inheritance question

[zildjohn01]

hi, i would like to be able to tell which type of derived class a base class is. pseudocode:

// pseudocode.cpp
#include <stdio.h>

class Base{};
class Derived : public Base {};

void test(Base &cls) {
	if(typeof(cls) == typeof(Derived))
		printf("cls is 'Derived'\n");
	else if(typeof(cls) == typeof(Base))
		printf("cls is 'Base'\n");
}

void main() {
	Base b;
	Derived d;
	test(b);
	test(d);
	printf("done\n");
}

any ideas if this is possible in standard c++ ??

 

[trollacious]

According to

http://www.codeproject.com/vcpp/stl/typeof.asp

One of missing operators in the C++ standard is the typeof operator which returns the type-of given expression. It is often implemeted as a compiler extension, but is still missing from the Visual Studio's C++ compiler (7.1).

Lee

 

[PerFnurt]

The whole point is that the function Test doesn't need to know how Base is instantiated.

/Per
[sub]
www.perfnurt.se[/sub]

 

[ArkM]

If the Base is not a polymorphic class (no virtual functions), no matter what's a class of test() argument (you can't access derived class members via cls reference.
Another case (it works in VC++ 6 etc):

class Base{public:virtual ~Base(){}};
class Derived : public Base {};
...
void test(Base &cls) 
{
  Derived d;
  const type_info& tyb = typeid(cls);
  const type_info& tyd = typeid(d);
  cout << (tyb==tyd?"Derived":"Base") << endl;
}

int main() { // not void main()!
    Base b;
    Derived d;
    test(b);
    test(d);
    cout << "done" << endl;
    return 0;
}

Don't forget to turn on RTTI support...

 

[PerFnurt]

A third case that is kind of cheating, but anyway:

// Forwvard declaration
class Derived;  

class Base
{
public:
  Base() {}
  virtual ~Base() {}

  Derived* getDerived() { return NULL; }
};

class Derived : public Base
{
	public:
	Derived():Base() {}
	virtual ~Derived() {}

	Derived* getDerived() { return this; };

	void derivedOnlyMehtod() {};
};


void test(Base& base)
{
	Derived* d = base.getDerived();
	if (d!=NULL) d->derivedOnlyMethod();
}

/Per
[sub]
www.perfnurt.se[/sub]

 

[cpjust]

I've never found typeid to be very reliable in VC 6.0. If I have 2 classes derived from the same base and I say:

if ( typeid( derived1 ) == typeid( derived2 ) )

it will always be true, because for some stupid reason the typeid always returns the type of the base class.