问如何在c++中将子类安全地转换为父类？

EN

#include <iostream>
using namespace std;

class base{
public:
    virtual bool test1() = 0;
};

class base2{
public:
    int a;

    base2(int p) : a(p) {}
};

class son:public base, public base2 {
public:
    bool test1() {
        return true;
    }
    son(int p) : base2(p) {}
};

void out(base2* p) {
    cout << p -> a;
}

int main()
{
    
    /* if I use this, it will print a wrong number,such as -1359748056
     base * o = new son(9);
     out((base2*)o);
     */
    
    /* if I use this, it will print 9
    out(new son(9));
    */
    return 0;
}