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#include <vector>
 
class Array {
    std::vector<int> Arr;
    int n;
public:
    Array(int _n = 0) : Arr(_n), n(_n) {
    }
    ~Array(void) {
    }
    friend void f(const Array &A) {
        A.Arr[0] = 3;  // why can the Arr[0], Arr[1] be changed the value ?
        A.Arr[1] = 4;
    //  A.n = 10;        // can not be changed because of 'const class type'
    }
};
 
void main()
{
    Array A(5);
    f(A);
}
 

prog.cpp: In function 'void f(const Array&)':
prog.cpp:12:18: error: assignment of read-only location 'A.Array::Arr.std::vector<_Tp, _Alloc>::operator[]<int, std::allocator<int> >(0u)'
         A.Arr[0] = 3;  // why can the Arr[0], Arr[1] be changed the value ?
                  ^
prog.cpp:13:18: error: assignment of read-only location 'A.Array::Arr.std::vector<_Tp, _Alloc>::operator[]<int, std::allocator<int> >(1u)'
         A.Arr[1] = 4;
                  ^
prog.cpp: At global scope:
prog.cpp:18:11: error: '::main' must return 'int'
 void main()
           ^

Standard output is empty