Can't use getline when reading from a file

Question

I want to read numbers from a file and I'm having problems with the getline. I have the following code, I'll post the part that matters:

main.cpp

int main() {
    GrafNoEtiquetat Graf;
    ifstream f_ent;
    ofstream f_sort;
    string str;

    cout << "Introdueix el nom del fitxer a llegir." << endl;
    cin >> str;
    char *cstr=new char[str.size()+1];
    strcpy(cstr, str.c_str());
    f_ent.open(cstr);
    if(f_ent.fail()) cerr << "El fitxer no s'ha pogut obrir." << endl;
    else {
        Graf(cstr);
        unidireccional(Graf);
        delete [] cstr;
        cout << "Introdueix el nom del fitxer de surtida" << endl;
        cstr = new char [str.size()+1];
        strcpy(cstr, str.c_str());
        f_sort.open(cstr);
        if(f_sort.fail()) cerr << "El fitxer no s'ha creat." << endl;
        else Graf.escriureGraf(f_sort);
    }
    return 0;
}

And here is the function that creates the Graf using const char * cstr:

GrafNoEtiquetat::GrafNoEtiquetat(const char * cstr) {
    char c[1000];
    int n1, n2;
    cstr.getline(c,80);
    while(c!="#") {
        nNodes++;
        cstr.getline(c,80);
    }
    arestes.resize(nNodes+1);
    while(!cstr.eof()) {
        cstr >> n1;
        cstr >> n2;
        afegirAresta(n1, n2);
    }
    cstr.close();
}

I'm getting errors in all the lines i use 'cstr', at the getline, cstr.eof(), when I read n1 and n2 and when i want to close the file. The errors are similar to the following:

error: request for member 'getline' in 'cstr', which is of non-class type 'const char*'

I don't know why is this happening, any clues?

Paweł Stawarz · Accepted Answer · 2013-11-28T19:53:16.313

The error message says exacly what the problem is. There is no getline method which would be a member of a const char*.

you define cstr as const char * cstr, and then try to call getline on it: cstr.getline(c,80);. You should use it to read contents from a istream, not an array of characters.

If you want to do it your way, do it as follows:

GrafNoEtiquetat::GrafNoEtiquetat(const char * cstr) {
    ifstream inputFile(cstr);
    char c[1000];
    int n1, n2;
    inputFile.getline(c,80);
    while(c!="#") {
        nNodes++;
        inputFile.getline(c,80);
    }
    arestes.resize(nNodes+1);
    while(!inputFile.eof()) {
        inputFile >> n1;
        inputFile >> n2;
        afegirAresta(n1, n2);
    }
    inputFile.close();
}

And you should also check if the file was openned properly. To do that use ifstream::is_open.

Can't use getline when reading from a file

1 Answers1