View types like span, string_view, FunctionRef: absolutely pass by value. They're designed to be cheap to copy, and they provide type erasure which can make you

View types like span, string_view, FunctionRef: absolutely pass by value. They're designed to be cheap to copy, and they provide type erasure which can make your API more flexible. You also usually want to pass smart pointers (unique_ptr, shared_ptr) by value to avoid breaking your ownership model, or pass by raw pointer or reference to the held data. The obvious case that comes to mind where you'd want to write const unique_ptr<T>& is when iterating over a vector<unique_ptr<T>>. Otherwise, pass ptr.get() for an optional param, or *ptr for a required param where you've already checked ptr != nullptr. Wrapper types like optional, variant, StatusOr? Usually pass by reference, unless the pointed-to type is small (<= 8 bytes). One common pattern where I see wide structs being passed by value is the use of option structs (https://abseil.io/tips/173 https://abseil.io/tips/173) that are usually constructed via designated initializers. There is some marginal benefit to being able to std::move() out individual fields but it's not really a big deal either way, as said computation is usually only done once on program initialization.