Monday, February 6, 2012

populating STL containers requires copy-ctor

See also http://bigblog.tanbin.com/2012/07/insert-into-vector-copy-ctor-constantly.html for detailed experiments.

I believe STL containers generally use the payload's copy-ctor during insertion.

vector? YES according to p21 [[effSTL]] vector example
deque at extremities? yes [1]
deque mid-stream???
list? yes
set? yes
map? yes

Let's examine vectors. A java Vector (or arrayList) holds nothing but pointers. A 5-element vector occupies about 32bits x 5. All pointee objects live on the heap.

A stl vector, in basic configuration, holds a bunch of non-ref objects. A 5-element vector of Trade where sizeof(Trade)=20 bytes occupies exactly 100 bytes. (No additional overhead due to nodes and links. Remember a linked container consists of nodes each containing a payload object.)

P22 [[effSTL]] suggests that an empty vector can have the 100 bytes reserved but no Trade instance constructed. Item 9 in [[effSTL]] too suggest that default allocators for all STL containers grab memory from heap and return it as uncooked (i.e. uninitialized) chunk of raw memory. [[c++nutshell] says an implementation of vector must allocate an uninitialized array of objects and initialize Individual elements when needed

When you push_back a Trade that lives in Address 8123, our 5-element vector needs to somehow copy that Trade into Position #6. Assume there's enough capacity reserved so no reallocation. Our vector can't use assignment operator, because Position #6 is not a Trade object, but just 20 raw bytes allocated by the default vector allocator. Can it use the copy constructor? I think so, yet it doesn't grab memory from stack or heap, but from the vector's allocator. Item 10 of [[effC++]] has a complete allocator class. If your vector uses such an allocator, then every memory allocation "within" the vector would be intercepted by this allocator.

Down-shift after removing an element calls the assignment operator. I feel there's no choice. Shifting Item 3 down to Position 2 -- you can't use copy-ctor or any ctor, since they use uncooked raw memory. In this case Slot 2 is already allocated and "cooked" (initialize).

I believe up-shift uses the assignment operator or copy-constructor.

[1] allocating memory from the "reserved packing lots". See other posts on deque memory model.

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