How to determine size of a class in C++ ?

Question

Below is the sample code, Where size of a is not defined at the time of compilation,
Then how compiler knows the size of a? If we run the same program in C then compilation will fail.

#include<iostream>
using namespace std;
main()
{
        string a;
        cin>>a;
        cout<<a;
}

Answer 1

If I understood your query correctly, your concern is about size of an empty class.
Size of an empty class is 1 byte. It is because if you create two objects of an empty class, there should be at least one byte difference between their addresses. Reason being, sizeof an empty class is not kept as zero.