C++ interview questions on patterns like thread safe singleton and bridge

Make changes in the singleton class for multithreading
Q4) Implement a Singleton pattern as a template such that, for any given class Foo, I can call Singleton::instance() & get a pointer to a singleton of type Foo. Assume the existence of a class Lock which has acquire() & release() methods. The implementation should be thread safe & exception safe

class foo
{
public:
static foo* instance();
private:
foo() {}
~foo() {}
static foo* smInstance;
static Lock smMutex;
};

foo* foo::smInstance = NULL;
Lock foo::smMutex;

foo* foo::instance()
{
LockLocker lock(&smMutex);

if (smInstance == NULL)
smInstance = new foo();

return smInstance;
}

i think they expect a double checking lock

template
class Singleton
{
public:
T* Instance()
{
if(!instance)
{
Lock mutex; // RAII object which aquires mutex lock

if(!instance) // Double check
{
instance = new T;
}
}

return instance;
}
private:
Singleton(){}
~Singleton(){}
static T *instance;
};

In .cpp

static Singleton::instance = 0
Venkat on September 27, 2009 |Edit | Edit

sorry… the Instance() function should be static
Reply to Comment
ac on November 14, 2009 |Edit | Edit

and you should also make the operator= private for this class.

exception safe and thread-safe

template
class Singleton
{
public:
T* Instance()
{
if(!instance)
{
Lock lock(m_dataMutex); // RAII object which aquires mutex lock

if(!instance) // Double check
{
try
{
instance = new T;
}
catch(…)
{
cout<<"Exception encountered in Singleton::MyInstance"< NOTE I now post my TRADING ALERTS into my personal FACEBOOK ACCOUNT and TWITTER. Don't worry as I don't post stupid cat videos or what I eat!