Singleton in C++
Why read if you can watch?
Watch Singleton's video tutorialBefore and after
BEFORE - A global variable is default initialized - when it is declared - but it is not ini- tialized in earnest until its first use. This requires that the initialization code be replicated throughout the appli- cation.
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(void)
{
if (!global_ptr)
global_ptr = new GlobalClass;
cout << "main: global_ptr is " << global_ptr->get_value() << '\n';
foo();
bar();
}
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(void)
{
cout << "main: global_ptr is " << GlobalClass::instance()->get_value() <<
'\n';
foo();
bar();
}
main: global_ptr is 0
foo: global_ptr is 1
bar: global_ptr is 2
