Question

I declared the following struct in my C++ program:

    struct person {
        char name[10];      /* first name */
        char id[10];        /* ID number */
        off_t pos;      /* position in file, for demonstration */
    } people[] = {
        { "arnold", "123456789", 0 },
        { "miriam", "987654321", 10240 },
        { "joe",    "192837465", 81920 },
    };

        j = sizeof(people) / sizeof(people[0]);     /* count of elements */

gives j = 3 here, i.e, no of elements in the array; always even if you add or reduce the elements…

But

         char b[8];
         i = sizeof(b)/sizeof(b[0]);

gives the value of i = a constant = 4 on my machine.

Now thats justified as the sizeof(char*) is constant on my machine and the sizeof(char) too is constant..

But as soon as I declare the struct person, the sizeof(person*) and sizeof(person) should also be constant, and it should also yield a constant value, isn’t it???

Answer 1

Your compiler is wrong.

char b[8];
i = sizeof(b)/sizeof(b[0]);

should yield i==8.

The result you’re getting for the struct size is correct. I would switch compilers if I were you.

If you were to pass b to a function calculating the size, than you’d be right. But as the code is now, no.

Also, if it was a function calculating sizeof(people) / sizeof(people[0]) which received people as a parameter, you’d also get a constant.

This is because arrays decay to pointers when passed as arguments.