I have a class with function called void deallocatorFunc(ClassA *p) I want using templates make it possible to write boost::shared_ptr< ClassA > ptr(new ClassA()); instead of boost::shared_ptr< ClassA > ptr(new ClassA(), deallocatorFunc);. I want it to spread on to my class and its inheritors. How to do such thing in C++? (I really need my peculiar destructor for that special class, while I want to keep super simple API).
2 Answers
You can use specialization for your class and wrap the standard implementation. Here's a self-contained compilable example.
#include <iostream> #include <boost/shared_ptr.hpp> using namespace std; class A { public: A() { cout << "Constructing A." << endl; }; virtual ~A() { cout << "Destructing A." << endl; } }; class B : public A { public: B() { cout << "Constructing B." << endl; }; virtual ~B() { cout << "Destructing B." << endl; } }; class C { public: C() { cout << "Constructing C." << endl; }; virtual ~C() { cout << "Destructing C." << endl; } }; void deallocatorFunc(A *p) { cout << "Deallocator function." << endl; delete p; }; namespace boost { template<> class shared_ptr<A> { private: shared_ptr<void> _ptr; public: shared_ptr(A* p) : _ptr(p, deallocatorFunc) { } }; } int main( int argc, const char* argv[] ) { boost::shared_ptr<A> ptrA(new A()); boost::shared_ptr<B> ptrB(new B()); boost::shared_ptr<C> ptrC(new C()); } Output:
Constructing A. Constructing A. Constructing B. Constructing C. Destructing C. Destructing B. Destructing A. Deallocator function. Destructing A.
Note
As given, the specialization doesn't work for derived classes of A!
For that to work, you need more trickery:
1 Comment
ildjarn
You're demonstrating explicit specialization, not partial specialization.
namespace boost { template<typename myclass> class shared_ptr { }; } Enjoy your redefinition of boost::shared_ptr. Anything which includes the above code in a header will get your shared_ptr instead of boost's.
Keep in mind this will not be compatible with code which includes boost on its own!!!
9 Comments
myWallJSON
Is it any how possible tojust overload/specialize boost shared_ptr creation for one class type keeping it compatible with code which includes boost on its own?
ildjarn
@myWallJSON : No, but you could make your constructor private and force people to use factory functions to allocate objects on the freestore (the factory functions would use the correct semantics).
Borealid
@ildjarn: of course it's possible, he just needs to override one template specialization of
shared_ptr and copy through the remainder.ildjarn
If you think that's true then you should elaborate more. I don't think it's true, but I don't know what you mean by "override one template specialization of
shared_ptr and copy through the remainder" because that doesn't make much sense to me. (And how would whatever that is avoid violating the ODR?)Borealid
@ildjarn: The ODR applies to template instantiations, not template declarations. You may have a template where "if the parameter is
MyClass, use this version, otherwise use the generic catch-all". See gotw.ca/gotw/049.htm as a random off-the-web resource. |
operator deletefor your type?