How to make boost::make_shared a friend of my clas

Question

I have written a class with protected constructor, so that new instances can only be produced with a static create() function which returns shared_ptr's to my class. To provide efficient allocation I'd like to use boost::make_shared inside the create function, however the compiler complains that my class constructor is protected inside boost::make_shared. I decided to my boost::make_shared a friend of my class but I'm puzzled about the syntax. I tried

template< class T, class A1, class A2 >
friend boost::shared_ptr<Connection> boost::make_shared(const ConnectionManagerPtr&, const std::string&);

but the compiler gived me syntax errors. Please help.

Answer 1

You don't need to template the friend part, but you need to signify that the friend function is a template:

friend boost::shared_ptr<Connection> boost::make_shared<>(/* ... */);
//                                                     ^^

That works with Comeau and current GCC versions but fails with VC. Better would be the following form:

friend boost::shared_ptr<Connection> boost::make_shared<Connection>(/* ... */);

That works across multiple compilers now - i tested it on VC8, VC10, GCC 4.2, GCC 4.5 and Comeau 4.3.

Alternatively using a qualified name to refer to a particular instance of the function template as Martin does should work and does with Comeau, but GCC chokes on it.

A useful alternative that doesn't depend on the implementation details of make_shared() (and thus also works with VC10s TR1 implementation) is to use the pass-key-idiom for access-protection of the constructor and to befriend the create() function instead, e.g.:

class Connection {
// ...
public:
    class Key {
        friend boost::shared_ptr<Connection> create(const ConnectionManagerPtr&, 
                                                    const std::string&);
        Key() {}
    };
    Connection(const ConnectionManagerPtr&, const std::string&, const Key&);
};

boost::shared_ptr<Connection> create(const ConnectionManagerPtr& p, 
                                     const std::string& s) 
{
    return boost::make_shared<Connection>(p, s, Connection::Key());
}

Answer 2

I think that is not the right place to use make_shared. Just construct your object with operator new and pass the pointer to shared_ptr constructor. That way you don't need to be friends with anyone.

BTW, why template arguments and function arguments are of different type?

Answer 3

Just a summary of how a complete version may look like:

#include <iostream>
#include <boost/make_shared.hpp>

class Foo {
  explicit Foo(int x) {
    std::cout << "Foo::Foo(" << x << ")\n";
  }
public:
  friend boost::shared_ptr<Foo> boost::make_shared<Foo, int>(const int& x);

  static boost::shared_ptr<Foo> create(int x) {
    return boost::make_shared<Foo, int>(x);
  }

  ~Foo() {
    std::cout << "Foo::~Foo()\n";
  }
};

int main(int argc, const char *argv[]) {
  Foo::create(42);
}

Answer 4

I would try without the template part. After all, you want a specific instantiation of the (template) function to be a friend of your class, aren't you? Does

friend boost::shared_ptr<Connection> boost::make_shared(const ConnectionManagerPtr&, const std::string&);

work?

If that's not the solution, it might be helpful to give us the compiler messages you're getting ...

Answer 5

I ended up using the below simple solution to enforce shared ownership. No friendship required.

class probe {
    probe() = default;
    probe(...) { ... }

    // Part I of III, private
    struct creation_token {};
    probe(probe const&) = delete;
    probe& operator=(probe const&) = delete;

public:
    // Part II of III, public
    template <class... Args>
    probe(creation_token&&, Args&&... args):
        probe(std::forward<Args>(args)...) {}

    // Part III of III, public
    template <class... Args>
    static auto create(Args&&... args) {
        return make_shared<probe>(creation_token(),
            std::forward<Args>(args)...);
    }
};

Answer 6

Below are some macros I wrote up to do this for you. In your case, you would use:

BOOST_MAKE_SHARED_2ARG_CONSTRUCTOR(Connection, const ConnectionManagerPtr&, const std::string&);

Macro definitions:

// Required includes
#include <boost/make_shared.hpp>
#include <boost/type_traits/add_reference.hpp>
#include <boost/type_traits/add_const.hpp> 

// Helper macro
#define CONST_REFERENCE(T) boost::add_reference<boost::add_const<T>::type>::type

/** BOOST_MAKE_SHARED_nARG_CONSTRUCTOR(CLASS_NAME, ARG1_TYPE, ARG2_TYPE, ...) 
  *
  * Use this macro inside the body of a class to declare that boost::make_shared
  * should be considered a friend function when used in conjunction with the
  * constructor that takes the given argument types.  This allows the constructor 
  * to be declared private (making it impossible to accidentally create an instance 
  * of the object without immediatly storing it in a boost::shared_ptr).  
  * Example usage:
  *
  * class Foo {
  *   private:
  *     Foo(int size, const char* name);
  *     MAKE_SHARED_2ARG_CONSTRUCTOR(Foo, int, const char*);
  * };
  * 
  * boost::shared_ptr<Foo> myFoo = boost::make_shared<Foo>(3, "Bob");
  *
  * Note that you need to explicitly specify the number of arguments 
  * that the constructor takes as part of the macro name.  Also, note that 
  * macros don't mix well with templated types that contain commas -- so 
  * if you have such a type, then you should typedef it to a shorter name 
  * before using it with this macro.
  */
#define BOOST_MAKE_SHARED_0ARG_CONSTRUCTOR(CLASS_NAME) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>()
#define BOOST_MAKE_SHARED_1ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1))
#define BOOST_MAKE_SHARED_2ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2))
#define BOOST_MAKE_SHARED_3ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3))
#define BOOST_MAKE_SHARED_4ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4))
#define BOOST_MAKE_SHARED_5ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4, ARG_TYPE5) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4), CONST_REFERENCE(ARG_TYPE5))
#define BOOST_MAKE_SHARED_6ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4, ARG_TYPE5, ARG_TYPE6) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4), CONST_REFERENCE(ARG_TYPE5), CONST_REFERENCE(ARG_TYPE6))
#define BOOST_MAKE_SHARED_7ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4, ARG_TYPE5, ARG_TYPE6, ARG_TYPE7) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4), CONST_REFERENCE(ARG_TYPE5), CONST_REFERENCE(ARG_TYPE6)), CONST_REFERENCE(ARG_TYPE7))
#define BOOST_MAKE_SHARED_8ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4, ARG_TYPE5, ARG_TYPE6, ARG_TYPE7, ARG_TYPE8) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4), CONST_REFERENCE(ARG_TYPE5), CONST_REFERENCE(ARG_TYPE6)), CONST_REFERENCE(ARG_TYPE7)), CONST_REFERENCE(ARG_TYPE8))
#define BOOST_MAKE_SHARED_9ARG_CONSTRUCTOR(CLASS_NAME, ARG_TYPE1, ARG_TYPE2, ARG_TYPE3, ARG_TYPE4, ARG_TYPE5, ARG_TYPE6, ARG_TYPE7, ARG_TYPE8, ARG_TYPE9) \
    friend boost::shared_ptr<CLASS_NAME> boost::make_shared<CLASS_NAME>(CONST_REFERENCE(ARG_TYPE1), CONST_REFERENCE(ARG_TYPE2), CONST_REFERENCE(ARG_TYPE3), CONST_REFERENCE(ARG_TYPE4), CONST_REFERENCE(ARG_TYPE5), CONST_REFERENCE(ARG_TYPE6)), CONST_REFERENCE(ARG_TYPE7)), CONST_REFERENCE(ARG_TYPE8)), CONST_REFERENCE(ARG_TYPE9))