模板元编程或操作 [英] template meta-programming OR operation
问题描述
我有一个类,可以用一组附加模板来装饰,以提供额外的功能。每个插件都有一个标识addon_value,基类需要知道。
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.
下面的代码是我想做的一个例子。显然,main()函数无法编译。目标是让CBase :: GetValueOfAddOns()知道每个附加组件的addon_value的OR值。该计算实际上不必在GetValueOfAddOns()中执行,它只能够获得结果。
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;
}
感谢任何帮助,
PaulH
Thanks for any help, PaulH
推荐答案
不确定这是最优雅的方式,但以下是相当直接:
Not sure this is the most elegant way, but the following is fairly straightforward:
将此添加到CMyClass:
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;
};
并且这是专门的 CMyClass< empty,empty,empty> code>:
and this to the specialized CMyClass<empty, empty, empty>
:
enum {AddonsValues = 0};
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