Question

#include <boost/smart_ptr.hpp>

class Base {
};

class Derived : public Base {
  public:
    Derived() : Base() {}
};

void func(/*const*/ boost::shared_ptr<Base>& obj) {
}

int main() {
  boost::shared_ptr<Base> b;
  boost::shared_ptr<Derived> d;
  func(b);
  func(d);
}

This compiles with the const in func’s signature but not without it. The error appears in the line with the call func(d);

Any hints for me?

Answer 1

When reading the documentation of boost::shared_ptr we find the following:

A shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*.

---

This means that boost::shared_ptr<Derived> is implicitly convertable to an object of type boost::shared_ptr<Base>.

When this conversion takes place upon executing func (d) a temporary will be created, though non-const references cannot be bound to temporary objects – which is why your compiler issues an error unless you make the argument to func a const&.