why the program is crushing [duplicate]

  #include <iostream>

    using namespace std;

    class Shallow {
    private:
        int *data;
    public:
        void set_data_value(int d) { *data = d; }
        int get_data_value() { return *data; }
        // Constructor
        Shallow(int d);
        // Copy Constructor
        Shallow(const Shallow &source);
        // Destructor
        ~Shallow();
    };

    Shallow::Shallow(int d) {
        data = new int;
        *data = d;
    }

    Shallow::Shallow(const Shallow &source) 
        : data(source.data) {
            cout << "Copy constructor  - shallow copy" << endl;
    }

    Shallow::~Shallow() {
        delete data;
        cout << "Destructor freeing data" << endl;
    }

    void display_shallow(Shallow s) {
        cout << s.get_data_value() << endl;
    }

    int main() {

        Shallow obj1 {100};
        display_shallow(obj1);

doing alright until here
Shallow obj2 {obj1}; here where my program stop doing ok obj2.set_data_value(1000); enter code here can anyone explain this to me in this point
return 0; } in the end of my program something going wrong