Singleton
- C++ course
- Advanced
- Lesson 31/83
- 5 min
Learn to write a single-instance class with the Singleton pattern.
What is the Singleton pattern?
Singleton is a design pattern that ensures a class has only one instance during the whole program. This single instance is accessible from everywhere. It is common for logging, configuration or connection pools.
The cleanest way in C++ is the Meyers singleton: a method that returns a static local variable. The compiler guarantees a single instance and thread safety since C++11.
Writing the Meyers singleton
In the Meyers singleton you declare a static local variable inside the instance() method. The variable is created on the first call and the same object is returned afterwards. The constructor is made private so a second instance cannot be created with new from outside.
Example: static Singleton& instance() { static Singleton s; return s; }. The line static Singleton s; runs only once. private: Singleton() = default; blocks external construction. Copying is also disabled with = delete.
Practice Write the Singleton class
Define a Singleton class. Add a static Singleton& instance() method under public that defines static Singleton s; and returns it. Make the constructor private. In main call instance() twice and compare their addresses; if equal print "Single instance".
Starter code
#include <iostream>
// Define the Singleton class here
int main() {
// Call the instance() method and print the result
return 0;
}Sample solution
#include <iostream>
class Singleton {
public:
static Singleton& instance() {
static Singleton s;
return s;
}
private:
Singleton() = default;
};
int main() {
auto& a = Singleton::instance();
auto& b = Singleton::instance();
std::cout << (&a == &b ? "Single instance" : "Many");
return 0;
}Expected output
Single instance
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does the Singleton pattern guarantee?
- The class runs faster
- All methods become static
- A single instance across the program Correct answer
- Private members become public
Explanation: Singleton ensures only one instance of a class is created.
2 Complete the instance method
static Singleton& ____() {
static Singleton s;
return s;
}static Singleton& instance() {
static Singleton s;
return s;
}Explanation: The instance() method returns the static local.
3 Match each piece to its role
| Term | Meaning |
|---|---|
static Singleton& instance() |
Returns single instance |
private Singleton() |
Blocks external construction |
static local variable |
Thread safe |
Meyers singleton |
Cleanest C++ way |