Synchronized is a Java

Synchronized means that the method cannot be executed by two threads at the same time and the JVM take care of enforcing that. 

In C++, you will have to use some synchronizationconstruct, like a critical section or a mutex

Another answer from Anders K.

In the (Java) example

public static synchronized Singleton getInstance()

means that only one thread at a time should be able to access the getInstance() method this to avoid a racing condition.

Synchronized Methods from Oracle Java Doc

The Java programming language provides two basic synchronization idioms: synchronized methods and synchronized statements. The more complex of the two, synchronized statements, are described in the next section. This section is about synchronized methods.

To make a method synchronized, simply add the synchronized keyword to its declaration:

public class SynchronizedCounter {    private int c = 0;

    public synchronized void increment() {        c++;    }

    public synchronized void decrement() {        c--;    }

    public synchronized int value() {        return c;    }}

• First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

• Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

If count is an instance of SynchronizedCounter, then making these methods synchronized has two effects:

Note that constructors cannot be synchronized — using the synchronized keyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.