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:

However, the two units can legally reference each other if one of the references is moved to the implementation section:

unit Unit2;

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.

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