Question

Well, Here is the C++ code I’ve got here and cant compile

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

const int n = 900;
class City{
string name;
double area;
int count, roads;


public:
City() {}
City(string a, double b, int c, int d ) { a=name; b=area; c=count; d=roads;}
string getname() {return name;}
double getarea() {return area;}
int getcount() {return count;}
int getroads() {return roads;}
friend ostream& operator << (ostream& , City& );
friend istream& operator >> (istream& , City& );

};

ostream& operator << (ostream& out, City& a) {
out<<"name "<<a.name<<", area "<<a.area<<", count "<<a.count<< ", roads   "<<a.roads<<endl;
return out; 
 }
istream& operator >> (istream& in, City& a) {
in>>a.name>>a.area>>a.count>>a.roads;
return in;
   }
void fill(int arr[], int size){
ifstream ifs("cities.txt.");
for (int i=0;i<size;i++) 
    ifs>>arr[i];
  }


 void func(City* arr){
ofstream ofs("density.out");
for(int i=0;i<n;i++){
    if(arr[i].getcount()/arr[i].getarea()>1000)
        ofs<<arr[i];
}
  }

  int main(){
City* hm;
hm = new City[n];
fill(hm, n);
func(hm);
system ("pause");
return 0;
 }

here is the error I get while compiling:

error C2664: ‘fill’ : cannot convert parameter 1 from ‘City *’ to ‘int
[]’

somehow I see that, it says something is wrong with the Class ‘City *” and int[] array, but can’t figure out. I change the ‘int’ with double but same prob.
that’s problem, otherwise it seems a simple array filling function from file.
any idea what’s wrong with it?

So, how would

void fill(City& arr, int size)
‘s body would like?

Answer 1

void fill(int arr[], int size) means that you need to pass an array of int to fill. However, you are passing it an instance of City, specifically hm in your main.

I am assuming that you are trying to read in a list of City descriptions from cities.txt (and thus have appropriate stream extraction/insertion operators). Change the signature of fill to accept a pointer to City objects so that you can fill in an array of City objects and not ints.

void fill(City *arr, int size); 

Make sure that the argument you pass to fill in main is a properly allocated array of City objects of size at least sz where sz is what you pass in as the second parameter. Keep in mind to call delete [] to free this array that you create.

A more idiomatic approach would be to use vector<City> so that you don’t have to worry about memory management issues. Your modified fill signature would then be:

void fill(std::vector<City>& c); // note we no longer need the second argument

You will need to #include <vector> in order to be able to use vector<City> though.

Finally, to take advantage of RVO, simply return a vector<City> by value instead of passing it in as a parameter. So, you’d do something like:

std::vector<City> fill(); // cleaner, faster