3. Programming language support for genericity

3.3. Advantages and limitations

The language syntax allows precise specification of constraints on generic formal parameters. For example, it is possible to specify that a generic formal type will only accept a modular type as the actual. It is also possible to express constraints between generic formal parameters; for example:

 generic
    type Index_Type is (<>); -- must be a discrete type
    type Element_Type is private; -- can be any nonlimited type
    type Array_Type is array (Index_Type range <>) of Element_Type;

In this example, Array_Type is constrained by both Index_Type and Element_Type. When instantiating the unit, the programmer must pass an actual array type that satisfies these constraints.

The disadvantage of this fine-grained control is a complicated syntax, but, because all generic formal parameters are completely defined in the specification, the compiler can instantiate generics without looking at the body of the generic.

Unlike C++, Ada does not allow specialised generic instances, and requires that all generics be instantiated explicitly. These rules have several consequences:

  • the compiler can implement shared generics: the object code for a generic unit can be shared between all instances (unless the programmer requests inlining of subprograms, of course). As further consequences:
    • there is no possibility of code bloat (code bloat is common in C++ and requires special care, as explained below).
    • it is possible to instantiate generics at run-time, as well as at compile time, since no new object code is required for a new instance.
    • actual objects corresponding to a generic formal object are always considered to be non-static inside the generic; see Generic formal objects in the Wikibook for details and consequences.
  • all instances of a generic being exactly the same, it is easier to review and understand programs written by others; there are no "special cases" to take into account.
  • all instantiations being explicit, there are no hidden instantiations that might make it difficult to understand the program.
  • Ada does not permit "template metaprogramming", because it does not allow specialisations.