(ddd-themes.info)Building Boxes from Data

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1.2 Building Boxes from Data
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To visualize data structures, each atomic type and each type constructor
from the programming language is assigned a VSL display function.
Atomic values like numbers, characters, enumerations, or character
strings are displayed using string boxes holding their value; the VSL
function to display them leaves them unchanged:

     // Atomic Values
     simple_value(value) = value;

   Composite values require more attention.  An array, for instance, may
be displayed using a horizontal alignment:

     // Array
     array(...) = frame(halign(...));

   When GDB sends DDD the value of an array, the VSL function `array()'
is invoked with array elements as values.  A GDB array expression `{1,
2, 3}' is thus evaluated in VSL as

     array(simple_value("1"), simple_value("2"), simple_value("3"))

which equals

     "1" & "2" & "3"

a composite box holding a horizontal alignment of three string boxes.
The actual VSL function used in DDD also puts delimiters between the
elements and comes in a vertical variant as well.

   Nested structures like multi-dimensional arrays are displayed by
applying the `array()' function in a bottom-up fashion.  First,
`array()' is applied to the innermost structures; the resulting boxes
are then passed as arguments to another `array()' invocation.  The GDB
output

     {{"A", "B", "C"}, {"D", "E", "F"}}

representing a 2 * 3 array of character strings, is evaluated in VSL as

     array(array("A", "B", "C"), array("A", "B", "C"))

resulting in a horizontal alignment of two more alignments representing
the inner arrays.

   Record structures are built in a similar manner, using a display
function `struct\_member' rendering the record members.  Names and
values are separated by an equality sign:

     // Member of a record structure
     struct_member (name, value) =
       name & " = " & value;

   The display function `struct' renders the record itself, using the
`valign()' function.(1)

     // Record structure
     struct(...) = frame(valign(...));

   This is a simple example; the actual VSL function used in DDD takes
additional effort to align the equality signs; also, it ensures that
language-specific delimiters are used, that collapsed structs are
rendered properly, and so on.

   ---------- Footnotes ----------

   (1) `valign()' is similar to `halign()', but builds a vertical
alignment.