By const reference, void f(const BigThing& x). Passing by value would copy the entire object on every call, which for something like a 256-byte sensor frame is real, repeated cost on an MCU. A const& passes an alias (effectively a pointer under the hood), so it's cheap regardless of size, and the const guarantees the function can't modify the caller's object, giving you the no-copy benefit without giving up safety. If the function needs to modify the caller's object, pass a non-const reference (BigThing&) as an out-parameter. Only small, cheap-to-copy types (int, float, a pointer, a small POD) should be passed by value, where a copy is as cheap as passing an address and avoids a layer of indirection.
C++ for Embedded · Interview question
How should you pass a large object to a function, and why?
A strong answer
What a weak answer sounds like
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From the lesson
References vs Pointers
References are aliases that can't be null and can't be reseated. When to reach for a reference, when you still need a pointer, and why const& is the default way to pass objects.