Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 18 | Threads and Locks
18.3 Implement a singleton design pattern as a template such that, for any given class
Foo, you can call Singleton::instance() and get a pointer to an instance of a singleton
of type Foo. Assume the existence of a class Lock which has acquire() and release()
methods. How could you make your implementation thread safe and exception safe?
SOLUTION
1 using namespace std;
2 /* Place holder for thread synchronization lock */
3 class Lock {
4 public:
5 Lock() { /* placeholder code to create the lock */ }
6 ~Lock() { /* placeholder code to deallocate the lock */ }
7 void AcquireLock() { /* placeholder to acquire the lock */ }
8 void ReleaseLock() { /* placeholder to release the lock */ }
9 };
10
11 /* Singleton class with a method that creates a new instance of the
12 * class of the type of the passed in template if it does not
13 * already exist. */
14 template class Singleton {
15 private:
16 static Lock lock;
17 static T* object;
18 protected:
19 Singleton() { };
20 public:
21 static T * instance();
22 };
23 Lock Singleton::lock;
24
25 T * Singleton::Instance() {
26 /* if object is not initialized, acquire lock */
27 if (object == 0) {
28 lock.AcquireLock();
29 /* If two threads simultaneously check and pass the first “if”
30 * condition, then only the one who acquired the lock first
31 * should create the instance */
32 if (object == 0) {
33 object = new T;
34 }
35 lock.ReleaseLock();
36 }
37 return object;
38 }
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