Composition of patterns and transitivity of friendship
I have a container with private data and a class of friends accessing the private data:
template<class T>
class Container
{
friend typename T::MyAccessor;
vector<T> _data;
};
template<class T>
class Accessor
{
public:
void doSomething(Container<T> *c)
{
cout << c->_data.size() << endl;
}
};
template<class T, template<typename> class CustomAccessor>
struct MyBase
{
typedef Container<T> MyContainer;
typedef CustomAccessor<T> MyAccessor;
};
Now I want to create an accessory composition like this:
template<
template<typename> class First
,template<typename> class Second
,class T
>
class Composite
{
public:
typedef First<T> MyFirst;
typedef Second<T> MySecond;
void doSomething(Container<T> *c)
{
MyFirst a;
a.doSomething(c);
MySecond b;
b.doSomething(c);
}
};
template<class T>
class DoubleAccessor : public Composite<Accessor, Accessor, T> {};
but the friendship is not transitive and is composed, and access to them is not available to the personal data of the access container. Is there a way to get around this without exposing the container's identity to everyone?
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1 answer
What if you just declare class Accessor
as friend
yours class Container
:
template<class T>
class Container {
template<class T1> friend class Accessor;
std::vector<T> _data;
};
template<class T>
class Accessor {
public:
void doSomething(Container<T> *c) {
std::cout << c->_data.size() << std::endl;
}
};
template<class T, template<typename> class CustomAccessor = Accessor>
struct MyBase {
typedef Container<T> MyContainer;
typedef CustomAccessor<T> MyAccessor;
};
template<class T, template<typename> class First = Accessor, template<typename> class Second = Accessor>
class Composite {
public:
typedef First<T> MyFirst;
typedef Second<T> MySecond;
void doSomething(Container<T> *c) {
MyFirst a;
a.doSomething(c);
MySecond b;
b.doSomething(c);
}
};
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