c++ downcasting

[zeny]

Hi everybody! I´m trying to make this program where i store in a vector different types of derived objects from a single base class. So i declare the vector like this:

vector<BaseClass> vec;

then, assuming that all derived objects are, naturally, base class objects i push a DerivedClass object into the vector:

vec.push_back(new DerivedClass(...));

When i pop one object from the vector, to know what kind of object i´m dealing with, i made the atribute "type" and the respective get method(getType()) in the base class. Then i do the following, wich gives a cast compilation error :

for(int i=0; i<vec.size(); i++){

if(vec[i].getType() == ...)

DerivedClass *obj = dynamic_cast<DerivedClass*> (vec[i]);
}

I´ve tried also the static cast and gives the same result. I´d be very gratefull for any help with this. Thanks for your attention.

[Banfa]

You have a pointer mis-match, you vector is declared

vector<BaseClass> vec;

but in your cast you use

DerivedClass *obj = dynamic_cast<DerivedClass*> (vec[i]);

vec[i] is of type BaseClass and you are trying to cast it to DerivedClass *

[zeny]

You have a pointer mis-match, you vector is declared

vector<BaseClass> vec;

but in your cast you use

DerivedClass *obj = dynamic_cast<DerivedClass*> (vec[i]);

vec[i] is of type BaseClass and you are trying to cast it to DerivedClass *

Yes, but as i said, aren´t the DerivedClass objects also automatically BaseClass objects???
In Java I know that this works perfectly even with arrays. For example:

BaseClass array[] = new BasClass[10];

array[0] = new DerivedClass(...);

DerivedClass obj = (DerivedClass)array[0];

The above code works in Java. How do i do the same in c++?

thanks

[Banfa]

Yes, but as i said, aren´t the DerivedClass objects also automatically BaseClass objects???

Yes. This is not about the type of object but how you are referencing it.

The DerivedClass objects are automatically BassClass objects, or rather have the functionality of the BaseClass objects and can be treated as such, however there is no circumstance in which an Object is a pointer to an object

This will never work

BaseClass objBC;
DerivedClass * pObjDC;

pObjDC = dynamic_cast<DerivedClass*>(objBC);

because objBC is not a pointer type and pObjDC is, you are trying to cast object into pointer to object.

If you are going to use pointers you need to dereference the pointer in question.

BaseClass objBC;
DerivedClass * pObjDC;

pObjDC = dynamic_cast<DerivedClass*>(&objBC);



You need to get your head round pointers, which are apparently one of the big differences between Java and C++, Java hides them away so you don't need to know about them.

[zeny]

Thank u so much. Now at least the program compiles sucessfully but when i try to access the methods of the pObjDC (pObjDC -> method()) it throws an exception that i haven´t yet managed to catch.
I must confess i´m a novice in c++ and i really have to get more into pointers.

[ima]

hi everybody...

how to limit the data in BYTE data type?
i have tried this:

if (strlen(passphrase)<8)

but it doesnt work and get error"invalid conversion from BYTE to char".actully, all my data is in BYTE.

thanks in advance.

-ima

[D_C]

That question sounds like it should go in it's own thread.

Are you familiar with the sizeof operator? passphrase is probably not a string (char*). Instead, try sizeof(passphrase), it should return how many bites the variable uses.