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STL and most other containers value initialize new elements in common operations
like vector::resize(size_type n) or explicit
vector::vector(size_type n).
In some performance-sensitive environments, where vectors are used as a replacement for variable-size buffers for file or network operations, value initialization is a cost that is not negligible as elements are going to be overwritten by an external source shortly after new elements are added to the container.
Boost.Container offers two new members for
vector, static_vector
and stable_vector: explicit container::container(size_type n, default_init_t) and container::resize(size_type n, default_init_t), where new elements are constructed using
default
initialization.
When filling associative containers big performance gains can be achieved
if the input range to be inserted is guaranteed by the user to be ordered
according to the predicate. This can happen when inserting values from a
set to a multiset
or between different associative container families ([multi]set/map
vs. flat_[multi]set/map).
Boost.Container has some overloads for constructors
and insertions taking an ordered_unique_range_t
or an ordered_range_t tag
parameters as the first argument. When an ordered_unique_range_t
overload is used, the user notifies the container that the input range is
ordered according to the container predicate and has no duplicates. When
an ordered_range_t overload
is used, the user notifies the container that the input range is ordered
according to the container predicate but it might have duplicates. With this
information, the container can avoid multiple predicate calls and improve
insertion times.
In the first C++ standard list::size() was not required to be constant-time, and
that caused some controversy in the C++ community. Quoting Howard Hinnant's
On
List Size paper:
There is a considerable debate on whether
std::list<T>::size()should be O(1) or O(N). The usual argument notes that it is a tradeoff with:
splice(iterator position, list& x, iterator first, iterator last);If size() is O(1) and this != &x, then this method must perform a linear operation so that it can adjust the size member in each list
C++11 definitely required size() to be O(1), so range splice became O(N).
However, Howard Hinnant's paper proposed a new splice
overload so that even O(1) list:size()
implementations could achieve O(1) range splice when the range size was known
to the caller:
void splice(iterator position, list& x, iterator first, iterator last, size_type n);Effects: Inserts elements in the range [first, last) before position and removes the elements from x.
Requires: [first, last) is a valid range in x. The result is undefined if position is an iterator in the range [first, last). Invalidates only the iterators and references to the spliced elements. n == distance(first, last).
Throws: Nothing.
Complexity: Constant time.
This new splice signature allows the client to pass the distance of the input range in. This information is often available at the call site. If it is passed in, then the operation is constant time, even with an O(1) size.
Boost.Container implements this overload
for list and a modified version
of it for slist (as slist::size()
is also O(1)).