C function to return true when pointer points to a specific polymorphic type-CodePudding

Other languages have an "is" operator that allow this, how do I emulate this in C and can it be done at runtime?

Say for example,

class A {
public:
// virtual methods..
};
class B : public A {};
class C: public A {};

A* x;
template<typename T>
bool foo(); // returns true if par is of type T

int main() {

 A* b = new B();
 A* c = new C();
 x = c;

 if(foo<B>(x))
 {
  // do something
 }
}

CodePudding user response：

At runtime, you could dynamic_cast and see if you have a nullpointer. A dynamic cast will return nullptr at runtime if the input pointed type is not compatible with the output pointed type.

A* b = new B();
A* c = new C();
x = c;

if(dynamic_cast<B*>(x) != nullptr)
{
  // do something
}

CodePudding user response：

You can just write

if (dynamic_cast<B*>(x)) {
  // ...
}

since it returns nullptr (which evaluates as false) if the dynamic type doesn't match.

Note that this doesn't guarantee that B is the most-derived type, only that it is an accessible base class of the most-derived type.

Since you actually asked how to write the function foo, it's simply

template<typename Dynamic, typename Static>
bool foo(Static *p)
{
  return dynamic_cast<Dynamic*>(p);
}

but this isn't very interesting, and the foo<B>(x) call is less clear at the call site - to an experienced C developer - than just writing the dynamic_cast directly.

NB. The comments suggesting that this is usually a bad idea are correct - it's generally better to use virtual dispatch to handle this kind of thing automatically.