The following code example is taken from the book
C++ Templates - The Complete Guide
by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
© Copyright David Vandevoorde and Nicolai M. Josuttis 2002
#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();
}