Convert from shared_ptr to weak_ptr polymorphism

Question

I have problems with this. Let them say that I have this vector

std::vector<std::shared_ptr<Car>> cars; 

The car is an abstract class. I want to be able to return weak pointers of different types, so I do the following.

 template<typename T> std::weak_ptr<T> GetCar() { for (std::vector<std::shared_ptr<Car>>::iterator it = cars.begin(); it != cars.end(); ++it) { T* derived = dynamic_cast<T*>((*it).get()); if (derived != nullptr) { std::weak_ptr<T> carPointer = *it; return carPointer; } } return std::weak_ptr<T>(); } 

I get the following error though when I try to use a function with a class that inherits from Car. Error C2440 'initializing': cannot convert from 'std::shared_ptr<Car>' to 'std::weak_ptr<Saab>' When asked, there cannot be a valid car. I tried using boost :: optional, but it does not handle polymorphism. I can go with rude pointers if I can't get this to work.

Answer 1

You cannot directly create your weak_ptr<Saab> from shared_ptr<Car> , since the Car template parameter must be implicitly converted to Saab to work.

But first you can convert shared_ptr<Car> to shared_ptr<Saab> , and then create your weak_ptr after. In my example below, I used std::dynamic_pointer_cast for this.

Here is what I came up with:

 #include <iostream> #include <vector> #include <memory> struct Car { virtual void name() = 0; }; struct Saab : Car { virtual void name() { std::cout << "Saab" << std::endl; } }; struct Renault : Car { virtual void name() { std::cout << "Renault" << std::endl; } }; template<typename T> std::weak_ptr<T> GetCar(std::vector<std::shared_ptr<Car>> cars) { for (std::vector<std::shared_ptr<Car>>::iterator it = cars.begin(); it != cars.end(); ++it) { auto derived = std::dynamic_pointer_cast<T>(*it); if (derived != nullptr) { std::weak_ptr<T> carPointer(derived); return carPointer; } } return std::weak_ptr<T>(); } int main() { std::vector<std::shared_ptr<Car>> cars; cars.push_back(std::make_shared<Saab>()); cars.push_back(std::make_shared<Renault>()); auto wp = GetCar<Saab>(cars); auto sp = wp.lock(); if (sp) { sp->name(); } auto wp2 = GetCar<Renault>(cars); auto sp2 = wp2.lock(); if (sp2) { sp2->name(); } } 

He prints:

Saab

Renault

Coliru Link: http://coliru.stacked-crooked.com/a/9dbb85b556b83597