I've been reading CppCoreGuidelines F.15 and I don't understand the following sentences from the table of parameter passing:
"Cheap" ≈ a handful of hot int copies
"Moderate cost" ≈ memcpy hot/contiguous ~1KB and no allocation
What does "hot int copy" mean?
"Hot" in this case likely refers to the likelihood of being cached. A particular piece of memory is "cold" if it is likely not in the cache, due to not having been touched recently within this thread of execution. Conversely, a piece of memory is "hot" if it likely has been touched recently, or is contiguous with memory that has been recently touched.
So it's talking about the cost of doing a memory copy of something that is currently in the cache and is therefore cheap in terms of actual memory bandwidth.
For example, consider a function that returns an array<int, 50>. If the values in that array were generated by the function itself, then those integers are "hot", since they're still almost certainly in the cache. So returning it by value is considered OK.
However, if there is some data structure that contains such a type, this function could have simply retrieved a pointer to that object. Returning it by value means doing several uncached memory accesses, since you have to copy to the return value. That is less than ideal from a memory cache perspective, so perhaps returning a pointer to the array would be more appropriate.
Obviously, uncached accesses will happen either way, but in the latter case, the caller gets to decide which accesses to perform and which not to perform.