Find size of a struct without using sizeof operator

How will you find the size of a structure in C/C++ language without using sizeof operator.

(Note: sum of sizes of all fields of a structure may not be equal to the exact memory allocated to the structure. See, Method-1 in solution).

Solution:

Method-1: (Erroneous)

You may be tempted to calculate the size of all the fields of structure, then compute the sum of all these sizes and get the result. For example:

        struct Student{
            int rollno;
            char name[25];
            float avgMarks;
        }obj;

You can say that

        size_of_obj = sizeof(int) + 25*sizeof(char) + sizeof(float);

There are 2 problems in this:

We are using sizeof operator to calculate the sizeof fields.. :)

There may be padding along the fields of a structure. This padding is an extra memory which will not be used to store anything, but to optimize the read/write of individual fields of a structure variable. (If a field is stored in two words of memory then it will take 2 reads, but if we leave the space in between so that the field is stored in just one word, the read & write operation will take only one cycle, read more about it here...).

Right method is to allocate an array of structure and compare the memory addresses of consecutive elements of an Array:

        struct Student arr[2];
        size_of_struct = (char*)&arr[1] - (char*)&arr[0];

Note that if we don't convert the address to char* then the answer will always be 1 (because it will talk about the memory block & not memory location).