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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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