Andreas Kling 1a71e20f93 AK: Add HashMap::ensure(key, callback) ...

This function ensures that a key is present in the HashMap.
If it's not present, it is inserted, and the corresponding value
is initialized with whatever the callback returns.

It allows us to express this:

    auto it = map.find(key);
    if (it == map.end()) {
        map.set(it, make_a_value());
        it = map.find(key);
    }
    auto& value = it->value;

Like this:

    auto& value = map.ensure(key, [] { return make_a_value(); });

Note that the callback is only invoked if we have to insert a missing
key into the HashMap. This is important in case constructing the default
value is expensive or otherwise undesirable.

2021-09-04 20:30:56 +02:00

4.6 KiB

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