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

There's a more hairy and interesting problem that happens when you have two strictly related classes A and B that work together.

Say A is the "master class" and B its "slave". If you want to restrict instantiation of B only to A, you'd make B's constructor private, and friend B to A like this

class B
{
public:
    // B your methods...

private:
    B();
    friend class A;
};

Unfortunately calling std::make_shared() from a method of A will make the compiler complain about B::B() being private.

My solution to this is to create a public Pass dummy class (just like nullptr_t) inside B that has private constructor and is friend with A and make B's constructor public and add Pass to its arguments, like this.

class B
{
public:
  class Pass
  {
    Pass() {}
    friend class A;
  };

  B(Pass, int someArgument)
  {
  }
};

class A
{
public:
  A()
  {
    // This is valid
    auto ptr = std::make_shared(B::Pass(), 42);
  }
};

class C
{
public:
  C()
  {
    // This is not
    auto ptr = std::make_shared(B::Pass(), 42);
  }
};