Numbers may contain offset-based sub-fields. E.g., a value like 12345 might consist of fields 12 and 345.
Such fields can be considered offset-based, using end- and begin offsets in the number representations.
In this example 345 begins at digit position 0 (indicating the least significant digit of 12345) and at
digit position 3 the next field starts. Likewise, the field 12 begins at digit position 3 and has ended
at digit position 5.
The Field class template provides facilities for retrieving and assigning position based values of
existing numeric values of (currently) at most 64 bits.
To represent such fields the following format is used:
Field<base, end, begin>::function(argument(s))
where base specifies the number system’s base value, end specifies the (0-based) index position where the
number field ends, and begin specifies the index position where the number field begins. Here are two
examples, using the decimal number system:
Field<10, 3, 0>::get(12345) // returns 345
Field<10, 5, 3>::get(12345) // returns 12
The decision to specify the end offset before (i.e., left of) the begin offset is based on the
consideration that this corresponds to the standard way of looking at digit positions in numbers, where
the end offset is found to the left of the begin offset.
Values of fields can be retrieved, but they can also be set: to set a field’s value the following format
is used:
Field<10, 3, 1>::set(12345, 99) // returns 12995
Field<10, 1, 0>::set(12345, 0) // returns 12450
When values are assigned to fields the maximum width of the destination field is taken into account. When
specifying 9999 instead of 99 in the above example the returned value will still be 12995, as the
destination field has a width of two digit positions. Likewise, specifying a smaller value sets the
remaining (more significant) digits to 0:
Field<10, 3, 1>::set(12345, 9) // returns 12095
The class templates themselves are unaware of bases of number systems. Since 0xdeaf equals the decimal
value 57007 and 0xd equals 13, calling the above function as
Field<16, 1, 0>::set(76007, 13)
returns the hexadecimal value 0xdead’.
The Field class template requires three non-type numeric arguments:
o base, specifying the base of the number system;
o end, specifying the 0-based offset of the digit position where the field has ended;
o begin, specifying the 0-based offset of the digit position where the field begins;
The class template is specialized for situations where base is a mere power of 2 (like 2, 4, 8, 16, ...)
because in those cases bit-operations can be used which are faster than multiplications, divisions and
modulo computation which are required when other number system bases are used.
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