Records

So far, all the types we have encountered have values that are not decomposable: each instance represents a single piece of data. Now we are going to see our first class of composite types: records.

Records allow composing a value out of instances of other types. Each of those instances will be given a name. The pair consisting of a name and an instance of a specific type is called a field, or a component.

Record type declaration

Here is an example of a simple record declaration:

type Date is record
   --  The following declarations are components of the record
   Day   : Integer range 1 .. 31;
   Month : Month_Type;
   Year  : Integer range 1 .. 3000; --  You can add custom constraints on fields
end record;

Fields look a lot like variable declarations, except that they are inside of a record definition. And as with variable declarations, you can specify additional constraints when supplying the subtype of the field.

type Date is record
   Day   : Integer range 1 .. 31;
   Month : Month_Type := January;
   --  This component has a default value
   Year  : Integer range 1 .. 3000 := 2012;
   --                                 ^ Default value
end record;

Record components can have default values. When a variable having the record type is declared, a field with a default initialization will be automatically set to this value. The value can be any expression of the component type, and may be run-time computable.

Aggregates

Ada_Birthday    : Date := (10, December, 1815);
Leap_Day_2020   : Date := (Day => 29, Month => February, Year => 2020);
--                ^ By name

Records have a convenient notation for expressing values, illustrated above. This notation is called aggregate notation, and the literals are called aggregates. They can be used in a variety of contexts that we will see throughout the course, one of which is to initalize records.

An aggregate is a list of values separated by commas and enclosed in parentheses. It is allowed in any context where a value of the record is expected.

Values for the components can be specified positionally, as in Ada_Birthday example, or by name, as in Leap_Day_2020. A mixture of positional and named values is permitted, but you cannot use a positional notation after a named one.

Component selection

To access components of a record instance, you use an operation that is called component selection:

with Ada.Text_IO; use Ada.Text_IO; procedure Record_Selection is type Month_Type is (January, February, March, April, May, June, July, August, September, October, November, December); type Date is record Day : Integer range 1 .. 31; Month : Month_Type; Year : Integer range 1 .. 3000 := 2032; end record; Some_Day : Date := (1, January, 2000); begin Some_Day.Year := 2001; Put_Line ("Day:" & Integer'Image (Some_Day.Day) & ", Month: " & Month_Type'Image (Some_Day.Month) & ", Year:" & Integer'Image (Some_Day.Year)); end Record_Selection;