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Reading a file named on the command line within a factory
Putting it all together example from a Well House Consultants training course
More on Putting it all together [link]

This example is described in the following article(s):
   • Reading files, and using factories to create vectors of objects from the data in C++ - [link]

Source code: flow.cpp Module: C239
#include <iostream>
#include <fstream>
#include <vector>

/* Practical example of reading from a file within a factory
and creating a vector of objects. Also include splitting attributes
from within each line of data as the object is defined */

// See examples flow2.cpp and flow4.cpp for earlier, shorter examples which
// we used in building up to this example.

using namespace std;

// -------------------------------

class Train {
                Train(char *);
                static Train* factory(char *);
                int getcapacity();
                static ifstream *datasource;
                int ncars;
                int spv;

// -------------------------------

ifstream * Train::datasource = NULL;

Train* Train::factory(char * source) {
        char trainline[257];

// Extra checking compared to earlier examples as file to read is named
// on the command line and so we're at the mercy of the user!

        if (! datasource) {
                datasource = new ifstream(source);
                if (! datasource->good()) {
                        cerr << "Unable to open data file" << endl;
                        return NULL;
        if (datasource->eof()) {
                return NULL;
        // Next line for demonstration
        cout << trainline << endl;
        return new Train(trainline);

Train::Train(char * source) {

// Source line example: Train 06:45 3 75

        int starts[4];
        int k=0;
        int param=0;
        while (source[k] != '\0') {
                if (source[k] == ' ') {
                        source[k] = '\0';
                        starts[param++] = k+1;
                if (param > 3) break;

        this->ncars = atoi(&source[starts[2]]);
        this->spv = atoi(&source[starts[1]]);

int Train::getcapacity() {
        return (int)(this->ncars * this->spv * 1.4);

// -------------------------------

int main (int argc, char **argv) {

        vector<Train *> *service = new vector<Train *>(0);
        int k;

        Train *current;

// As we're now looking for a file named on the command line, we had
// best check there is one there!

        if (argc != 2) {
                cerr << "Usage: " << argv[0] << " filename" << endl;
                return 1;

        while (1) { // Pass in file named on the command line
                current = Train::factory(argv[1]);
                if (! current) break;


        for (k=0; k<service->size(); k++) {
                cout << service->at(k)->getcapacity() << endl;

        return 0;


/* Sample Output

munchkin:cj12 grahamellis$ ./flow trainsdata.txt
Unable to open data file
munchkin:cj12 grahamellis$ ./flow
Usage: ./flow filename
munchkin:cj12 grahamellis$ ./flow trains.txt
Train 06:45 3 75
Train 07:17 2 74
Train 19:11 1 68
Train 19:47 2 77
munchkin:cj12 grahamellis$


Learn about this subject
This module and example are covered on the following public courses:
 * Learning to program in C and C++
 * C++ for C Programmers
 * C and C++ Programming
 * Learning to program in C and C++
 * C and C++ Programming
Also available on on site courses for larger groups

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