Inheritance & polymorphism

[placid]

Hi all,

I was just wondering, in Java there is an operator called instanceOf,
whats the equivalent in c++?

Thanks in advance to all.

 

[puzzlecracker]

Hi all,

I was just wondering, in Java there is an operator called instanceOf,
whats the equivalent in c++?

Introduction to C++.

 

[David Hilsee]

Hi all,

I was just wondering, in Java there is an operator called instanceOf,
whats the equivalent in c++?

Thanks in advance to all.

The closest equivalent is dynamic_cast (the pointer variant), but it
resembles C#'s "as" more than it does instanceof.

 

[placid]

The closest equivalent is dynamic_cast (the pointer variant), but it
resembles C#'s "as" more than it does instanceof.

is there another way then a dynamic_cast ?

 

[Werner]

is there another way then a dynamic_cast ?

Hi,

Unlike Java, C++ does not have a common polymorphic ancestor. This does
not stop you from doing the following, what may be more or less what
java does behind the scenes, I guess:

#include <typeinfo>
#include <iostream>

struct Base
{
bool instanceOf( Base const& other ) const
{
//Just doing this for clarity...
const std::type_info& myInfo( typeid(*this) );
const std::type_info& otherInfo( typeid(other) );

//Could have used: return( typeid(*this)==typeid(other) );
return ( myInfo == otherInfo );
}
virtual ~Base(){}
};

struct Derived : Base
{
};


int _tmain(int argc, _TCHAR* argv[])
{
Base base;
Derived d1;
Derived d2;
std::cout << "D1, D2:" << d1.instanceOf( d2 ) << std::endl;
std::cout << "D1, Base: " << base.instanceOf( d1 ) << std::endl;
std::cout << "Base, D1: " << d1.instanceOf( base ) << std::endl;

std::cin.get();
}

//Output:
D1, D2:1
D1, Base: 0
Base, D1: 0

You go figure!

Regards,

Werner

 

[mlimber]

Hi all,

I was just wondering, in Java there is an operator called instanceOf,
whats the equivalent in c++?

Thanks in advance to all.

The typeid operator is similar but not equivalent. There is also the
Boost TypeTraits library
(http://boost.org/doc/html/boost_typetraits.html), which is also part
of the TR1 tentative extension to the C++ standard library and which
gives a compile-time value for type queries such as "Is class A a base
class of class B?"

Cheers! --M

 

[Josh Mcfarlane]

is there another way then a dynamic_cast ?

Not built in, although you could code a static function to return the
class type. The question you have to ask yourself is why do you need it?

 

[guyarad]

I'm not familier with java, so can you explain what does instanceOf do?

 

[David Hilsee]

I'm not familier with java, so can you explain what does instanceOf do?

It allows you to determine if an object is an instance of a class or one of
the class's children. It's usually used to determine if a you can cast an
object to a certain type. Its usage will often look like this:

Something x = ...;
if ( x instanceof SomethingElse ) { // is x a SomethingElse?
SomethingElse x2 = (SomethingElse)x; // cast it
// do something with x2
}

 

[red floyd]

It allows you to determine if an object is an instance of a class or one of
the class's children. It's usually used to determine if a you can cast an
object to a certain type. Its usage will often look like this:

Something x = ...;
if ( x instanceof SomethingElse ) { // is x a SomethingElse?
SomethingElse x2 = (SomethingElse)x; // cast it
// do something with x2
}

if (SomethingElse x2 = dynamic_cast<SomethingElse*>(&x))
{
// do something with x2
}
else
{
// x is not a SomethingElse
}

 

[David Hilsee]

if (SomethingElse x2 = dynamic_cast<SomethingElse*>(&x))
{
// do something with x2
}
else
{
// x is not a SomethingElse
}

Yes, ignoring a missing *, that's roughly the C++ equivalent of the Java
code I provided.