How do I call ::std::make_shared on a class with only protected or private constructors?

Question

I have this code that doesn\'t work, but I think the intent is clear:

testmakeshared.cpp

#include 

class A {
 public:
   stat         


        

Answer 1

Looking at the requirements for std::make_shared in 20.7.2.2.6 shared_ptr creation [util.smartptr.shared.create], paragraph 1:

Requires: The expression ::new (pv) T(std::forward(args)...), where pv has type void* and points to storage suitable to hold an object of type T, shall be well formed. A shall be an allocator (17.6.3.5). The copy constructor and destructor of A shall not throw exceptions.

Since the requirement is unconditionally specified in terms of that expression and things like scope aren't taken into account, I think tricks like friendship are right out.

A simple solution is to derive from A. This needn't require making A an interface or even a polymorphic type.

// interface in header
std::shared_ptr make_a();

// implementation in source
namespace {

struct concrete_A: public A {};

} // namespace

std::shared_ptr
make_a()
{
    return std::make_shared();
}