1.6.9.3 Object handles
Many of the “opaque object handles” , such as atom_t,
term_t, and functor_t are integers.24Typically uintptr_t
values, which the C standard defines as “an unsigned integer type
with the property that any valid pointer to void can be converted to
this type, then converted back to pointer to void, and the result will
compare equal to the original pointer.’ As such,
there is no compile-time detection of passing the wrong handle to a
function.
This leads to a problem with classes such as PlTerm -
C++ overloading cannot be used to distinguish, for example, creating a
term from an atom versus creating a term from an integer. There are a
number of possible solutions, including:
- A subclass for each kind of initializer;
- A tag for each kind of intializer;
- Change the C code to use a
structinstead of an integer.
It is impractical to change the C code, both because of the amount of edits that would be required and also because of the possibility that the changes would inhibit some optimizations.
There isn't much difference between subclasses versus tags; but as a matter of design, it's better to specify things as constants than as (theoretically) variables, so the decision was to use subclasses.