派生自任意数量的类

Question

我有一个类的功能,我想依赖于一组插件策略.但是,我不确定如何从一个任意数量的类派生一个类.

下面的代码是我想要实现的一个例子.

// insert clever boost or template trickery here
template< class ListOfPolicies >
class CMyClass : public ListOfPolicies 
{
public:
    CMyClass()
    {
        // identifiers should be the result of OR-ing all 
        // of the MY_IDENTIFIERS in the TypeList.
        DWORD identifiers; 

        DoSomeInitialization( ..., identifiers, ... );
    }

    int MyFunction()
    {
        return 100;
    }

    // ...
};

template< class T >
class PolicyA
{
public:
    enum { MY_IDENTIFIER = 0x00000001 };

    int DoSomethingA()
    {
        T* pT = static_cast< T* >( this );
        return pT->MyFunction() + 1;
    };

    // ...
};

template< class T >
class PolicyB
{
public:
    enum { MY_IDENTIFIER = 0x00000010 };

    int DoSomethingB()
    {
        T* pT = static_cast< T* >( this );
        return pT->MyFunction() + 2;
    };

    // ...
};

int _tmain(int argc, _TCHAR* argv[])
{
    CMyClass< PolicyA > A;
    assert( A.DoSomethingA() == 101 );

    CMyClass< PolicyA, PolicyB > AB
    assert( AB.DoSomethingA() == 101 );
    assert( AB.DoSomethingB() == 102 );

    return 0;
}

谢谢,PaulH

Answer 1

使用Boost.MPL库:

//Warning: Untested
namespace bmpl = boost::mpl;
template<class Typelist>
class X : bmpl::inherit_linearly<Typelist, bmpl::inherit<bmpl::_1, bmpl::_2> >::type
{
...
};

用作:

X<bmpl::vector<Foo, Bar, Baz> > FooBarBaz;

对于"OR-ing all MY_IDENTIFIER"部分,有以下几点:

//Warning: still not tested:
enum {OR_ED_IDENTIFIERS = 
    bmpl::fold<Typelist, bmpl::int_<0>, bmpl::bitor_<_1, _2> >::value;
}