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Factory method and file input to seed objects
I/O in C++ example from a Well House Consultants training course
More on I/O in C++ [link]
Source code: flow2.cpp Module: C235
#include <iostream>
#include <fstream> using namespace std; /* A factory method implemented in C++, reading from a data file within the factory with each call returning the next object pointer, and a NULL return once there's no more data. This example has a fixed size array to hold the objects that are returned by the factory - see NEXT EXAMPLE (flow4) for a Vector based example */ // ------------------------------- class Train { public: Train(char *); static Train* factory(); int getcapacity(); private: static ifstream *datasource; int ncars; int spv; }; // ------------------------------- /* We're using a static method (a factory pattern) to generate our objects rather than calling the constructor directly. This allows us to handle the reading of data from a file (in the form of an iterator and return a NULL when we have run out of data */ ifstream * Train::datasource = NULL; Train* Train::factory() { char trainline[256]; if (! datasource) { // Only on the first call do you open the file datasource = new ifstream("trains.txt"); } // read the (next) line from the file datasource->getline(trainline,256); 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 // This constructor takes a character string record containing all the // initial values for an object, separated them out ans stores the ones // it needs into variables within the object int starts[4]; int k=0; int param=0; // Split the string by replacing the incoming separators with nulls so that // you have in effect a series of null terminated strings while (source[k] != '\0') { if (source[k] == ' ') { source[k] = '\0'; starts[param++] = k+1; } k++; if (param > 3) break; } // Get the integers from the appropriate fields this->ncars = atoi(&source[starts[2]]); this->spv = atoi(&source[starts[1]]); } int Train::getcapacity() { // 40% standing allowed in a train without it being overcrowded ... return (int)(this->ncars * this->spv * 1.4); } // ------------------------------- int main () { Train *service[10]; int traincount = 0; int k; // Main program is quite short and straightforward - clever stuff is in the classes Train *current; while (1) { current = Train::factory(); if (! current) break; service[traincount++] = current; } for (k=0; k<traincount; k++) { cout << service[k]->getcapacity() << endl; } return 0; } /* Sample running munchkin:cj12 grahamellis$ g++ -o flow2 flow2.cpp munchkin:cj12 grahamellis$ ./flow2 Train 06:45 3 75 Train 07:17 2 74 Train 19:11 1 68 Train 19:47 2 77 315 207 95 215 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 Books covering this topic
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