What happens if I read a map’s value where the key does not exist?

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The map::operator[] searches the data structure for a value corresponding to the given key, and returns a reference to it.

If it can’t find one it transparently creates a default constructed element for it. (If you do not want this behaviour you can use the map::at function instead.)

You can get a full list of methods of std::map here:

http://en.cppreference.com/w/cpp/container/map

Here is the documentation of map::operator[] from the current C++ standard…

23.4.4.3 Map Element Access

T& operator[](const key_type& x);

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

  2. Requires: key_type shall be CopyConstructible and mapped_type shall be DefaultConstructible.

  3. Returns: A reference to the mapped_type corresponding to x in *this.

  4. Complexity: logarithmic.

T& operator[](key_type&& x);

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

  2. Requires: mapped_type shall be DefaultConstructible.

  3. Returns: A reference to the mapped_type corresponding to x in *this.

  4. Complexity: logarithmic.

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