polymorphism and protected inheritance issue

Question

I have this code, and in the line tp = new asp_table , it won't let me to compile saying I don't have access. I don't understand why? I tried to make a pointer from base class to derived class, but it wouldn't let me. I would like to understand why.

class table {    
    int size;     
    int priority;   

    public:       
    table(int s=0,int p=0):size(s),priority(p){ }
    virtual void print(){}
};

class stud_table:public table {   
    char * name;  
    int gr;

    public: 
    void print() {cout<<"students table"<<endl;}  
    ~stud_table() {delete []name;} 
};

class asp_table:protected table { 
    char* thesis;
}; 

int main() {  
    stud_table st;   
    table * tp = &st; 
    tp = new asp_table;
    stud_table * stp = &st;   
    cout<<"Program"<<endl;    
    return 0;
} 

Answer 1

The error message says it all:

cannot cast 'asp_table' to its protected base class 'table'

protected inheritance means only asp_table and its derived classes know of the inheritance. Therefore tp = new asp_table is not possible outside the class or its derived classes.