Singleton object using unique_ptr - c++

Question

I have the below CPP source code for creating a Singleton Object of a class using unique_ptr:
#include <iostream>
#include <memory>
class A
{
public:
std::unique_ptr<A> getInstance(int log);
~A();
private:
static bool instanceFlag;
static std::unique_ptr<A> single;
A(int log);
int mLog;
};
bool A::instanceFlag = false;
std::unique_ptr<A> A::single = NULL;
std::unique_ptr<A> A::getInstance(int log)
{
if(!instanceFlag)
{
//single = std::make_unique<A>(log);
single = std::unique_ptr<A>(new A(log));
instanceFlag = true;
return std::move(single);
}
else
{
return std::move(single);
}
}
A::A(int log) :
mLog(log)
{
std::cout << "Called A cons" << std::flush << std::endl;
}
int main()
{
std::unique_ptr<A> mA = A::getInstance(5);
}
But when I compile the code I get below error:
$ c++ -std=c++11 try2.cpp
try2.cpp: In function 'int main()':
try2.cpp:45:41: error: cannot call member function 'std::unique_ptr<A> A::getInstance(int)' without object
std::unique_ptr<A> mA = A::getInstance(5);
^
However I have exactly the same format of code in my project I get an error :
Source code Line 39: single = std::make_unique<A>(log);
Compilation error:
39: required from here
single = std::make_unique<A>(log);
error: A(int log)' is private
A::A(int log) :
^

Answer 1

First, the static issue. This:
std::unique_ptr<A> getInstance(int log);
is an instance method. You need an instance of A to call it on, but you can't get an instance of A without calling this method first, so ...
The reason for using instance methods is that they have access to the instance they're invoked on. Your method only uses the following members:
static bool instanceFlag;
static std::unique_ptr<A> single;
which, since they're static, don't need an instance. Just make the method static too, and you'll be able to call it without first getting an A from somewhere.
Second, the logic issue. Your getInstance method returns a unique_ptr to your single instance. The entire point of unique_ptr is that's it's unique: exactly one pointer owns and controls the lifetime of an object. When you returned a unique_ptr, you transferred ownership of the singleton object from the singleton class itself, to the caller. You even explicitly called move to make it really clear that this is happening.
Now, after calling getInstance once, your singleton is completely broken. If you call getInstance again, the singleton believes it has an instance (because instanceFlag), but the unique_ptr is in an indeterminate state. The only thing we can say for sure is that it doesn't control an instance of A.
Just return a raw pointer (or reference) to A instead of transferring ownership.

Answer 2

Short answer: Don't use singletons.
Long answer: std::make_unique() uses constructor of an object it tries to create, and in your code it's private. Since it is an external function, it's not possible. You can create an object on your own and pass it to std::unique_ptr to manage, like you do on the line below the commented one.
The most important problem is, when you do std::move(), your class member single is gone. It becomes empty unique_ptr (aka. std::unique_ptr{}, it doesn't manage anything). With any next call to getInstance, you're essentially returning nullptr. You should use std::shared_ptr, which you'll want to return by copy or return reference to your std::unique_ptr.

Answer 3

Even after making the getInstance static, it won't compile because the definition for destructor is missing. Either give empty definition or leave it as default.
~A() = default;
Moving single will work first time and invokes UB for subsequent calls to getInstance. Instead return by reference & and remove std::move
static std::unique_ptr<A>& getInstance(int log);
std::unique_ptr<A>& A::getInstance(int log)
{
if(!instanceFlag)
{
//single = std::make_unique<A>(log);
single = std::unique_ptr<A>(new A(log));
instanceFlag = true;
return single;
}
else
{
return single;
}
}
Inside main() you can get by reference
std::unique_ptr<A>& mA = A::getInstance(5);

Answer 4

The problem is that, as error log suggests, you are calling member function i.e. getInstance without any instance. Function std::unique_ptr<A> getInstance(int log); should be declared as static.
Declare it static as follow:
static std::unique_ptr<A> getInstance(int log);
Note: In singleton pattern getInstance function is always static function.
Following is an extract from Wikipedia:
An implementation of the singleton pattern must:
ensure that only one instance of the singleton class ever exists; and
provide global access to that instance.
Typically, this is done by:
declaring all constructors of the class to be private; and
providing a static method that returns a reference to the instance.
The instance is usually stored as a private static variable; the instance is created when the variable is initialized, at some point before the static method is first called.