Andrei Alexandrescu <seewebsiteforem...@erdani.org> napisał(a):
TDPL has an example that can be reduced as follows:
void main() {
uint[string] map;
foreach (line; stdin.byLine()) {
++map[line];
}
}
byLine reuses its buffer so it exposes it as char[]. Therefore,
attempting to use map[line] will fail. The program compiled and did the
wrong thing because of a bug.
The challenge is devising a means to make the program compile and work
as expected. Looking up an uint[string] with a char[] should work, and
if the char[] is to be inserted, a copy should be made (via .idup or
to!string). The rule needs to be well defined and reasonably general.
The effect is something like this:
void main() {
uint[string] map;
foreach (line; stdin.byLine()) {
auto p = line in map;
if (p) ++*p;
else map[line.idup] = 1;
}
}
Ideally the programmer would write the simple code (first snippet) and
achieve the semantics of the more complex code (second snippet). Any
ideas?
To be honest I find the 2nd snippet more readable, it's just one line more:
uint[string] map;
foreach (line; stdin.byLine()) {
if (auto p = line in map) ++*p;
else map[line.idup] = 1;
}
You know exactly what it does and it's const correct. I don't see value in
implicit cloning.
The first example smells of STL map[key] -- if key is not found, it
implicitly inserts key, initializes the value, and returns a reference.
Let's not go there.
--
Tomek
