Lazy Callback

General Usage
1

In the callback example provided on the readme of CPLEX.jl we have:

# You can select where the callback is run
if context_id != CPX_CALLBACKCONTEXT_CANDIDATE
    return
end

and then

ispoint_p = Ref{Cint}()
ret = CPXcallbackcandidateispoint(cb_data, ispoint_p)
if ret != 0 || ispoint_p[] == 0
    return  # No candidate point available or error
end

I do not understand the usefulness of the second part. Can we have the context equal to CPX_CALLBACKCONTEXT_CANDIDATE without having any candidate point available?

2

Welcome!
The reason is explained in the CPLEX documentation here.
Btw: you are more likely to get help on such issues, if you post them in the category Optimization (Mathematical).