How to make_shared a derived class?

问题

I want to use the make_shared<T> function with a derived class, like below

class Base {
     public:
     typedef std::shared_ptr<Base> Ptr;
};

class Derived : public Base {};

Base::Ptr myPtr = std::make_shared(/* Derived() */ );

How can I tell make_shared to build such an object?

I want to avoid the classical

Base::Ptr ptr = Base::Ptr(new Derived());

To make use of the single alloc in the make_shared function.

回答1:

std::shared_ptr has a converting constructor that can make a shared_ptr<Base> from a shared_ptr<Derived>, so the following should work:

#include <memory>
class Base {
    public:
    typedef std::shared_ptr<Base> Ptr;
};
class Derived : public Base {};

int main() {
    Base::Ptr myPtr = std::make_shared<Derived>();
}

来源：https://stackoverflow.com/questions/24968294/how-to-make-shared-a-derived-class