The class MachineInt is intended to help you write functions which
accept arguments whose type is a machine integer (see Why? below).
We recommend that you use MachineInt only to specify function
argument types; other uses may result in disappointing performance.
You cannot perform arithmetic directly with values of type MachineInt.
The primary operations are those for extracting a usable value from a
MachineInt object:
Arithmetic directly with MachineInt values is not possible. The
value(s) must be converted to long or unsigned long before
operating on them.
IsNegative(N) -- true iff N is negative,
if false the value can be extracted as an unsigned long,
if true the value can be extracted as a signed long
IsSignedLong(N) -- true iff N can be extracted as a signed long
AsUnsignedLong(N) -- extract N as an unsigned long -- see NOTE!
AsSignedLong(N) -- extract N as a signed long -- see NOTE!
IsInRange(lo,x,hi) -- true iff lo <= x <= hi
You should not call AsUnsignedLong if the value is negative, nor should
you call AsSignedLong if the value is large and positive --- currently, an
error is signalled only if debugging is active. Here's an outline of the
recommended usage:
void SomeProcedure(const MachineInt& N)
{
if (IsNegative(N))
{
long n = AsSignedLong(N);
...
}
else // N is non-negative
{
unsigned long n = AsUnsignedLong(N);
...
}
}
The class MachineInt was created in an attempt to circumvent C++'s
innate automatic conversions between the various integral types; most
particularly the silent conversion of negative signed values into unsigned ones.
Various C++ programming style guides recommend avoiding unsigned integer types. Unfortunately values of such types appear frequently as the result of various counting functions in the STL. So it is somewhat impractical to avoid unsigned values completely.
The class MachineInt employs automatic user-defined conversions to
force all integral values into the largest integral type, viz. long or
unsigned long. An extra "sign bit" inside a MachineInt indicates
whether the value is negative (i.e. must be regarded as a signed long).
Passing an argument as a MachineInt is surely not as fast as using a
built in integral type, but should avoid "nasty surprises" which can
arise with C+'s automatic conversions (e.g. a large unsigned long could
be viewed as a negative long).
On the whole everything is very simple; the hard part was establishing a reasonable design that interoperates with C++'s overload resolution rules.
An object of type MachineInt contains two data fields:
myValue -- the original integer value converted to unsigned long
IamNegative -- true iff the original value was (signed and) negative
The flag IamNegative allows the field myValue to be
interpreted correctly: if IamNegative is true then the correct
value of myValue may be obtained by casting it to a (signed)
long; conversely, if IamNegative is false then the value
of myValue is correct as it stands (i.e. as an unsigned long).
Most functions are so simple that an inline implementation is appropriate.
The implementation of the function abs will work correctly even if
the value being represented is the most negative signed long.
Note that the C++ standard allows the system to produce an error when
negating a long whose value is the most negative representable
value; in contrast, operations on unsigned long values will never
produce errors (except division by zero).
The impl of IsInRange is a bit involved; it must avoid overflow,
and may not assume anything about the internal representations of
signed and unsigned long values.
My biggest doubt is whether this is really the right way to tackle the
problem of silent automatic conversion between long and unsigned long.
Anyway, I'm using it (until a better solution comes along).
2011