Question

Please check out the following func and its output

void main()
{
    Distance d1;
    d1.setFeet(256);
    d1.setInches(2.2);

    char *p=(char *)&d1;
    *p=1;

    cout<< d1.getFeet()<< " "<< d1.getInches()<< endl;
}

The class Distance gets its values thru setFeet and setInches, passing int and float arguments respectively. It displays the values through through the getFeet and getInches methods.

However, the output of this function is 257 2.2. Why am I getting these values?

Answer 1

This is a really bad idea:

char *p=(char *)&d1;
*p=1;

Your code should never make assumptions about the internal structure of the class. If your class had any virtual functions, for example, that code would cause a crash when you called them.

I can only conclude that your Distance class looks like this:

class Distance {
    short feet;
    float inches;
public:
    void setFeet(...
};

When you setFeet(256), it sets the high byte (MSB) to 1 (256 = 1 * 2^8) and the low byte (LSB) to 0. When you assign the value 1 to the char at the address of the Distance object, you’re forcing the first byte of the short representing feet to 1. On a little-endian machine, the low byte is at the lower address, so you end up with a short with both bytes set to 1, which is 1 * 2^8 + 1 = 257.

On a big-endian machine, you would still have the value 256, but it would be purely coincidental because you happen to be forcing a value of 1 on a byte that would already be 1.

However, because you’re using undefined behavior, depending on the compiler and the compile options, you might end up with literally anything. A famous expression from comp.lang.c is that such undefined behavior could “cause demons to fly out of your nose”.