There's no way to set thread priorities via the C++11 library. I don't think this is going to change in C++14, and my crystal ball is too hazy to comment on versions after that.

In POSIX, pthread_setschedparam(thread.native_handle(), policy, {priority});

I don't know the equivalent Windows function, but I'm sure there must be one.

My quick implementation...

#include <thread>
#include <pthread.h>
#include <iostream>
#include <cstring>

class thread : public std::thread
{
  public:
    thread() {}
    static void setScheduling(std::thread &th, int policy, int priority) {
        sch_params.sched_priority = priority;
        if(pthread_setschedparam(th.native_handle(), policy, &sch_params)) {
            std::cerr << "Failed to set Thread scheduling : " << std::strerror(errno) << std::endl;
        }
    }
  private:
    sched_param sch_params;
};

and this is how I use it...

// create thread
std::thread example_thread(example_function);

// set scheduling of created thread
thread::setScheduling(example_thread, SCHED_RR, 2);

The standard C++ library doesn't define any access to thread priorities. To set thread attributes you'd use the std::thread's native_handle() and use it, e.g., on a POSIX system with pthread_getschedparam() or pthread_setschedparam(). I don't know if there are any proposals to add scheduling attributes to the thread interface.

In Windows processes are organized in class and level priority. Read this: Scheduling Priorities, it gives a good overall knowledge about thread and process priority. You can use the following functions to control the priorities even dynamically: GetPriorityClass(), SetPriorityClass(), SetThreadPriority(), GetThreadPriority().

Apperantly you can also use std::thread's native_handle() with pthread_getschedparam() or pthread_setschedparam() on a windows system. Check this example, std::thread: Native Handle and pay attention to the headers added!