C / C++ / MFC

//  Demonstrate dynamic_cast on template classes.
#include <iostream>
using namespace std;

template <class T>
class Num {
protected:
    T val;
public:
    Num(T x) { val = x;}
    virtual T getval() {    return val;    }
};

template <class T>
class SqrNum : public Num<T> {
public:
    SqrNum(T x) : Num<T>(x) {}
    T getval()  {   return (val * val);   }
};

int main(){
    Num<int> *bp, numInt_ob(2);
    SqrNum<int> *dp, sqrInt_ob(3);
    Num<double> numDouble_ob(3.3);

    bp = dynamic_cast<Num<int>*> (&sqrInt_ob);          //sqrInt_ob is from a derived class of Num<int>.
    if(bp) {
        cout << "Cast from sqrNum<int>* to Num<int>* OK.\n"
             << "Value is " << bp->getval() << endl;
    }
    else
        cout << "Error !\n\n";

    dp = dynamic_cast<SqrNum<int>> (&numInt_ob);        //numInt_ob is base class object and SqrNum is a derived class.
    if(dp)
        cout << "Error !\n";
    else {
        cout << "Cast from Num<int>* to SqrNum<int>* not OK.\n"
             << "Can't cast a pointer to a base object into\n"
             << "a pointer to a derived object\n\n";
    }

    bp = dynamic_cast<Num<int>*> (&numDouble_ob);        //Num<int> of type int & numDouble_ob is of type Num<double>
    if(bp)
        cout << "Error" << endl;
    else {
        cout << "Can't cast from Num<double>* to Num<int>*.\n"
             << "These are 2 different types.\n\n";
    }

    return 0;
}

Quote:

error: 'val' was not declared in this scope
T getval() { return (val val); }
^~~

Quote:

class SqrNum : public Num<t>

//  Demonstrate dynamic_cast on template classes.
#include <iostream>
using namespace std;

template <class T>
class Num {
protected:
    T val;
public:
    Num(T x) { val = x;}
    virtual T getval() {    return val;    }
};

template <class T>
class SqrNum : public Num<T> {
public:
    SqrNum(T x) : Num<T>(x) {}
    T getval()  {   return (Num<T>::val * Num<T>::val);   }
};

int main(){
    Num<int> *bp, numInt_ob(2);
    SqrNum<int> *dp, sqrInt_ob(3);
    Num<double> numDouble_ob(3.3);

    bp = dynamic_cast<Num<int>*> (&sqrInt_ob);          //sqrInt_ob is from a derived class of Num<int>.
    if(bp) {
        cout << "Cast from sqrNum<int>* to Num<int>* OK.\n"
             << "Value is " << bp->getval() << endl;
    }
    else
        cout << "Error !\n\n";

    dp = dynamic_cast<SqrNum<int>*> (&numInt_ob);        //numInt_ob is base class object and SqrNum is a derived class.
    if(dp)
        cout << "Error !\n";
    else {
        cout << "Cast from Num<int>* to SqrNum<int>* not OK.\n"
             << "Can't cast a pointer to a base object into\n"
             << "a pointer to a derived object\n\n";
    }

    bp = dynamic_cast<Num<int>*> (&numDouble_ob);        //Num<int> of type int & numDouble_ob is of type Num<double>
    if(bp)
        cout << "Error" << endl;
    else {
        cout << "Can't cast from Num<double>* to Num<int>*.\n"
             << "These are 2 different types.\n\n";
    }

    return 0;
}