CPP Synchronized

When I first discovered Java's synchronized blocks, I was intrigued. It's pretty easy to forget to unlock a mutex, though RAII and unique_lock aid with this, and easy to follow the structure of a block. I thought for a while about how I might implement something like this in C++11, and eventually accomplished this in two different ways.

synctable

My favorite of the two implementations. Allows for a synchronized block on any pointer using the syntax:

synchronized(&obj) {
    // ... critical section
}

Note that this could have been done using an object rather than a pointer with something more resembling Java synchronized(obj) but it seemed to be more in the spirit of C++ to use the address.

This is accomplished with a #define, and a for loop that initializes an object to lock and unlock a std::mutex on for entry/exit. The mutexes themselves are stored in a global std::unordered_map of void * to mutex.

An alternative use is a tablesynchronized block. This requires the programmer to create a SyncTable and pass that as the first argument to tablesynchronized

SyncTable table;
// ... later on
tablesynchronized(table, &obj) {
    // ... critical section
}

The idea here is a group of threads can use a local SyncTable rather than the global one to avoid contention on the global table.

lockable_objects

A non-intrusive use of multiple inheritance. A "lockable" version of any class can be instantiated with

Lockable<Object> lockable_object(/* Object ctor args */);

Any lockable object can be used within a synchronized block. This approach avoids the SyncTables entirely

synchronized(lockable_object){
    // ... critical section
}

Can then be used without the overhead of a lookup for the mutex. Additionally, no & is used for lockable objects.