std::shared_ptr and Inheritance

Question

I am having some problems with automatic typecasting between shared_ptr of inherited classes.

My class structure is as follows, a base class Base and two derived classes Derived1 and Derived2.

// Base class
class Base {
protected:
  ...
  ...
public:
  Base() = default;
  virtual ~Base() = default;
  virtual void run() = 0;
  ...
  ...
};

// Derived class
class Derived1: Base {
protected:
  ...
  ...
public:
  Derived1() = default;
  virtual ~Derived1() = default;
  void run() {...}
  ...
  ...
};

// Derived class
class Derived2: Base {
protected:
  ...
  ...
public:
  Derived2() = default;
  virtual ~Derived2() = default;
  void run() {...}
  ...
  ...
};

I have a function doSomething()

void doSomething(std::shared_ptr<Base> ptr) {
  ptr->run();
  ...
}

I call the function with the derived classes like so -

doSomething(make_shared<Derived1>())
doSomething(make_shared<Derived2>())

But I get an error saying -

no viable conversion from 'shared_ptr<class Derived1>' to 'shared_ptr<class Base>'
no viable conversion from 'shared_ptr<class Derived1>' to 'shared_ptr<class Base>'

What am I doing wrong? Is it safe just to use static_pointer_cast to the Base type? Like -

doSomething(static_pointer_cast<Base>(make_sahred<Derived2>()))

SOLUTION My bad... The problem was that I was inheriting the base class privately.

score 10 · Accepted Answer · answered Jul 07 '14 at 05:10

10

As far as I can tell, the code that you've presented compiles fine: http://ideone.com/06RB2W

#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() {}
};

class Derived2: public Base {
    public:
        Derived2() = default;
        virtual ~Derived2() = default;
        void run() {}
};

void doSomething(std::shared_ptr<Base> ptr) {
    ptr->run();
}

int main() {
    doSomething(std::make_shared<Derived1>());
    doSomething(std::make_shared<Derived2>());
}

answered Jul 07 '14 at 05:10

Bill Lynch

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13

Sorry my bad... I was inheriting the class privately in my code! That was a really rookie mistake! – ssb Jul 07 '14 at 05:57
2

Does shared_ptr automatically (without any type-casting) converts to base class shared_ptr? Do they share same reference count? – abhiarora Jun 26 '17 at 00:32
It does not automatically type cast, but if you manually cast it, then the reference count is shared. – Bill Lynch Jun 26 '17 at 01:04
1

In your code you didn't manually type cast, so are they sharing the reference count? – Mudit Jain Oct 07 '17 at 00:07

std::shared_ptr and Inheritance

1 Answers1