Why does copy constructor not need to check whether the input object pointers to itself or not？

As the code below, the copy assignment operator has to check whether the input object pointers to itself or not. I wonder why copy constructor does not need to do the same check.

I am novice in C++.I would be grateful to have some help on this question.

  class rule_of_three
    {
        char* cstring; // raw pointer used as a handle to a dynamically-allocated memory block

        void init(const char* s)
        {
            std::size_t n = std::strlen(s) + 1;
            cstring = new char[n];
            std::memcpy(cstring, s, n); // populate
        }
     public:
        rule_of_three(const char* s = "") { init(s); }

        ~rule_of_three()
        {
            delete[] cstring;  // deallocate
        }

        rule_of_three(const rule_of_three& other) // copy constructor
        { 
            init(other.cstring);
        }

        rule_of_three& operator=(const rule_of_three& other) // copy assignment
        {
            if(this != &other) {
                delete[] cstring;  // deallocate
                init(other.cstring);
            }
            return *this;
        }
    };