Boost Mutex Scoped Lock

Now I understand the point of a Mutex is to prevent two threads from accessing the same resource at the same time, but I don't see the correlation between io_mutex and std::cout.

std::cout is a global object, so you can see that as a shared resource. If you access it concurrently from several threads, those accesses must be synchronized somehow, to avoid data races and undefined behavior.

Perhaps it will be easier for you to notice that concurrent access occurs by considering that:

std::cout << x

Is actually equivalent to:

::operator << (std::cout, x)

Which means you are calling a function that operates on the std::cout object, and you are doing so from different threads at the same time. std::cout must be protected somehow. But that's not the only reason why the scoped_lock is there (keep reading).

Does this code just lock everything within the scope until the scope is finished?

Yes, it locks io_mutex until the lock object itself goes out of scope (being a typical RAII wrapper), which happens at the end of each iteration of your for loop.

Why is it needed? Well, although in C++11 individual insertions into cout are guaranteed to be thread-safe, subsequent, separate insertions may be interleaved when several threads are outputting something.

Keep in mind that each insertion through operator << is a separate function call, as if you were doing:

std::cout << id;
std::cout << ": ";
std::cout << i;
std::cout << endl;

The fact that operator << returns the stream object allows you to chain the above function calls in a single expression (as you have done in your program), but the fact that you are having several separate function calls still holds.

Now looking at the above snippet, it is more evident that the purpose of this scoped lock is to make sure that each message of the form:

<id> ": " <index> <endl>

Gets printed without its parts being interleaved with parts from other messages.

Also, in C++03 (where insertions into cout are not guaranteed to be thread-safe) , the lock will protect the cout object itself from being accessed concurrently.

A mutex has nothing to do with anything else in the program (except a conditional variable), at least at a higher level. A mutex has two effeccts: it controls program flow, and prevents multiple threads from executing the same block of code simultaneously. It also ensures memory synchronization. The important issue here, is that mutexes aren't associated with resources, and don't prevent two threads from accessing the same resource at the same time. A mutex defines a critical section of code, which can only be entered by one thread at a time. If all of the use of a particular resource is done in critical sections controled by the same mutex, then the resource is effectively protected by the mutex. But the relationship is established by the coder, by ensuring that all use does take place in the critical sections.