Implement ctl::unique_ptr #1216
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The way unique_ptr is supposed to work is as a purely compile-time check that your raw pointers are getting deleted when they go out of scope. It should ideally emit the same exact machine code as if you were using raw pointers with manual deletes. Part of what this means is that under normal circumstances, a unique_ptr shouldn’t take up more space than a raw pointer - in other words, sizeof unique_ptr<T> should == sizeof(T*). The STL approach of having a deleter as a second template parameter makes this slightly tricky but doable. It works like this: normally each member of a struct needs to take at least 1 byte, but this doesn’t apply to inheriting from a base struct that has no members. So practically speaking, what we do is, when it will save us space, we inherit from our deleter instead of including it as a member. The above is what our slightly complicated ctl::compressed_pair class is doing. Honestly we could just elide compressed_pair and inherit from the pair_elem for our deleter since a T* always takes up a word, but I wrote the whole thing anyway - maybe it’ll come in handy somewhere else. The present PR doesn’t bother with the specialization for array types. I also left a couple other parts of the STL API unimplemented. I’d love to see someone else implement these, or I’ll get to them at some point.
For that we need [[no_unique_address]]. Actually wait, I think maybe we have [[no_unique_address]]! Maybe we can get rid of compressed_pair entirely...
This felt so good. Omg. Bye. You won’t be missed.
jart
approved these changes
Jun 16, 2024
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The way unique_ptr is supposed to work is as a purely compile-time check that your raw pointers are getting deleted when they go out of scope. It should ideally emit the same exact machine code as if you were using raw pointers with manual deletes.
Part of what this means is that under normal circumstances, a unique_ptr shouldn’t take up more space than a raw pointer - in other words,
sizeof unique_ptr<T>should== sizeof(T*).The present PR doesn’t bother with the specialization for array types. I also left a couple other parts of the STL API unimplemented. I’d love to see someone else implement these, or I’ll get to them at some point.