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I'm using `std::multimap` with tens of millions of elements.  The key is a pointer, and the data is a shared pointer to a class.  Sometimes I need to remove a few elements, sometimes many elements, and sometimes all elements.  I'm currently using `std::erase_if` and I'm wondering about performance for the case where all elements are removed.

The first part of the question is: If all elements are being removed, would `clear` be substantially faster than `erase_if`?

I'm guessing the answer is "yes," which leads to the next question.  A problem is that, at the point of the `erase_if` call, I don't know whether or not all elements will be removed.  So, I could do something like `count_if` first, and if the count equals the map size then do `clear`, and otherwise do the `erase_if`.  When removing a few elements, I'm guessing the use of `count_if` followed by `erase_if` would roughly double the time, which I think would be ok.  But when removing all elements, how much would the `count_if` reduce the gain of using `clear`?

(I'm using gcc 12.3.0.)

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