How to return the greatest key strictly less than the given key in a std::multimap?

Question

multimap offers the methods lower_bound and upper_bound. Both may return an iterator to a value with key greater than the desired, with lower_bound possibly yielding exactly the desired.

Now I want an iterator to a value where the key is strictly less the requested. If it were a map rather than multimap, this would be relatively simple to achieve as described here: Returning the greatest key strictly less than the given key in a C++ Map. But in a multimap, decrementing an iterator is not guaranteed to make it point to a strictly smaller key. So I would need to decrement repeatedly, until a smaller key is found. Not particularly nice.

Is there a more elegant way of doing this?

The keys will in general be floating-point.

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My apologies, it turns out that you can actually do it with a single decrement. I just placed it wrong in my program, that was the real error.

Answer 1

lower_bound points to the smallest element greater than or equal to the argument (or end). Thus decrementing it once gives you the desired element (if it exists).

Answer 2

AFAIK, lower/upper_bound will return iterator to FIRST element of this value, So you can decrease it