模板元编程或操作

Question

I have a class that can be decorated with a set of add-on templates to provide additional functionality. Each add-on has an identifying addon_value that the base class needs to know.

The code below is an example of what I would like to do. Obviously, the main() function fails to compile. The goal is for CBase::GetValueOfAddOns() to know the value of OR-ing the addon_value for each add-on. The calculation does not actually have to be performed in GetValueOfAddOns(), it just has to be able to get at the result.

template< class T >
class AddOn_A : public T
{
public: 
    AddOn_A( int x ) : T( x ) 
    {};

    enum { addon_value = 0x00000001 };
};

template< class T >
class AddOn_B : public T
{
public: 
    AddOn_B( int x ) : T( x ) 
    {};

    enum { addon_value = 0x00000010 };
};

class CBase
{
public:
    explicit CBase( int x ) : x_( x ) 
    {
        // error LNK2001: unresolved external symbol "public: virtual int __thiscall CBase::GetValueOfAddOns(void)const " (?GetValueOfAddOns@CBase@@UBEHXZ)
        int z = GetValueOfAddOns();
    };

    virtual int GetValueOfAddOns() const = 0;

private:
    int x_;
};

// define an empty AddOn
template< class > class empty 
{
public:
    enum { addon_value = 0x00000000 };
};

// forward declaration and Add-On defaults
template< template< class > class AddOn1 = empty,
          template< class > class AddOn2 = empty,
          template< class > class AddOn3 = empty >
class CMyClass;

// specialized template for the default case
template<> class CMyClass< empty, empty, empty > : public CBase
{
public:
    CMyClass( int x ) : CBase( x ) 
    {};

    enum { addon_value = 0x00000000 };
};

// actual definition
template< template< class > class AddOn1,
          template< class > class AddOn2,
          template< class > class AddOn3 >
class CMyClass : public AddOn1< CBase >,
                 public CMyClass< AddOn2, AddOn3 >
{
public:
    CMyClass( int x ) : AddOn1< CBase >( x ),
                        CMyClass< AddOn2, AddOn3 >( x )
    {};

    enum { addon_value = AddOn1< CBase >::addon_value | CMyClass< AddOn2, AddOn3 >::addon_value };

    int GetValueOfAddOns() const
    {
        return addon_value;
    }
};

int _tmain( int argc, _TCHAR* argv[] )
{
    CMyClass< AddOn_A > A( 0 );
    _ASSERT( A.GetValueOfAddOns() == AddOn_A< CBase >::addon_value );

    CMyClass< AddOn_A, AddOn_B > AB( 0 );
    _ASSERT( AB.GetValueOfAddOns() == ( AddOn_A< CBase >::addon_value | AddOn_B< CBase >::addon_value ) );

    return 0;
}

Thanks for any help, PaulH

Answer 1

Not sure this is the most elegant way, but the following is fairly straightforward:

Add this to CMyClass:

enum {AddonsValues = AddOn1<CBase>::addon_value | CMyClass<AddOn2, AddOn3>::AddonsValues};

int GetValueOfAddOns()
{
    // return the result of OR-ing the addon_value of each add-on.
    return AddonsValues;
};

and this to the specialized CMyClass<empty, empty, empty> :

enum {AddonsValues = 0};

Answer 2

If you make that function pure virtual , you can implement it in CMyClass , where you have all information available. Just change your empty class to define enum { addon_value = 0x00000000 }; , which will make this easier.

Answer 3

Let's make it better:

See your new question about the problem with hierarchy here I have solved the AddOnValues problem there as well, and there is no need for template meta-programming, really.