Feature Request: Read-Only References / Pointer-to-Const Support

dclements — Tue, 01 Sep 2026 14:15:28 -0000

I would like to suggest adding support for persistent read-only references or pointers in CODESYS, similar to a C++ pointer-to-const (const T*).

For example, something conceptually similar to:

POINTER TO CONSTANT ST_MyData
or
REFERENCE TO CONSTANT ST_MyData

The important distinction is that the reference itself may still be passed around or reassigned, but the referenced object cannot be modified through that reference.

CODESYS currently supports VAR_IN_OUT CONSTANT, which provides read-only reference semantics for parameters, but there does not appear to be an equivalent type that can be stored in a variable or returned from a method.

A practical use case is a registry that owns long-lived definitions.

The registry needs to retain ownership of the data while downstream systems maintain persistent references to it.

Currently, the main options are:

Return the struct by value, which loses reference semantics and requires a copy.
Return POINTER TO ST_Definition, which allows consumers to modify registry-owned data.
Convert the structure into a Function Block and expose a getter-only interface, which provides the desired encapsulation but introduces more complexity and memory overhead.

A read-only reference/pointer type would allow libraries to expose persistent, zero-copy access to internally owned data while preserving encapsulation.

This would be particularly useful for library APIs and object-oriented CODESYS architectures where ownership of data should remain with one component while other components are allowed to observe it.

Similar requests for constant references/pointers have been discussed on CODESYS Forge previously, so this appears to be a recurring use case rather than something specific to one application.

Recent posts by dclements

Feature Request: Read-Only References / Pointer-to-Const Support