Re: templates: policy-based class design and overloading operator?

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Fri, 08 Oct 2010 11:29:18 -0400

Message-ID:

<i8ndcg$41h$1@news.eternal-september.org>

On 10/8/2010 10:51 AM, Rui Maciel wrote:

Let's say that we have a host class foo which has part of it's
interface implemented through a policy class of type BarPolicy, which
could be anyone between BarPolicy1 and BarPolicyN:

<code>

template<typename scalar> class BarPolicy1 { public: BarPolicy()
{}; ~BarPolicy() {};

scalar value(const size_t a, const size_t b); };

// snipped: other BarPolicy classes

template<typename scalar, template<typename> class BarPolicy> class
Foo : public BarPolicy<scalar> { public: Foo() {}; ~Foo()
{};

scalar value(const size_t a, const size_t b) {return 1; }; };

</code>

Now, I intend to overload operator<< to output the contents of class
Foo to the standard output streams, no matter what policy class has
been used in assembling a host class Foo and no matter what type has
been passed as Foo's scalar. The thing is, overloading operator<<
considering only the case where Foo is implemented with a specific
policy (for example, BarPolicy1) is a trivial task to accomplish.
Yet, I'm not getting any luck in overloading operator<< so that it
accepts any generic Foo class, independent of what policy has been
adopted.

So, can anyone help? Is it possible to declare such an operator? If
so, can anyone provide with an example?

What do you need that operator to do?

#include <ostream>

template<class scalar, template<class> class Policy>
class UsesPolicy : public Policy<scalar>
{
    friend std::ostream& operator << (std::ostream& os, UsesPolicy const&)
    {
        return os << "Generic output operator\n";
    }
};

template<class T> class Policy1 {};
template<class T> class PolicyN {};

#include <iostream>

int main()
{
    UsesPolicy<int,Policy1> up_int_P1;
    UsesPolicy<char,PolicyN> up_char_PN;

    std::cout << up_int_P1;
    std::cout << up_char_PN;
}

V
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