Summary

All Attributes available in C++17

Attribute	Description
[[noreturn]]	a function does not return to the caller
[[carries_dependency]]	extra information about dependency chains
[[deprecated]]	an entity is deprecated
[[deprecated("reason")]]	provides additional message about the deprecation
[[fallthrough]]	indicates a intentional fall-through in a switch statement
[[nodiscard]]	a warning is generated if the return value is discarded
[[maybe_unused]]	an entity might not be used in the code

Each compiler vendor can specify their syntax for attributes and annotations. In Modern C++, the ISO Committee tries to extract common parts and standardise it as [[attributes]].

There’s also a relevant quote from Bjarne Stroustrup’s C++11 FAQ about suggested use:

There is a reasonable fear that attributes will be used to create language dialects. The recommendation is to use attributes to only control things that do not affect the meaning of a program but might help detect errors (e.g. [[noreturn]]) or help optimisers (e.g. [[carries_dependency]]).