Quick start manual

3-8 Delphi Language Guide
Unit references and the uses clause
Circular unit references
When units reference each other directly or indirectly, the units are said to be
mutually dependent. Mutual dependencies are allowed as long as there are no
circular paths connecting the uses clause of one interface section to the uses clause of
another. In other words, starting from the interface section of a unit, it must never be
possible to return to that unit by following references through interface sections of
other units. For a pattern of mutual dependencies to be valid, each circular reference
path must lead through the uses clause of at least one implementation section.
In the simplest case of two mutually dependent units, this means that the units
cannot list each other in their interface uses clauses. So the following example leads
to a compilation error:
unit Unit1;
interface
uses Unit2;
ƒ
unit Unit2;
interface
uses Unit1;
ƒ
However, the two units can legally reference each other if one of the references is
moved to the implementation section:
unit Unit1;
interface
uses Unit2;
ƒ
unit Unit2;
interface
ƒ
implementation
uses Unit1;
ƒ
To reduce the chance of circular references, it’s a good idea to list units in the
implementation uses clause whenever possible. Only when identifiers from another
unit are used in the interface section is it necessary to list that unit in the interface
uses clause.