MySQL  8.0.19
Source Code Documentation

Namespaces

 dd
 The version of the current data dictionary table definitions.
 

Detailed Description

Declarations of internal functions which operate on Sdi_context objects. Conceptually these are member functions of Sdi_context, but declaring them as such would mean that the definition of Sdi_context would have had to be made available in every translation unit where these functions get called (most data dictionary object implementation files).

This is essentially a modification of the pimpl (Pointer to IMPLementation) idiom where we avoid the need to create separate api and implementation classes and avoid the extra indirection of going through the pimpl pointer.