Converting any class into a Singleton class in C++
Singleton is one of the creational design patterns. A singleton class has only one instance and provides a global point of access to that instance. It's commonly used when you want to restrict the instantiation of a class to a single object, which can be useful for scenarios where you need a single point of control or coordination.
Following are some of the prominent features of Singleton class:
Private constructor: The key idea behind the Singleton pattern is to make the class's constructor private, preventing external code from directly creating instances of the class.
Static instance: Inside the class, you have a static object that holds the single instance of the class. This instance is usually created the first time it's requested and then reused.
Static method: You provide a static method that acts as the global access point to the single instance. This method checks if an instance already exists and either returns the existing one or creates a new instance if needed.
Thread safety: In multithreaded environments, it's important to ensure that the creation of the instance is thread-safe.
Global access: The Singleton instance can be accessed globally, allowing different parts of your codebase to interact with the same instance of the class.
In this article, I am introducing an approach using which we can convert any class into a Singleton class with minimum effort. To achieve this we are going to create what is called a mixin. Without further ado let's create the SingletonMixin
.
SingletonMixin
is going to be a template classThe default constructor is protected to prevent direct instantiation
The destructor is virtual and protected so that it can be overridden in a derived class
The copy constructor and copy assignment constructor are also disabled to ensure a single instance is present at any point in time
The core logic of the singleton instance creation is implemented inside the
getInstance
method
#include <iostream>
using namespace std;
template <typename T>
class SingletonMixin {
public:
static T& getInstance() {
static T instance; // Guaranteed to be created and destroyed once in a thread-safe way
return instance;
}
SingletonMixin(const SingletonMixin&) = delete; // Disable copy constructor
SingletonMixin& operator=(const SingletonMixin&) = delete; // Disable copy assignment constructor
protected:
SingletonMixin() {
// Protected constructor to prevent direct instantiation
cout << "SingletonMixin constructor called: " << this << endl;
}
virtual ~SingletonMixin() {
cout << "SingletonMixin destructor called: " << this << endl;
}
};
Now let's take for example the following code. It represents a simple class called MyClass
which has a default constructor and a single public method implemented. Whereas in real-world use cases, we can have many more methods.
class MyClass {
public:
void showMessage() {
cout << "Hello from MyClass!" << endl;
}
MyClass() {
cout << "MyClass constructor called: " << this << endl;
};
};
We can convert MyClass
into a singleton class in the following way:
First, we inherit from
SingletonMixin<MyClass>
Make the default constructor private and
SingletonMixin<MyClass>
a friend class ofMyClass
, so thatMyClass
can only be instantiated from theSingletonMixin<MyClass>
class.
class MyClass : public SingletonMixin<MyClass> {
public:
void showMessage() {
cout << "Hello from MyClass!" << endl;
}
private:
MyClass() {
cout << "MyClass constructor called: " << this << endl;
};
friend class SingletonMixin<MyClass>;
};
To create/access the singleton instance of MyClass
we can call the getInstance
method on MyClass
class. We can call it as many times as we want, we are going to receive the same instance of the MyClass
class. We can verify it printing out the address of the instance.
#include <thread>
void thread_function(const char * s) {
this_thread::sleep_for(chrono::milliseconds(1000));
MyClass& singleton = MyClass::getInstance();
cout << s << ": " << &singleton << endl << flush;
}
int main() {
thread thread_1(thread_function, "thread 1");
thread thread_2(thread_function, "thread 2");
MyClass& singleton1 = MyClass::getInstance();
MyClass& singleton2 = MyClass::getInstance();
MyClass& singleton3 = MyClass::getInstance();
cout << &singleton1 << ", " << &singleton2 << ", " << &singleton3 << endl;
thread_1.join();
thread_2.join();
return 0;
}
MyClass
.