C++ - passing references to std::shared_ptr or boost::shared_ptr

Question

If I have a function that needs to work with a shared_ptr, wouldn\'t it be more efficient to pass it a reference to it (so to avoid copying the shared_ptr

Answer 1

One thing that I haven't seen mentioned yet is that when you pass shared pointers by reference, you lose the implicit conversion that you get if you want to pass a derived class shared pointer through a reference to a base class shared pointer.

For example, this code will produce an error, but it will work if you change test() so that the shared pointer is not passed by reference.

#include 

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

// ONLY instances of Base can be passed by reference.  If you have a shared_ptr
// to a derived type, you have to cast it manually.  If you remove the reference
// and pass the shared_ptr by value, then the cast is implicit so you don't have
// to worry about it.
void test(boost::shared_ptr& b)
{
    return;
}

int main(void)
{
    boost::shared_ptr d(new Derived);
    test(d);

    // If you want the above call to work with references, you will have to manually cast
    // pointers like this, EVERY time you call the function.  Since you are creating a new
    // shared pointer, you lose the benefit of passing by reference.
    boost::shared_ptr b = boost::dynamic_pointer_cast(d);
    test(b);

    return 0;
}