卡在模板需求循环中 [英] stuck in a template requirement loop
问题描述
我有一个类使用附加模板添加如下的附加功能:
template<类T>
class AddOn_A
{
public:
int SomeFuncA()
{
T * pT = static_cast< T *>(这);
return pT-> DoSomething()+ 1;
};
};
class CMyClass:public AddOn_A< CMyClass>
{
public:
int DoSomething()
{
return 100;
};
};
int _tmain(int argc,_TCHAR * argv [])
{
CMyClass A;
_ASSERT(A.SomeFuncA()== 101);
return 0;
}
现在我想扩展这个CMyClass可以接受不同的加载项如AddOn_B。
模板<类T>
class AddOn_B
{
public:
int SomeFuncB()
{
T * pT = static_cast< T *>(这);
return pT-> DoSomething()+ 2;
};
};
template< class AddOn>
class CMyClass:public AddOn
{
public:
int DoSomething()
{
return 100;
};
};
int _tmain(int argc,_TCHAR * argv [])
{
//错误C3203:'AddOn_A':非特定类模板不能用作模板参数对于模板参数AddOn,期望一个真实类型
//错误C2955:'AddOn_A':使用类模板需要模板参数列表
CMyClass< AddOn_A>一个;
_ASSERT(A.SomeFuncA()== 101);
//这里有相同的错误
CMyClass< AddOn_B> B;
_ASSERT(B.SomeFuncB()== 102);
return 0;不幸的是,每个Add_On需要CMyClass作为模板参数,它需要一个Add_On等等。 ...我在一个需求循环。
有没有一些模板魔法,我可以用来获得我寻找的功能?有更好的方法吗?
谢谢,
PaulH
解决方案显然,您尝试使用着名的奇怪的循环模板模式。
我不知道你确实想要做什么,但你可能会得到另一个解决方案:
如果你使用了两个类,该怎么办:
class Base {};
class MyClass:public AddOn< Base> {};
您也可以使用基于策略的方法:
class PolicyA_A {};
class PolicyA_B {};
class PolicyB_A {};
class PolicyB_B {};
template< class PolicyA,class PolicyB>
class MyClass:private PolicyA,private PolicyB {};
typdef MyClass< PolicyA_A,PolicyB_A> MyClassAA;
这个想法是将工作的一部分委托给政策以增加灵活性。
最后但同样重要的是,您可以使用装饰方法:
class Base {};
template< class T>
class AddOn_A:public T {};
class MyClass:public AddOn_A< AddOn_B < Base> > {};
它允许你通过抑制多重继承和使层次结构线性化来摆脱虚拟继承。
I have a class that uses an "add-on" template to add additional functionality as below:
template< class T >
class AddOn_A
{
public:
int SomeFuncA()
{
T* pT = static_cast< T* >( this );
return pT->DoSomething() + 1;
};
};
class CMyClass : public AddOn_A< CMyClass >
{
public:
int DoSomething()
{
return 100;
};
};
int _tmain(int argc, _TCHAR* argv[])
{
CMyClass A;
_ASSERT( A.SomeFuncA() == 101 );
return 0;
}
Now I would like to extend this such that CMyClass can accept different add-ons like AddOn_B.
template< class T >
class AddOn_B
{
public:
int SomeFuncB()
{
T* pT = static_cast< T* >( this );
return pT->DoSomething() + 2;
};
};
template< class AddOn >
class CMyClass : public AddOn
{
public:
int DoSomething()
{
return 100;
};
};
int _tmain(int argc, _TCHAR* argv[])
{
// error C3203: 'AddOn_A' : unspecialized class template can't be used as a template argument for template parameter 'AddOn', expected a real type
// error C2955: 'AddOn_A' : use of class template requires template argument list
CMyClass< AddOn_A > A;
_ASSERT( A.SomeFuncA() == 101 );
// same errors here
CMyClass< AddOn_B > B;
_ASSERT( B.SomeFuncB() == 102 );
return 0;
}
Unfortunately, each Add_On requires CMyClass as a template parameter which requires an Add_On, etc... I'm in a requirement loop.
Is there some template magic I can use to get the functionality I'm looking for? Is there a better method of doing this?
Thanks,
PaulH
解决方案 Apparently you are trying to use the famous Curiously Recurring Template Pattern.
I am not sure of what you exactly want to do, but you might get away with another solution:
What if you used two classes:
class Base {};
class MyClass: public AddOn<Base> {};
You may also use a Policy Based approach:
class PolicyA_A {};
class PolicyA_B {};
class PolicyB_A {};
class PolicyB_B {};
template <class PolicyA, class PolicyB>
class MyClass: private PolicyA, private PolicyB {};
typdef MyClass<PolicyA_A, PolicyB_A> MyClassAA;
The idea is to delegate part of the job to policies to add flexibility.
Last but not least you may use a Decorator approach:
class Base {};
template <class T>
class AddOn_A: public T {};
class MyClass: public AddOn_A< AddOn_B< Base > > {};
It allows you to get rid of the virtual inheritance by suppressing the Multi Inheritance and making the hierarchy linear.
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