using boolean_t = BooleanType;
The type used to store JSON booleans.
RFC 8259 implicitly describes a boolean as a type which differentiates the two literals #!json true and #!json false.
To store objects in C++, a type is defined by the template parameter BooleanType which chooses the type to use.
With the default values for BooleanType (#!cpp bool), the default value for boolean_t is #!cpp bool.
Boolean values are stored directly inside a basic_json type.