Compile error when try to create singleton using shared_ptr - multithreading

Question

I try to create a singleton object using shared_ptrs. However the code does not compile when the constructors/destructor are private for the specific object
The code is below.h
//ThreadPool.h
class ThreadPool
{
public:
static std::shared_ptr<ThreadPool> & getInstance();
inline static std::shared_ptr<ThreadPool> m_threadPoolInstance;
private:
ThreadPool() =default;
~ ThreadPool() = default;
ThreadPool(ThreadPool const &) = default;
};
//ThreadPool.cpp
#include "pch.h"
#include <ThreadPool.h>
std::shared_ptr<ThreadPool> & ThreadPool::getInstance()
{
if (! m_threadPoolInstance)
{
ThreadPool * p_ThreadPool = new ThreadPool();
m_threadPoolInstance.reset(p_ThreadPool);
}
return m_threadPoolInstance;
}
I am using VS17 compiler
The error that is created is the following
error C2440: '': cannot convert from '_Ux *' to
'std::shared_ptr'
with
[
_Ux=ThreadPool
] include\memory(1462): note: No constructor could take the source type, or constructor overload resolution was ambiguous
threadpool.cpp(9): note: see reference to function template
instantiation 'void std::shared_ptr::reset(_Ux
*)' being compiled
with
[
_Ux=ThreadPool
] threadpool.cpp(9): note: see reference to function template instantiation 'void std::shared_ptr::reset(_Ux
*)' being compiled
with
[
_Ux=ThreadPool
]
When I set the constructors/destructor in public section, the compilation is succesfull.
However running the same code using gcc compiler , compiles succesfully

Answer 1

The conversion fails because your ThreadPool class has a private destructor.
Calling .reset(ptr) will use the delete expression (delete ptr;) as the deleter, which requires that the destructor be public.
Refer to overload (2) here: https://en.cppreference.com/w/cpp/memory/shared_ptr/reset
2-4) Replaces the managed object with an object pointed to by ptr. Y must be a complete type and implicitly convertible to T. Additionally:
2) Uses the delete expression as the deleter. A valid delete expression must be available, i.e. delete ptr must be well formed, have well-defined behavior and not throw any exceptions. Equivalent to shared_ptr(ptr).swap(*this);.
You either need to make the destructor public or provide a custom deleter.