Question

im trying to write a program that reads from a text file into a linked list

here is the list structure.

#include <iostream> 
#include <string>
#include <fstream>
using namespace std;


struct Video { 
char video_name[1024];      //  video name
int ranking;                // Number of viewer hits
char url[1024];             //  URL
Video *next;  // pointer to Video structure
}  *head = NULL;        // EMPTY linked list

here is the read-in code:

void load()
{
struct Video *temp;
temp = (Video*)malloc(sizeof(Video)); //allocate space for node 
temp = head;
ifstream rankFile ("Ranking.dbm");
if (rankFile.is_open())
{
    while ( rankFile.good() )
    {
        cin.getline(rankFile, temp->video_name, "\n");
        cin.getline(rankFile, temp->ranking, "\n");
        cin.getline(rankFile, temp->url, "\n");
        temp = temp->next; 

    }
    myfile.close();
}

else cout << "Unable to open file"; 

return ;

}

it is reading from a text file Ranking.dbm which looks like this:

bagheera
20
bagheera.com
sushi
60
sushi.com
wicket
99
wicket.com
teek
100
teek.com

however i am getting an error saying: Invalid conversion from void* to char* on all 3 of my cin.getline() statements while it is reading from the file. I need to be able to read in line by line from my file (Ranking.dbm) and store each set of 3 lines to temp->video_name , temp->ranking and temp->url and then create a new nodes and save the next 3 lines… so on and so forth till ive read in everything from the file.

how can i do this? am i going about this in a completely wrong manner or is this just a syntax error? i’m still getting the hang of C++ :/

Answer 1

This is incorrect use of std::istream::getline():

cin.getline(rankFile, temp->video_name, "\n");

and does not make any sense as there are two input streams involved: cin and rankFile. The correct invocation (but not the most preferable) is:

rankFile.getline(temp->video_name, 1023);

Suggest:

• using std::string instead of char[] and use std::getline(in, std::string&).
• use operator>> to read the int as you cannot use std::getline() for this.
• check result of every read operation.
• don’t use malloc() in C++ use new and delete.
• don’t dynamically allocate if you don’t have to.
• use one of the STL containers for holding a list rather than implementing that yourself, std::vector<Video> for example.

For example:

struct Video { 
    std::string video_name;
    int ranking;
    std::string url;
};

std::vector<Video> load()
{
    std::vector<Video> result;
    std::ifstream rankFile("Ranking.dbm");
    if (rankFile.is_open())
    {
        Video temp;
        std::string line;
        while (std::getline(rankFile, temp.video_name) &&
               rankFile >> temp.ranking &&
               std::getline(rankFile, line) && // need to skip 'ranking's
                                               // unread new-line
               std::getline(rankFile, temp.url))
        {
            result.push_back(temp);
        }
    }
    else
    {
        std::cerr << "Unable to open file"; 
    }

    return result;
}