Rationale for Ada 2012

John Barnes

9.3.1 Aggregates for private types

The <> notation was introduced in Ada 2005 for aggregates to mean the default value if any. A curiosity is that we can write

type Secret is private;

type Visible is
   record
      A: Integer;
      S: Secret;
   end record;

X: Visible := (A => 77; S => <>);

but we cannot write

S: Secret := <>; -- illegal

The argument is that this would be of little use since the components take their default values anyway.

For uniformity it was proposed that we might allow

S: Secret := (others => <>);

for private types and also for task and protected types. One advantage would be that we could then write

S: constant Secret := (others => <>);

whereas it is not possible to declare a constant of a private type because we are unable to give an initial value.

However, discussion of this issue led into a quagmire in Ada 2005 and so was abandoned. It remains abandoned in Ada 2012!