std::map<string,int> default initialization of value [duplicate]

Question

This piece of code seems work well, with default value for they value_type (int) as 0; does it work for all cases?

std::map<std::string,int> w;
for (const auto& t: str)
   w[t]++;

What about double? map? default 0.0?

Answer 1

Yes, this code would work for any type of the key, including double. The reason this works is that the non-const operator [] returns a reference to the value at the key, not a copy of that value. It is that reference to which the ++ operator gets applied.

The code fragment that you show works as follows:

• For each key t of type string in the str container,
• The map w is searched for the given key
• Since the entry is not there, a new one gets inserted into the map
• Since the key of the entry is known, but the value is not, a default (value-initialized, e.i. 0 for int) object for the value gets created
• A reference to the newly created object (in this case, int& initialized to zero) is returned to the caller
• The ++ operator is applied to the reference returned from the [], which changes 0 to 1 (or 0.0 to 1.0, etc.)

Answer 2

Yes. When you use the []-operator on a map and no element with the desired key exists, a new element is inserted which is value-initialized. For an integer, this means initialized to zero.