Re: Template and function pointer question

From:
=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?= <Erik-wikstrom@telia.com>
Newsgroups:
comp.lang.c++
Date:
Mon, 15 Oct 2007 19:50:47 GMT
Message-ID:
<rwPQi.11416$ZA.7619@newsb.telia.net>
On 2007-10-15 21:09, webinfinite@gmail.com wrote:

This is my first post here, please tell me if I did anything wrong.

in the following code snippet:
1. template <class In, class Pred>
2. In find_if(In begin, In end, Pred f) {
3. while (begin != end && !f(*begin))
4. ++begin;
5. return begin;
6. }

7. bool is_negative(int n) {
8. return n < 0;
9. }

10. vector<int>::iterator i = find_if(v.begin(), v.end(),
is_negative);

find_if is the template needs a class Pred as its third argument but
in line 10's function call, it takes a function poniter is_negative,
does that mean in C++, function point is equivalent to class?


No. If you rewrite the template thingie like this instead

  template <typename In, typename Pred>
  In find_if(In begin, In end, Pred f)

it might become a little more clear, the types of In and Pred are
determined when the code is compiled, and it does not have to be a
class, anything that is a type will do.

If we take you code and add (before line 10) the following:

  struct IsNegative {
    bool operator()(int n) { return n < 0; }
  };

And after line 10 we add

  IsNegative pred;
  vector<int>::iterator i2 = find_if(v.begin(),v.end(), pred);

Now, in the first call the type of Pred in find_if() was a pointer to a
function, while in the second call the type is IsNegative. This process
where the types of the parameters are decided is called instantiation.
The template is not code that can be executed, it is just a template
describing a generic algorithm (or class), to be useful we must use this
description and fill in the parameters with real types, which created a
real function (or class). So with the additions that I made your program
will now contain two find_if functions, the first has the following
signature:

  typedef vector<int>::iterator iter;
  iter find_if(iter, iter, bool (*)(int));

Notice that the last argument is a function pointer. The second find_if
function have this signature:

  iter find_if(iter, iter, IsNegative);

The last argument is now an instance of the IsNegative class.

Hope this clear some things up for you.

--
Erik Wikstr??m

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