A Brief History of C++ Concurrency

At first, no direct support for threads in C++
Programmers could only use threads through C interfaces
- pthreads library for POSIX systems
- Windows API: provided access to threads in the "Base Services"
- Various Unix API's
  - System calls provided access to operating system's thread support
  - Varied between providers
- OpenMP (numeric computing)
  - Process C-style array loops in parallel
Later, C++ libraries appeared
- ACE
  - Cross platform networking library with support for threads
- Boost
  - Project to develop new features for C++
  - Basically a wrapper around the operating system's threading facilities
  - Provided a consistent interface
  - Used as basis for C++11's thread support
- Poco
  - More modern and lightweight version of ACE

Multi threading with Standard C++

C++ added support for Concurrency in 2011
Programmers can write threaded code which is
- Portable and efficient
- Has well defined semantics
- Allows fine-grained controls at low levels
Standard library changes
- Features added to create and manage threads
- All classes and functions made thread safe

Multi threading with C++11

The std::thread class is the base level of concurrency
- Rather low-level implementation
- Maps onto a software thread
- Managed by the operating system
- Similar to Boost threads, but with some important differences
  - No thread cancellation
  - Different argument passing semantics
  - Different behavior on thread destruction

Multi threading with later C++

C++14: Read-write locks
C++17: Many standard algorithms can execute in parallel
C++20:
- Joining threads
- Thread cancellation
- Coroutines, semaphores, latches and barriers
C++23: Improved support for coroutines

Threads and Tasks

The words are often used interchangeably
In this course:
- A thread is:
  - A software thread
  - An object of the C++ std::thread class
- A task is:
  - A higher level abstraction
  - Some work that should be performed concurrently