Re: Why can't pass vector of "Derived" to function that takes vector of "Base"?

On Apr 28, 6:05 pm, Rob <someidunknown1...@yahoo.com> wrote:


A template instantiated twice, one holding type T1 and the other type
T2, are completely unrelated types according to the rules of the C++
type system. As such, it's no different than trying to pass a Person
structure to a function expecting a Shape object.

Now logically, you might think that if the relationship between two
types T1 and T2 are polymorphic, then arrays or vectors of them ought
to be also, but it just isn't so.

Fist, be glad you used a vector and not a raw array (passed as a
pointer) as that *would* compile, and would have devastating results.
Consider the issue: How do you index into an array? The way it works
is through pointer arithmetic on the size of the array element.

Since an array of N objects of type T live in consecutive memory, the
Ith element is calculated by multiplying the size of T by I, and
adding that offset to the base address of the array.

Now suppose each object of your base class (T1) was 10 bytes in size,
but each derived object (T2) was 24 bytes. *If* you could use vectors
(or arrays) polymorphically, then element 0 would "line up" since they
both would have the same address, but when you try to access any
element after that, the arithmetic is wrong. Suppose you index
element 3:

   Let ary_base be the address of element 0 in the array or vector.

   Then:

   calculated address: sizeof(T1)*3 + ary_base ==> 30 + ary_base

   desired address: sizeof(T2)*3 + ary_base ==> 72 + ary_base

Since ary_base is the same in both cases, we can ignor it, and just
look at the offset to it. We want 72, we get 30. See the problem?

This is a well-known problem with arrays, and it extends to vectors
too, except that with vectors you have the benefit of a compile-time
detection of this subtle error, and with arrays you have runtime
failures. There are many places you can get detailed discussion about
this issue, but the FAQ is a particularly good one:
http://www.parashift.com/c++-faq-lite/proper-inheritance.html#faq-21.4

Depending on your situation, you can possibly work around this problem
it by changing your create() code into a function template, which
operates on a vector of the actual type you are using:

     template <typename T>
     void create(std::vector<T>& arr)
     {
     // ...
     }

--
Chris

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