C++: how to initialize vector in map with non-zero size

I have a map of vectors:

std::map<int, std::vector<bool>> mymap

At times, I need to insert a new element:

auto& newvec = mymap[42];
// Add stuff to newvec

As far is I understand (and assuming that 42 is not yet in the map), this will give me newvec with length 0 (constructed as std::vector<bool> {}) which I can then extend.

Is there a way to initialize the vector to some size n right away?

(I am not concerned about performance, just wondering if there is a way to do this).