Dealing with existing code

Dealing with existing code can be a challenge on its on. How can you fix or extend code your do not really understand?

understanding existing code

To start to understand code one approach to consider is to refacter it!

move declarations of variables to where they are used first
initialize all variables at declararion
make variables const if possible
if the initialization of a variable is the result of multiple statements, consider moving this code into a function.
create lambda's to group pieces of code you understand and can now give a name, preverably a verb to express what it is doing.
rename variables that do not have descriptive names, change the name as soon as your think "aha, but it is really X".
prefer the most fitting tool for the job (unique_ptr iso shared_ptr, lock_guard iso unique_lock, etc)
Use braces and indentation to express code flow
remove dead code such as unreferences variables
remove comments that do not add value
add [[nodiscard]]
change deeply nested code to fail early and factor out functions, the happy path is left-aligned
move repeated code into function, or keep the result in a wider scope.
add documentation to API functions once you understand them, for public API's

to be continued...