How to multithread core-schedule onto different cores (ideally in C++)

I have a large C++11 multithreaded application where the threads are always active, communicating to each other constantly, and should be scheduled on different physical CPUs for reasonable performance.

The default Linux behavior AFAIK is that threads will typically/often get scheduled onto the same CPU, causing horrible performance.

To solve this, I understand how to attach threads to specific physical CPUs in C++, e.g.:

    std::cout << "Assign to thread cpu " << cpu << "\n";
    cpu_set_t cpuset;
    CPU_ZERO(&cpuset);
    CPU_SET(cpu, &cpuset);
    int rc = pthread_setaffinity_np(thread.native_handle(), sizeof(cpu_set_t), &cpuset);

and can use this to pin to specific CPUs, e.g. attach 4 threads to CPUs 0,2,4,6.

However this approach requires a specific CPU number which is a problem in that there may be many programs running on a host using other CPUs. These might be my program or other programs. As just one example an 8 core machine might have two copies of my 4-threaded application so obviously having both of those two programs pick the same 4 CPUs is a problem.

I'd thus like a way to say "schedule the threads in this set all on different CPUs without caring of the CPU number". Is this possible in C++(11)?

If not, is this possible with numactl or another utility? E.g. I don't want "numactl -C 0,2,4,6" but rather "numactl -C W,X,Y,Z" where the scheduler can pick arbitrary W,X,Y,Z subject to W!=X!=Y!=Z.

I'm most interested in Linux behavior. I cannot change the OS configuration. I don't want the separate applications to cross communicate (nor can they as they might be other applications I do not control.)

Once I have the answer to this, the follow up is how do I modify this to add a e.g. fifth thread I do want to schedule on the same CPU as the first thread?

Thanks!