C ++ primer-string: size_type

The string type and many other library types define some ancillary types.
With these types, the use of database types can be unrelated to machines.
For example, string: size_type:
String. Size () returns a value of the size_type type. The return value cannot be assigned to an int variable.
We do not know the exact type of size_type, but we know it is an unsigned type. The value is much larger than the signed type.
To avoid overflow, the safest way to save the size of a string object is to use size_type.

On the other hand, when the following standard value is used to obtain characters in the string, such

String A = "test"

Char c = A [Index]

The size_type type is also recommended for index.