using dynamic_cast with template classes.

Tarun Jha

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

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

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

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

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

dp = dynamic\_cast\> (&numInt\_ob);        //numInt\_ob is base class object and SqrNum is a derived class.
if(dp)
    cout << "Error !\\n";
else {
    cout << "Cast from Num\* to SqrNum\* 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\*> (&numDouble\_ob);        //Num of type int & numDouble\_ob is of type Num
if(bp)
    cout << "Error" << endl;
else {
    cout << "Can't cast from Num\* to Num\*.\\n"
         << "These are 2 different types.\\n\\n";
}

return 0;

}

in the code above i m getting error that

Quote:

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

but

Quote:

class SqrNum : public Num

according to this the protected value in the base class should just work fine as protected value in the derived class, but it isn't ? How do i solve this problem ? EDIT : below is the working code:

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

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

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

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

bp = dynamic\_cast\*> (

Lost User

dp = dynamic_cast*> (&numInt_ob); //numInt_ob is base class object and SqrNum is a

Needs an '*' after <int>. Code now runs successfully.

Tarun Jha

i added * after SqrNum but it is still giving that error .

CPallini

Quote:

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

Should be

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

Tarun Jha

Quote:

return (Num::val * Num::val);

but i have already inherited all the members of the base class to the derived class, then why i need to use scope resolution operator to point where to look for val ?

Lost User

Did not give that error in the original code that I compiled.

CPallini

See [35.19] Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?[^]

Tarun Jha

thank's

CPallini

You are welcome.