Before the C++ 2011 (C++11) standard, many different threading implementations were developed, many of which are limited to a specific software platform. Some of these are still relevant today, such as Windows threads. Others have been superseded by standards, of which POSIX Threads (Pthreads) has become the de facto standard on UNIX-like OSes. This includes Linux-based and BSD-based OS, as well as OS X (macOS) and Solaris.
Many libraries were developed to make cross-platform development easier. Although Pthreads helps to make UNIX-like OS more or less compatible one of the prerequisites to make software portable across all major operating systems, a generic threading API is needed. This is why libraries such as Boost, POCO, and Qt were created. Applications can use these and rely on the library to handle any differences between platforms.
