Displaying garbage value - c++14

Question

When the friendly function add is used to print the value of sum variable of two classes, the correct output is printing. But, when then display function of their respective class are used, garbage value is printing.
What's wrong with the code?
#include <iostream>
using namespace std;
class DB;
class DM {
int m, cm;
float sum;
public:
void read()
{
cout << "Enter meters and centimeters respectively\n";
cin >> m >> cm;
}
void display(void)
{
cout << sum << " meters\n";
}
friend void add(DM p1, DB p2);
};
class DB {
int feet, inch;
float sum;
public:
void read()
{
cout << "Enter feets and inches respectively\n";
cin >> feet >> inch;
}
void display(void)
{
cout << sum << " feets\n";
}
friend void add(DM p1, DB p2);
};
void add(DM p1, DB p2)
{
float a = p2.feet * 12;
float b = a + p2.inch;
float c = b * .3048 + p1.m + p1.cm * .01;
//cout << c << endl;
p1.sum = c;
//cout << p1.sum << endl;
//p2.sum = (c/12)/.3048;
cout << p2.sum << endl;
}
int main()
{
DM obj1;
DB obj2;
obj1.read();
obj2.read();
add(obj1, obj2);
obj1.display();
obj2.display();
}

Answer 1

The problem is that you print sum but you never initialize the sum of obj1 and obj2.
You initialize sum only inside add() but you pass values to add() by copy, so you initialize sum only in temporary objects.
When you print obj1.sum and obj2.sum (through display()), the sum member are still not-initialized.
So the garbage.
Try passing objects by reference
// ..........V........V
void add (DM & p1, DB & p2)
This way the initialization of sum inside add() has effect also for calling objects.