// Documentation link:

// https://sourcemaking.com/design_patterns/singleton/cpp/1

#include <iostream>

using namespace std;

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();

return 0;

}

// g++ -o singleton singleton.cpp && valgrind --leak-check=full --show-leak-kinds=all ./singleton