Could somebody elaborate 'use a const pointer when the non-optional form would have used a reference'?

[junsoopark]

in cppguide.html,
Could somebody please elaborate 'use a const pointer when the non-optional form would have used a reference'? It will be also helpful if some examples can be added.

My understanding is as follows,

• Non-optional input parameters - values or const references
• Non-optional output and input/output parameters - references
• Optional input parameters(by value) - absl::optional
• the non-optional form would have used a reference(<= This means Optional input parameters by references?) - a const pointer
• Optional outputs and optional input/output parameters - non-const pointers

Thanks in advance.

Parameters are either inputs to the function, outputs from the function, or both. Non-optional input parameters should usually be values or const references, while non-optional output and input/output parameters should usually be references (which cannot be null). Generally, use absl::optional to represent optional by-value inputs, and use a const pointer when the non-optional form would have used a reference. Use non-const pointers to represent optional outputs and optional input/output parameters.

[gou4shi1]

I can't understand this too, does it mean if you can't use const &, then use const * as non-optional input?

[tituswinters]

It's trying to clarify the comparisons: T* is to T& what optional<T> is to T. If you would be passing by value, but want optionality, you pass optional<T>. If you would be passing by reference but want optionality, pass T*.