How to have a policy class implement a virtual function?
I'm trying to design a policy-based class, where a certain interface is implemented by the policy itself, so the class derives from the policy, which itself is a template (I got this kind of thinking from Alexandrescu's book):
#include <iostream>
#include <vector>
class TestInterface {
public:
virtual void test() = 0;
}开发者_JS百科;
class TestImpl1 {
public:
void test() {std::cerr << "Impl1" << std::endl;}
};
template<class TestPolicy>
class Foo : public TestInterface, TestPolicy {
};
Then, in the main()
function, I call test()
on (potentially) various different objects that all implement the same interface:
int main() {
std::vector<TestInterface*> foos;
foos.push_back(new Foo<TestImpl1>());
foos[0]->test();
delete foos[0];
return 0;
}
It doesn't compile, though, because
the following virtual functions are pure within ‘Foo<TestImpl1>’:
virtual void TestInterface::test()
I thought TestInterface::test()
is implemented because we derive from TestImpl1
?
For this to work the policy class needs to inherit from the interface class:
class TestInterface {
public:
virtual void test() = 0;
};
template< class Interface >
class TestImpl1 : public Interface {
public:
void test() {std::cerr << "Impl1" << std::endl;}
};
template<class TestPolicy>
class Foo : public TestPolicy<TestInterface> {
// ...
};
You could also try a boost::mpl approach:
keeping your TestInterface and TestImpl as they are:
#include <boost/mpl/inherit.hpp>
using namespace boost::mpl;
template <class TestPolicy>
class Foo: public inherit2< TestPolicy, inherit2< TestInterface , empty_base >::type >::type
{
...
}
should work
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