P4045

Code

P4045

Message

Function block is ABSTRACT and cannot be instantiated

This error occurs when a variable is declared with the type of a function block marked ABSTRACT (the CODESYS/TwinCAT dialect extension). An ABSTRACT function block exists only to be extended via EXTENDS – it cannot be instantiated directly.

Example

The following code will generate error P4045:

FUNCTION_BLOCK ABSTRACT FB_Base
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_User
VAR
    inst : FB_Base;
END_VAR
END_FUNCTION_BLOCK

To fix this error, declare the variable with a concrete (non-abstract) function block type that extends the abstract base instead:

FUNCTION_BLOCK ABSTRACT FB_Base
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_Concrete EXTENDS FB_Base
END_FUNCTION_BLOCK

FUNCTION_BLOCK FB_User
VAR
    inst : FB_Concrete;
END_VAR
END_FUNCTION_BLOCK

Extending an abstract function block via EXTENDS is unaffected by this error – only directly declaring a variable of the abstract type itself is rejected.

Think IronPLC is wrong about this?

If you believe this diagnostic is incorrect, open an issue on GitHub with a small sample that demonstrates the problem.