Polymorphism and shared_ptr passed by reference - c++

Question

I try to send to function a shared_ptr with polymorphic class.
My objective is to find a best way to send my shared_ptr
without increase ref_count.
EDIT: I don't search solution where my shared_ptr is replaced because I want to call shared_ptr.reset() for example.
Currently, void doGenericTemplate(std::shared_ptr<CLASS>& ptr) is what I want in result BUT I prefer a single function in program.
Do you have another solution ?
Moreover, I don't understand why the function void doGeneric(std::shared_ptr<Base>& ptr) doesn't compile (equivalent without shared_ptr work fine: please check doClassic in complete code).
Do you have an explain ?
Thanks you !
Partial code
#include <iostream>
#include <memory>
class Base
{
public:
Base() = default;
virtual ~Base() = default;
virtual void run() = 0;
};
class Derived1: public Base
{
public:
Derived1() = default;
virtual ~Derived1() = default;
void run()
{
std::cout << " Derived1";
}
};
class Derived2: public Base
{
public:
Derived2() = default;
virtual ~Derived2() = default;
void run()
{
std::cout << " Derived2";
}
};
// This function works but increase count
void doGenericCopy(std::shared_ptr<Base> ptr)
{
ptr->run();
std::cout << " Ref count: " << ptr.use_count() << std::endl;
}
// This function works without increase count = OK !
void doSpecificD1(std::shared_ptr<Derived1>& ptr)
{
ptr->run();
std::cout << " Ref count: " << ptr.use_count() << std::endl;
}
// Compilation error = FAILED !
void doGeneric(std::shared_ptr<Base>& ptr)
{
ptr->run();
std::cout << " Ref count: " << ptr.use_count() << std::endl;
}
// Working fine for all Derivate = OK !
template<typename CLASS>
void doGenericTemplate(std::shared_ptr<CLASS>& ptr)
{
ptr->run();
std::cout << " Ref count: " << ptr.use_count() << std::endl;
}
int main()
{
auto d1 = std::make_shared<Derived1>();
auto d2 = std::make_shared<Derived2>();
std::cout << "With copy: " << std::endl;
doGenericCopy(d1);
doGenericCopy(d2);
std::cout << "Specific: " << std::endl;
doSpecificD1(d1);
std::cout << "Template: " << std::endl;
doGenericTemplate(d1);
doGenericTemplate(d2);
// Compilation issue
//doGeneric(d1);
}
Complete code
https://ideone.com/ZL0v7z
Conclusion
Currently in c++, shared_ptr has not in language a specific tools to use polymorphism of class inside template.
The best way is to refactor my code and avoids to manage shared_ptr (ref_count, reset).
Thanks guys !

Answer 1

Do you have another solution ?
Pass object by reference or const reference instead of shared_ptr.
void doGeneric(Base& r)
{
r.run();
}
Firstly - this shows explicitly that you do not store or cache pointer somwhere. Secondly - you avoid ambiguities like the one you presented here.
Do you have an explain ?
Passing shared_ptr<Derived> to function causes implicit cast to shared_ptr<Base>. This new shared_ptr<Base> is temporary, so it can not be cast to shared_ptr<Base> &. This implicit cast would increase ref count even if you could pass it.

Answer 2

A shared_ptr<Base> and shared_ptr<Derived> are unrelated types, except you can implicitly create a shared_ptr<Base> from a shared_ptr<Derived>.
This creation adds a reference count.
If you really, really want to avoid that reference count...
template<class T>
struct shared_ptr_view {
template<class D>
shared_ptr_view( std::shared_ptr<D>& sptr ):
vtable( get_vtable<D>() ),
ptr( std::addressof(sptr) )
{}
shared_ptr_view( shared_ptr_view const& ) = default;
shared_ptr_view() = default;
shared_ptr_view& operator=( shared_ptr_view const& ) = delete;
T* get() const { if(vtable) return vtable->get(ptr); return nullptr; }
void clear() const { if(vtable) vtable->clear(ptr); }
std::shared_ptr<T> copy() const { if(vtable) return vtable->copy(ptr); return {} }
operator std::shared_ptr<T>() const { return copy(); }
T* operator->() const { return get(); }
T& operator*() const { return *get(); }
explicit operator bool() const { return get(); }
std::size_t use_count() const { if (vtable) return vtable->use_count(ptr); return 0; }
private:
struct vtable_t {
T*(*get)(void*) = 0;
std::shared_ptr<T>(*copy)(void*) = 0;
void(*clear)(void*) = 0;
std::size_t(*use_count)(void*) = 0;
};
vtable_t const* vtable = 0;
void* ptr = 0;
template<class D>
static vtable_t create_vtable() {
return {
[](void* ptr)->T*{ return static_cast<std::shared_ptr<D>*>(ptr)->get(); },
[](void* ptr)->std::shared_ptr<T>{ return *static_cast<std::shared_ptr<D>*>(ptr); },
[](void* ptr){ static_cast<std::shared_ptr<D>*>(ptr)->reset(); },
[](void* ptr){ return static_cast<std::shared_ptr<D>*>(ptr)->use_count(); }
};
}
template<class D>
static vtable_t const* get_vtable() {
static const auto vtable = create_vtable<D>();
return &vtable;
}
};
then
void doGeneric( shared_ptr_view<Base> ptr ) {
ptr->run();
std::cout << " Ref count: " << ptr.use_count() << std::endl;
}
does not increase the reference count. I think it is raw insanity.
shared_ptr_view.clear() works, but shared_ptr_view.reset(T*) cannot: a shared_ptr_view<Derived> cannot be reset to point to a Base*.