Problem casting from a vector

[DJ.precario]

Hi,

I have a vector (myVector) of BaseClass objects. I store in it a
DerivedClass object (where DerivedClass inherits from BaseClass). Later
I want to get the last element in the vector and store it in a
DerivedClass variable, doing:

DerivedClass d =
(DerivedClass)myVector[(int)dataTablesVector.size()-1];

this gives me the following error:

error C2440: 'type cast' : cannot convert from
'std::allocator<_Ty>::value_type' to 'DerivedClass'
with
[
_Ty=BaseClass *
]
Where am I going wrong?

Thanks.

[Rolf Magnus]

DJ*********@gmail.com wrote:

Hi,

I have a vector (myVector) of BaseClass objects. I store in it a
DerivedClass object (where DerivedClass inherits from BaseClass). Later
I want to get the last element in the vector and store it in a
DerivedClass variable, doing:
That doesn't make much sense. If it is a BaseClass variable, how would you
copy its contents into a DerivedClass one? What would the myVector of the
copy be initialized to?
DerivedClass d =
(DerivedClass)myVector[(int)dataTablesVector.size()-1];
Don't use C style casts. They are a big no-no in C++.
this gives me the following error:

error C2440: 'type cast' : cannot convert from
'std::allocator<_Ty>::value_type' to 'DerivedClass'
with
[
_Ty=BaseClass *
]
Where am I going wrong?

You are trying to copy a BaseClass object into a DerivedClass one. That's
only possible if you declare a conversion constructor or conversion
operator that does this. But I don't see how that could work. Maybe you
actually want a reference?

[Peter Julian]

<DJ*********@gmail.com> wrote in message
news:11**********************@o13g2000cwo.googlegr oups.com...

Hi,

I have a vector (myVector) of BaseClass objects. I store in it a
DerivedClass object (where DerivedClass inherits from BaseClass). Later
I want to get the last element in the vector and store it in a
DerivedClass variable, doing:

DerivedClass d =
(DerivedClass)myVector[(int)dataTablesVector.size()-1];

this gives me the following error:

error C2440: 'type cast' : cannot convert from
'std::allocator<_Ty>::value_type' to 'DerivedClass'
with
[
_Ty=BaseClass *
]
Where am I going wrong?

Forget the code. The logic doesn't make sense. Why are you storing base
class objects in a Derived class? Can you store a vector of vehicles in a
car? Would you store a vector of shapes in a triangle? Why would you store
Animals in a Chicken?

Inheritence only works when a strict relationship exists between the base
and its derivative. Its an exceptional relationship.

Storing a type in a class is called composition. A Car has a motor. A truck
has a vector of wheels. An airplane has a landing gear. A building has a
vector of floors. A Ferry can hold vehicles, etc...

Also, as already mentioned to you, casting is not guesswork in C++. This is
a language with strong type checking which extends to its casting mechanisms
(static_cast, dynamic_cast, etc). Casting from a base class to a derived
class implies that the object is a derived object to begin with.

[Rolf Magnus]

Peter Julian wrote:

I have a vector (myVector) of BaseClass objects. I store in it a
DerivedClass object (where DerivedClass inherits from BaseClass). Later
I want to get the last element in the vector and store it in a
DerivedClass variable, doing:

DerivedClass d =
(DerivedClass)myVector[(int)dataTablesVector.size()-1];

this gives me the following error:

error C2440: 'type cast' : cannot convert from
'std::allocator<_Ty>::value_type' to 'DerivedClass'
with
[
_Ty=BaseClass *
]
Where am I going wrong?

Forget the code. The logic doesn't make sense. Why are you storing base
class objects in a Derived class? Can you store a vector of vehicles in a
car?

Well, there are trucks that are used to transport cars.
Would you store a vector of shapes in a triangle?

Not in a triangle, but what about a compound object that is composed of
several simple shapes? Most 2D drawing programs offer something like that.

This structure is also called the composite design pattern, and it's not so
uncommon.