User Defined Atomics

This lesson gives an overview of user-defined atomics used from the perspective of concurrency in C++.

There are a lot of deep restrictions on a user-defined type MyType if you use it for an atomic type std::atomic<MyType>. These restrictions are on the type MyType, but also on the operations that std::atomic<MyType> can perform.

Here are the restrictions for MyType to become an atomic type:

  • The copy assignment operator for MyType (all base classes of MyType and all non-static members of MyType) must be trivial. This means that you must not define the copy assignment operator but request it by default from the compiler.
  • MyType must not have virtual methods or virtual base classes.
  • MyType must be bitwise comparable so that the C functions memcpy or memcmp can be applied.

Check the type properties at compile time

The type properties on MyType can be checked at compile time, by using the following functions: std::is_trivially_copy_constructible, std::is_polymorphic and std::is_trivial. All these functions are part of the very powerful type-traits library.

The user-defined atomic type std::atomic<MyType> supports only a limited interface.

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