Add implicit conversion from unique_ptr to T*

Question

General Question: Without going into whether or not it's a good idea, how can I add an implicit conversion operator to a class that has already been defined? For example, let's say that I want unique_ptr<T> to implicitly convert to T*, but I can't just add a member conversion operator because I can't change the definition of the unique_ptr class.

Options:

1. Is there some c++ voodoo that I can use to make this happen without creating a member function?
   Answer-So-Far: NO.
   There is no way to add an implicit conversion away from a type that you can't modify in code.
   Just ... sadness.

2. Could I derive from std::unique_ptr and add my own member conversion function? Are there any serious downsides to this?
   Answer-So-Far: Yes (from vsoftco)
   Downsides are yet to be determined. So far inheriting from std::unique_ptr, inheriting its constructors, and declaring an implicit conversion operator has worked splendidly with hardly any code needing to be written.

3. Am I just going to have to live without this the rest of my life?
   Answer-So-Far: We'll see...
   If I can get option 2 up and running without any serious side-effect or burdens, I'll test it out for a while and report back on whether I think it's worth it. We'll see!

Example code:

#include <algorithm>
#include <memory>
#include <vector>

struct MyClass
{
    MyClass(int v) : value(v) {}

    int value;
};

int main()
{
    auto vec = std::vector<std::unique_ptr<MyClass>>();

    vec.push_back(std::make_unique<MyClass>(1));
    vec.push_back(std::make_unique<MyClass>(2));

    // error C2664: 'void (__vectorcall *)(MyClass *)' : cannot convert argument 1 from 'std::unique_ptr<MyClass,std::default_delete<_Ty>>' to 'MyClass *'
    std::for_each(std::begin(vec), std::end(vec), [](MyClass* myClass)
    {
        myClass->value += 3;
    });
}

Answer 1

If you don't want to use std::unique_ptr<>::get() function, you can:

1. Define a free function that takes a std::unique_ptr and returns the raw pointer returned by get, although I don't think it really makes your code better, like:

   // free function
   template<typename T>
   T* get_raw_ptr(const std::unique_ptr<T>& up)
   {
       return up.get();
   }
   

   Conversions of unique_ptr to raw pointers are OK, but they have to be explicit. Implicit conversion may lead to lots of headaches, since they may happen when you least expect them.

2. It is a bad idea to derived from std::unique_ptr, as the latter is not made to be used as a base class (doesn't have a virtual destructor). In general, it is bad to derive from Standard Library classes. However, if you really insist, you can use a wrapper in which you define the implicit conversion operator, like:

   // wrapper
   template <class T, class Deleter = std::default_delete<T>> 
   class unique_ptr_wrapper: public std::unique_ptr<T, Deleter>
   {
   public:
       using std::unique_ptr<T, Deleter>::unique_ptr; // inheriting base ctors
       operator T* () const {return this->get();}
   };
   

and use is simply like

    // wrapper usage:
    unique_ptr_wrapper<int> upw{new int{42}};
    int* p = upw; // implicit conversion OK

3. 1 and 2 can help you, so you may improve your life ;)