Re: How do I put a pointer to an operator?

From:
Rolf Magnus <ramagnus@t-online.de>
Newsgroups:
comp.lang.c++
Date:
Wed, 03 May 2006 14:39:35 +0200
Message-ID:
<e3a8a7$56u$03$1@news.t-online.com>
Victor Bazarov wrote:

Rolf Magnus wrote:

leriaat@elte.hu wrote:

I would like to create a calculator, and I thought that it would be a
good idea to write a universal "do_it" function that would have a
pointer to the operator and the two operands like this:

double do_it(double a, doulbe b, doulbe (*op)(doulbe, doulbe) ) {
return op(a, b); }

Well, the idea is great, but I don't know how could I put a pointer
to the + operator... Is it possible at all? I mean something like
this:

cout << do_it(1, 2, &(operator+) );


    cout << do_it( 1, 2, std::plus<int>() );

You can't get pointers to the built-in operators. You can write
functions that themselves use those operators and then get pointers
to those functions. But I think it would be better to make an
abstract Operator base class and for each operation that your
calculator can do add a class that derives from it. Something like:

#include <iostream>

class Operator
{
[..]
};

class Plus : public Operator
{
   [..]
};

class Minus : public Operator
{
   [..]
};

void calc_result(double a, double b, const Operator& op)
{
   std::cout << a << ' ' << op.name()
             << ' ' << b << " = "
             << op(a, b) << '\n';
}


I always thought that 'std::plus' and 'std::minus' were specifically
for that purpose (except the "name", of course). See 20.3.2, and the
documentation on your library implementation, of course.


They are made for generic algorithms and don't provide run-time
polymorphism.
When writing a calculator in C++, you usually waht the user to be able to
specify the operation to be done at run-time.

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