Coupling between objects (CBO)

Title short form	CBO
Categories	Maintainability, reusability
Definition	Number of additional function blocks that are instantiated and used in a function block
Further information	CBO = Coupling Between Objects A function block with a high level of coupling between objects is likely to be involved in many different tasks and therefore violates the principle of clear responsibility.
Standard upper limit for the associated rule SA0179: Coupling between objects	30

Sample:

FUNCTION_BLOCK FB_Base
VAR
    fb3  : FB3;  // +1 instantiated here
END_VAR

FUNCTION_BLOCK FB_Sub EXTENDS FB_Base   // +0 for EXTENDS
VAR
    fb1  : FB1;  // +1: instantiated here
    fb2  : FB2;  // +1: instantiated here
END_VAR

fb3();           // +0: instantiated in FB_Base, no increment for call

The extension of a function block does not increase the coupling between objects.
fb3 is instantiated in the implementation of FB_Base and inherited by FB_Sub. The call in FB_Sub does not increase the coupling between the objects.
The metric Coupling Between Objects for FB_Sub is therefore : 2