llvm/polly/lib/External/isl/doc/interface

Design guidelines for the isl interfaces
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# Constructors

A function that constructs an isl object can be exposed in two ways, as
an unnamed constructor or as a named static function. Aiming for an interface
that is explicit and close to the isl C interface, all such functions
are exported as named static functions, except in the following cases.
Unnamed constructors are generated for functions where even without the
function name an object is identified uniquely by its arguments. For
example, there is a single isl_val that corresponds to a given integer
(isl_val_int_from_si), but both a NaN or a zero value can be constructed
from an isl_local_space (isl_aff_zero_on_domain and isl_aff_nan_on_domain).
Only function that create objects that are fully constructed by the function
and do not require further information to be added for typical use cases
are exposed as unnamed constructors.
Functions that commonly require more information to be provided
(isl_union_access_info_from_sink, isl_schedule_constraints_on_domain)
are exported as named static functions.
Typical examples of function that are generated as unnamed constructors
are the following:

- Conversion constructors
- Constructors from std::string
- Constructors where all arguments by themselves uniquely identify
  a complete object (e.g., isl_val_int_from_si)