cast one object to another

Question

0.00/5 (No votes)

See more:

Hi, what is the best way to cast one object to another? (no pointers).

e.g., I have a function:

C++

void app:f(ClassA object)
{

ClassB object2 = (ClassB) object;
}

would this work? We can assume ClassB is derived from ClassA.

thanks.

Posted 24-Feb-13 23:39pm

Matra Divat

Add a Solution

Comments

Philippe Mori 27-Feb-13 19:14pm

Do you want a new object or the same object interpreted differently?

Do you wants to assume that the object is of type ClassB or verify it?

4 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Fredrik Bornander · Answer 1 · 2013-02-24T23:41:00

Solution 1

If ClassB derives ClassA, then you should do;

C++

ClassB object2 = *dynamic_cast<ClassB*>(&object);

But you don't want to do that. Pass references and cast them using dynamic_cast without the referencing and dereferencing instead.
Fredrik

Posted 24-Feb-13 23:41pm

Fredrik Bornander

Updated 25-Feb-13 0:22am

v3

Comments

Stefan_Lang 25-Feb-13 6:06am

The right way, but the wrong types: dynamic_cast can only be used on references and pointers.

Fredrik Bornander 25-Feb-13 6:22am

Right you are.

Philippe Mori 27-Feb-13 19:10pm

You also need a reference on object2 otherwise you would have new object...

Stefan_Lang · Answer 2 · 2013-02-25T00:25:00

Matra Divat wrote:
We can assume ClassB is derived from ClassA.

In that case fix the function prototype:

C++

void app::f(ClassB b) {
   // ...
}

Casting a type is always a consequence of inappropriate interface design. If you have the ability to fix the interface, do that before having to introduce type casts!

If for any reason you cannot avoid the cast, the safe and correct way is not to assume, but to verify. Doing so is possible using dynamic_cast[^], but this cast only works on pointers and references. In your case, you should be doing sth like this:

C++

class ClassA {
   // ...
};
class ClassB : public class ClassA {
   // ...
};

void app::f(ClassA a) // 'a' is passed by value - is that intended?
{
   ClassB* pb = dynamic_cast<ClassB*>(&object);
   assert (0 != pb);  // verify your assumption - pointer will be NULL on failure
   if (0 == pb) {
      // this code will only be processed in release mode
      // issue an error message or throw an exception
   }
   else {
      ClassB& b = *pb;        // using reference to memory location of a
      // continue using your converted object ...
   }
}

H.Brydon · Answer 3 · 2013-02-25T08:56:00

Solution 3

void app:f(ClassA object)
{
 
ClassB object2 = (ClassB) object;
}

This won't do a cast (in C++). The syntax used in your question looks a lot like something that came from some java code (or C#?).

Study the difference between "ClassA" and "ClassA*". Likewise "ClassB" and "ClassB*".

You might have more success with the following:

void app:f(ClassA* object)
{
 
ClassB* object2 = (ClassB*) object;
}

but there are a lot of ways to go off the tracks with this too.

Posted 25-Feb-13 8:56am

H.Brydon

Comments

Stefan_Lang 26-Feb-13 8:48am

Actually the cast will work - at least it will compile. It is a C-style cast and will simply reinterpret the memory. You'd have the same effect (without cast) if you define a union:

union AB {
   ClassA object;
   ClassB object2;
};
void app::f(ClassA object) {
   AB objectab;
   objectab.object = object;
   ClassB object2 = objectab.object2;
}

H.Brydon 26-Feb-13 11:32am

You say "Actually the cast will work" - actually no - you are not doing a cast. If your code looks like:

void app:f(ClassA object)
{
ClassB object2 = (ClassB) object;
}

the "(ClassB)" does not do a cast. What happens is that 'object' is copied to a temporary "ClassB" object (shallow copy in this case) and assigned to 'object2'. There is no cast. Your union example is wrong. The objects are different. In C++, they are not given the same address (as they would in a union). Objects 'object' and 'object2' are distinct, where 'object2' is a shallow copy of 'object'.

If you did something similar in java, then object2 would be an alias of object. Not in C++.

Stefan_Lang 27-Feb-13 4:12am

Ok, it seems I've confused this with reinterpret_cast. I remembered code that did similar casts, and therefore assumed it must work the same way,

However, I just tried and the cast indeed only works the other way round (from ClassB to ClassA in this case).

Obviously the union example is not a good one then, given the fact the 'cast' wouldn't even compile. Sorry for the confusion.

H.Brydon 27-Feb-13 9:46am

No, it didn't work. Either way around. Look at the difference between the following 2 statements:

ClassB object2 = (ClassB) object;
ClassB* pObject2 = (ClassB*) pObject;

The first is not a cast no matter what the objects are. It is a shallow copy (usually, depending on the class definition).

Stefan_Lang 27-Feb-13 10:03am

For me, a "cast" is whatever the compiler makes of a cast statement. If your definition is different, then we're talking apples and oranges. Anyway, I don't see any remaining disagreements.

samadhan_kshirsagar · Answer 4 · 2013-02-27T01:13:00

Solution 4

union AB {
ClassA object;
ClassB object2;
};
void app::f(ClassA object) {
AB objectab;
objectab.object = object;
ClassB object2 = objectab.object2;
}

Posted 27-Feb-13 1:13am

samadhan_kshirsagar

Comments

Philippe Mori 27-Feb-13 19:15pm

Why not use reinterpret_cast if you want to pretent that the object is an unrelated type (or static_cast if typoes are related).

cast one object to another

4 solutions

Solution 1

Solution 2

Solution 3

Solution 4

Add your solution here

Preview 0

Existing Members

...or Join us