/ Design Patterns / Creational patterns / Singleton

Singleton Design Pattern in C++: Before and after

Before

A global variable is default initialized - when it is declared - but it is not initialized in earnest until its first use. This requires that the initialization code be replicated throughout the application. 

class GlobalClass
{
    int m_value;
  public:
    GlobalClass(int v = 0)
    {
        m_value = v;
    }
    int get_value()
    {
        return m_value;
    }
    void set_value(int v)
    {
        m_value = v;
    }
};

// Default initialization
GlobalClass *global_ptr = 0;

void foo(void)
{
  // Initialization on first use
  if (!global_ptr)
    global_ptr = new GlobalClass;
  global_ptr->set_value(1);
  cout << "foo: global_ptr is " << global_ptr->get_value() << '\n';
}

void bar(void)
{
  if (!global_ptr)
    global_ptr = new GlobalClass;
  global_ptr->set_value(2);
  cout << "bar: global_ptr is " << global_ptr->get_value() << '\n';
}

int main()
{
  if (!global_ptr)
    global_ptr = new GlobalClass;
  cout << "main: global_ptr is " << global_ptr->get_value() << '\n';
  foo();
  bar();
}

Output

main: global_ptr is 0
foo: global_ptr is 1
bar: global_ptr is 2


After

Make the class responsible for its own global pointer and "initialization on first use" (by using a private static pointer and a public static accessor method). The client uses only the public accessor method. 

class GlobalClass
{
    int m_value;
    static GlobalClass *s_instance;
    GlobalClass(int v = 0)
    {
        m_value = v;
    }
  public:
    int get_value()
    {
        return m_value;
    }
    void set_value(int v)
    {
        m_value = v;
    }
    static GlobalClass *instance()
    {
        if (!s_instance)
          s_instance = new GlobalClass;
        return s_instance;
    }
};

// Allocating and initializing GlobalClass's
// static data member.  The pointer is being
// allocated - not the object inself.
GlobalClass *GlobalClass::s_instance = 0;

void foo(void)
{
  GlobalClass::instance()->set_value(1);
  cout << "foo: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
}

void bar(void)
{
  GlobalClass::instance()->set_value(2);
  cout << "bar: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
}

int main()
{
  cout << "main: global_ptr is " << GlobalClass::instance()->get_value() << '\n';
  foo();
  bar();
}

Output

main: global_ptr is 0
foo: global_ptr is 1
bar: global_ptr is 2

Code examples

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