A reference is another name for an existing object, while a pointer is an object that stores an address.
That difference drives everything else. A reference must be initialized when declared and can never be reseated to a different object. A pointer can be null, can be reassigned, and can be moved through an array with arithmetic. A pointer also has its own storage and its own address, and a reference usually has neither.
Use a reference when the thing must exist and will not change identity. Use a pointer when absence is meaningful, when you need to rebind, or when you walk memory.
References make the contract clearer at the call site. A function taking const std::string& always receives a real object, so it needs no null check.
Rewriting in plainer words…
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