C 中的 Ada 类型声明

Question

Is there a way to declare a new integer type in C?

In Ada,

type a is new integer;

means that a is not equivalent to say, an integer variable x.

I know that typedef can create an alias of an integer, but it is not the same as the above Ada code. It is more like a subtype in Ada.

Does anyone know?

Answer 1

The type systems in C and Ada are different.

C uses a structural typing system for primitive types and a nominative typing system for structs and unions. C enums are simply named integer values, not a separate type from int. Since all int instances in C have the same structural representation on a given hardware platform, all typedefs of int are the same type.

Ada uses a nominative typing system for all types. Types are distinguished by their name and not by their structural layout. Thus type Integer is different than

type My_Int is new Integer;

My_Int is a type with a different name than Integer, even though it is derived from Integer. Therefore My_Int is not an Integer in Ada even though the two types share the same operations and the same memory layout.

AC typedef is similar to an Ada subtype inasmuch as a subtype is a member of its parent type. Subtypes and typedefs differ because a subtype of a scalar type may be defined with a restricted range such as:

subtype Natural is Integer range 0..Integer'Last;
subtype Positive is Integer range 1..Integer'Last;

Every instance of Natural or Positive is an Integer value. The subtypes have a restricted range of valid values compared to their base type.

There is no simple way a typedef statement in C can declare a restricted range of values. Proposals to that effect in the past have been rejected by the C standard committee.