virtual template inheritance : template inheritance « template « C++ Tutorial

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3. template inheritance

/* The following code example is taken from the book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>

// PolicySelector<A,B,C,D> creates A,B,C,D as base classes
// - Discriminator<> allows having even the same base class more than once

template <typename Base, int D>
class Discriminator : public Base {
};

template <typename Setter1, typename Setter2,
          typename Setter3, typename Setter4>
class PolicySelector : public Discriminator<Setter1,1>,
                       public Discriminator<Setter2,2>,
                       public Discriminator<Setter3,3>,
                       public Discriminator<Setter4,4> {
};


// default policies

class DefaultPolicy1 {};
class DefaultPolicy2 {};
class DefaultPolicy3 {
  public:
    static void doPrint() {
        std::cout << "DefaultPolicy3::doPrint()\n";
    }
};
class DefaultPolicy4 {};


// define default policies as P1, P2, P3, P4
class DefaultPolicies {
  public:
    typedef DefaultPolicy1 P1;
    typedef DefaultPolicy2 P2;
    typedef DefaultPolicy3 P3;
    typedef DefaultPolicy4 P4;
};


// class to define a usage of the default policy values
// - avoids ambiguities if we derive from DefaultPolicies more than once
class DefaultPolicyArgs : virtual public DefaultPolicies {
};


// class templates to override the default policy values

template <typename Policy>
class Policy1_is : virtual public DefaultPolicies {
  public:
    typedef Policy P1;  // overriding typedef
};

template <typename Policy>
class Policy2_is : virtual public DefaultPolicies {
  public:
    typedef Policy P2;  // overriding typedef
};

template <typename Policy>
class Policy3_is : virtual public DefaultPolicies {
  public:
    typedef Policy P3;  // overriding typedef
};

template <typename Policy>
class Policy4_is : virtual public DefaultPolicies {
  public:
    typedef Policy P4;  // overriding typedef
};


// create class template with four policies and default values

template <typename PolicySetter1 = DefaultPolicyArgs,
          typename PolicySetter2 = DefaultPolicyArgs,
          typename PolicySetter3 = DefaultPolicyArgs,
          typename PolicySetter4 = DefaultPolicyArgs>
class BreadSlicer {
    typedef PolicySelector<PolicySetter1, PolicySetter2,
                           PolicySetter3, PolicySetter4>
            Policies;
    // use Policies::P1, Policies::P2, //... to refer to the various policies.
  public:
    void print () {
        Policies::P3::doPrint();
    }
    //...
};


// define a custom policy
class CustomPolicy {
  public:
    static void doPrint() {
        std::cout << "CustomPolicy::doPrint()\n";
    }
};

int main()
{
    BreadSlicer<> bc1;
    bc1.print();

    BreadSlicer<Policy3_is<CustomPolicy> > bc2;
    bc2.print();
}

DefaultPolicy3::doPrint()
CustomPolicy::doPrint()