very helpful for preventing memory leaks.

Answer 2

In getMessage you can restore the file position after reading the line:

std::streampos pos = ifs.tellg();
std::getline(ifs, line, n);
ifs.seekg(pos);