Re: C++ Templates on MS VS C++

From:
coder_lol@yahoo.com
Newsgroups:
comp.lang.c++
Date:
14 May 2007 00:09:45 -0700
Message-ID:
<1179126585.682043.134250@e65g2000hsc.googlegroups.com>
Hi Ben,

You got me worried :)

Per your example,
Array<int> buff(100); // array of 100 int's

Yes, that's what I want. An array of 100 ints. However, I'd like to
allocate the array of 100 ints dynamically with new. I thought that
Array<int> *buf = new Array<int>(100) was the right way to go. Sorry
for asking this basic question, but how do I allocate an array of ints
using new with my template, so the memory allocated will be on the
heap? Does the quesiton make sense at all?

Thanks!!!

On May 13, 10:07 pm, benben <benhonghatgmaildotcom@nospam> wrote:

coder_...@yahoo.com wrote:

I put together a quick array template for MS Visual Studio 2003, but I
ran into some trouble. It's been quite a while since I had to roll my
own templates, so I'd appreciate all help.

Example 1
Array<int> *ibuf = new Array<int>(100);
ibuf[0] = 100; //O K


The above is equivalent to:

Array<int>* ibuf = new Array<int>(100); // an array of 100 Array<int>'s

{
     const Array<int>& temp(100); // a Array<int> of 100 int's
     ibuf[0] = temp; // assign temp to first slot in ibuf

} // destroy temp

But judging your confusion in the following context, I doubt the above
is what you really want to achieve. You might have wanted to write:

   Array<int> buff(100); // array of 100 int's
   buff[0] = 21;

Example 2
String *sptr = NULL;
Array<String*> *locales = new Array<String*>(50);
locales[0] = sptr; // ERROR, WHY?


Because locales is not an Array. It's a POINTER to Array. And by
convention locales[0] refers to the first Array<String*> object out of
50 you have just created. So basically you are trying to assign an
Array<String*> with a String*. Read on.

(*locales)[0] = sptr; // OK, but WHY ???


The * dereferences locales so the expression (*locale) is an Array, and
hence the [] operator apply.

Example 3 - OK: forward declaration of template class without
including template header.
template<class int8>
class Array;

Example 4 - ERROR: why ??? forward declaration of template class
without including template header.
template<class String*>
class Array;


Example 3 worked. But I suspect that it doesn't work in a way you may
have expected. Basically to forward declare a class template you do:

     template <class T>
         class C;

Where T is just an identifier to be used like a typename in the template
definition.

So in Example 3, the declaration is valid, you have just substituted T
with int8 and C with Array.

In Example 4, the declaration is invalid. It's because String* is an
identifier followed by a * operator.

Thank-you all!

=== ARRAY TEMPLATE ===
template<class T=int32>
class Array : public Object
{


Although this is ok, I'd strongly recommend you not to give a default
type parameter and not to inherit from Object. So you'd be better off by
using:

    template <class T>
       class Array{

public:
   Array(int32 aSize);


For better portability, consider using std::size_t instead of int32.
(int32 is not a built-in type anyway.) Also, to avoid confusion you had
in Example 1, make the constructor explicit:

      explicit Array(std::size_t aSize);

   virtual ~Array();


If you drop the inheritance, then virtual destructor is not needed.

   int32 length() const;

   T& operator[](int32 i);


For const use, also provide:

      const T& operator[](std::size_t) const;

   T *getData() const;


The above member function is ill-declared. Consider instead:

    T* getData();
    const T* getData() const;

private:
   T *data;
   int32 size;


Before you finish the class, make sure you also provide a copy
constructor and an assignment operator:

     template <class U>
         Array(const Array<U>&);

     template <class U>
         Array<T>& operator=(const Array<U>&);

};


Regards,
Ben

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