为什么程序崩溃

  #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);

一直做到这里为止
浅obj2 {obj1}; 这是我的程序停止执行的地方obj2.set_data_value（1000）; enter code here在这一点上谁能向我解释这一点，
返回0；在我的程序结束时出现了问题