Singleton Template (C++)

• Author: Terence J. Grant
• License: MIT License
• Last Update: 2015-11-15
• Donate: Your donations are appreciated!

Before downloading or using this product, make sure you understand and accept the terms of the license.

After downloading, make sure to follow the how to use instructions below; they're worth reading.

The Singleton Template is a C++ template class that allows you to define your own singleton classes as inheriting from a single base class.

This provides a few benefits over other techniques:

• Consistent method interface among all singletons (all singletons will have a ::getInstance() method)
• Never having to copy / paste code for singleton creation (which is good if, for instance, best practices change)

The singleton creation method in ::getInstance() is via the "Meyers Singleton" technique, described in the book Effective C++ by Scott Meyers. So essentially, this class is a "Meyers Singleton C++ template class."

• First, copy the header into your project.
• Next, include the header somewhere in your project or precompiled header if you use one.

#include "singleton.h"

• Create your own singleton class, ensuring that:

1. You inherit from tjgrant::singleton<MyClass>, where MyClass is the name of your class
2. Your constructor's access method is protected
3. Your destructor is virtual
4. You have the friend class statement similar to the example below

class MyClass
: public tjgrant::singleton<MyClass>
{
protected:
    //Members go here
 
protected:
    MyClass();
public:
    virtual ~MyClass();
 
public:
    //Methods go here
 
    friend class tjgrant::singleton<MyClass>;
};

Your MyClass class will now have a ::getInstance() method, which you can use to construct the class or get it's instance at runtime.

• Construct the instance like so:

MyClass::getInstance();

• Get the instance of the class at any point in time like so and you can do work with it:

MyClass* c = MyClass::getInstance();
 
if (c)
{
    c->doStuff(); // Assuming you have a method called ::doStuff, this is how you'd call it
}

• You can also call methods directly without using a temporary / local variable:

MyClass::getInstance()->doStuff();

That's it! You should be good to go.

If you have a question not answered here, please feel free to contact me and ask.

• Q: My class already inherits from another class; can I still use this?
• A: Yes; in fact this is generally the case
  • C++ allows for multiple inheritance so this not a problem; just comma-separate your inheritance classes and you're set
  • As an example, say your class MyClass originally inherited from MyBase; you would still follow the example above in setup with just a minor change at the beginning:

class MyClass
: public MyBase,
  public tjgrant::singleton<MyClass>

• Q: Is this library thread safe? (Advanced: Won't apply to most users.)
• A: It's Conditionally Safe
  • The first time you call ::getInstance, memory will be written; this is not thread-safe so your first call to ::getInstance should be on the main thread
  • After the first time, ::getInstance only reads memory, so this should be safe on any thread
• Note: That said, I would suggest only accessing ::getInstance on the main thread whenever possible

If you've found this project useful, please donate today!

November 15, 2015

• First public release