Negative Dec to Binary

Below is my program that reads decimal numbers and outputs them in binary code.  Know I am trying to read negative numbers and output them in 2's complement form.  For example, 8 is 0000000000001000 but -8 is 1111111111111000 in 2's complement form.  How can I do this.  I tried changing the <> but this did not work.  
my input has these numbers 252,8,9,45,63,74 (uses these numbers for negative as well).

#include <iostream>
#include <fstream>

using namespace std;

void dectobin(int num, int base, int count = 0);

int main()
{
      ifstream inFile;
      inFile.open("input3.txt");
      
      int decimalnum[6];
      int base;
      int i;
      base = 2;
      
      cout<<"Enter the number in decimal: "<<endl;
      if(!inFile)
      {
            cout<<"Cannot open input file!"<<endl;
            return 1;
      }
      else
            for (i=0 ;i<6;i++) {
                  inFile>>decimalnum[i];
                  cout<<"Decimal: "<<decimalnum[i]<<" = ";
                  dectobin(decimalnum[i], base);
                  cout<< " in Binary" <<endl;
            }
            
            return 0;
            
}
void dectobin(int num, int base, int count)
{
      if(num > 0 || count < 16)
      {
            dectobin(num/base, base, ++count);
            cout<<num % base;
            
      }
      
}