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

Yes, the value inserted by [] is guaranteed to be zero. From C++11 23.4.4.3/1:

Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map.

T() specifies value-initialisation which, for numeric types, means it's initialised with the value zero.

Answer 2

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.

More information in this reference.

Answer 3

This is a guarantee by the behavior of std::map's operator [] overload. If the key does not exist then it will be value-initialized for you. Since int, when value-initialized is assigned to zero, you're safe here.