Re: Initializing a map...
On Feb 21, 7:37 pm, Jeff Schwab <j...@schwabcenter.com> wrote:
Sam wrote:
Jeff Schwab writes:
#include <iostream>
#include <ostream>
int main()
{
std::map<int, int> m(map_initializer(3,4)(5,6)(7,8));
...
return 0;
}
That's a neat idea. It could probably be made a little more efficient
by replacing the calls to map::operator[] with calls to map::find and
operator[] takes only one argument.
Right, but what's the relevance?
You can use a std::pair, but it'll make this even more ugly.
Any ugliness would be hidden in map_initializer, so who cares.
I don't understand what you mean. I was pointing out that in:
m[k] = v;
The map first has to default-construct the value (assuming
this is the first time the map entry has been accessed), then
assign to it.
That's not really the issue---you're going to deep copy the map
anyway, so a little more or less shouldn't matter. The issue is
really one of semantics. Using insert or operator[] have
different semantics in the case of duplicates, and in the case
of insert, you can easily check the return value, and generate
an error (exception or assertion failure) in case of duplicates,
which is probably the most useful semantics.
One could instead check for the correct position of the value
with lower_bound (I mistakenly said map::find), then use that
iterator as a hint to insert the new value.
You could, but I don't see what that buys you. Insertion
without the hint (or with the wrong hint) is O(ln n). With the
hint, it is O(1), but lower_bound is O(ln n), so there's no
point in using it just to provide a hint.
This way, the value in the map can be copy-constructed in the
first place, rather than default-constructed, then assigned.
Which could also be a very important advantage if one of the
types didn't support default construction.
All in all: forget about optimization---good design requires the
use of insert.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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