dynamic_cast: 'a' is not a polymorphic type

[Maurice Termeer]

Hi, suppose i've got this:

class a {
public:
int n;
};

class b : public a {
public:
};

and I next do this:

a *x = new b();
b *y = dynamic_cast<b *>(x);

If I compile this, the compiler (msvc++ 2005) says:

error C2683: 'dynamic_cast' : 'a' is not a polymorphic type

and gnu g++ 3.2.3 says:

cannot dynamic_cast `x' (of type `class a*') to type `class b*' (
source type is not polymorphic)

I don't understand this. Why can't I dynamically cast a pointer of type
base class to some derived class? Probably I have to tell the compiler
that class a has some subclasses. Anyone got an idea?

Maurice Termeer

 

[Sharad Kala]

I don't understand this. Why can't I dynamically cast a pointer of type
base class to some derived class? Probably I have to tell the compiler
that class a has some subclasses. Anyone got an idea?

You got to have at least one virtual function in your base class to use
dynamic_cast or to make it polymorphic.

Sharad

 

[Ron Natalie]

You got to have at least one virtual function in your base class to use
dynamic_cast or to make it polymorphic.

You need to have a polymorphic class to use dynamic_cast IN THIS FASHION.

The presence of the virtual function in the base class is what makes it "polymorphic"
by DEFINITION.

 

[John Harrison]

Hi, suppose i've got this:

class a {
public:
int n;

Add this

virtual ~a() {}

and it will work

};

class b : public a {
public:
};

and I next do this:

a *x = new b();
b *y = dynamic_cast<b *>(x);

If I compile this, the compiler (msvc++ 2005) says:

error C2683: 'dynamic_cast' : 'a' is not a polymorphic type

john

 

[Rolf Magnus]

Hi, suppose i've got this:

class a {
public:
int n;
};

class b : public a {
public:
};

and I next do this:

a *x = new b();
b *y = dynamic_cast<b *>(x);

If I compile this, the compiler (msvc++ 2005) says:

error C2683: 'dynamic_cast' : 'a' is not a polymorphic type

and gnu g++ 3.2.3 says:

cannot dynamic_cast `x' (of type `class a*') to type `class b*' (
source type is not polymorphic)

I don't understand this. Why can't I dynamically cast a pointer of type
base class to some derived class?

Just as the compiler says: because your base class is not polymorphic, i.e.
doesn't use dynamic typing. Therefore, dynamic type casting can't work.

Probably I have to tell the compiler that class a has some subclasses.

No. You just have to make it polymorphic by adding a virtual member function
or making the destructor virtual.