Create A histogram using C++ with map/unordered_map: the default value for a non-existant key

Question

I am defining a small function to create a histogram of a vector of integers, Initially I defined the following function that first test whether the key exists in the map be

Answer 1

It is guaranteed to be zero-initialized for built-in types, and default constructed for user-defined types. The guarantee is that if an element for a given key does not exist, one is inserted, with the mapped_type being value initialized. For built-in types such as int, this means zero initialization.

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