Declaration of a class in ST

Syntax

Syntax
`CLASS (* optional_begin ) FINAL\|ABSTRACT ( optional_end ) class_name_2` `( optional_begin ) USING Namespace_1;` `USING Namespace_2; ( optional_end )` `( optional_begin ) EXTENDS class_name_1 ( optional_end )` `( optional_begin ) IMPLEMENTS interface_1, interface_2, ... interface_n ( optional_end )` `( optional: declaration of variables/instances )` `( optional: declaration of methods *)` `END_CLASS`

CLASS (* optional_begin *) FINAL|ABSTRACT (* optional_end *) class_name_2

(* optional_begin *) USING Namespace_1;

USING Namespace_2; (* optional_end *)

(* optional_begin *) EXTENDS class_name_1 (* optional_end *)

(* optional_begin *) IMPLEMENTS interface_1, interface_2, ... interface_n (* optional_end *)

(* optional: declaration of variables/instances *)

(* optional: declaration of methods *)

END_CLASS

Meaning

Meaning
declaration of a →class a feature of the →object-oriented programming), `class_name_2` must be an →IEC-identifier. The declaration is possible within an ST-object – the declaration is done either within the global →namespace or within a declared namespace. `CLASS` and `END_CLASS` are →keywords for the declaration of the class. Observe that a class is similar to a →function block that is using the features of the object-oriented programming. According to the →IEC-standard, there are the following differences: The keywords `FUNCTION_BLOCK` and `END_FUNCTION_BLOCK` are replaced by `CLASS` and `END_CLASS` respectively. Variables are only declared in the `VAR` section . The sections `VAR_INPUT`, `VAR_OUTPUT`, `VAR_IN_OUT`, and `VAR_TEMP` are not allowed. According to the IEC-table 48 "Class" (line 4a and 4b), variables are declared in the `VAR_EXTERNAL` section as well. A class has no body. A class may define only methods. A call of an instance of a class is not possible. Only the methods of a class can be called. For details on the optional keywords `FINAL`, `ABSTRACT`, `EXTENDS`, `IMPLEMENTS`, see "Declaration of a function block in ST" for their explanation. Just observe that: `EXTENDS` derives the class from a different class (this will be the base class). `IMPLEMENTS` implements one or more →interfaces in the class. The `USING` namespace directive after the name of the class is optional as well. See "Namespaces in ST: usage" for details on this directive.

declaration of a →class a feature of the →object-oriented programming), class_name_2 must be an →IEC-identifier.
The declaration is possible within an ST-object – the declaration is done either within the global →namespace or within a declared namespace. CLASS and END_CLASS are →keywords for the declaration of the class.

Observe that a class is similar to a →function block that is using the features of the object-oriented programming. According to the →IEC-standard, there are the following differences:

The keywords FUNCTION_BLOCK and END_FUNCTION_BLOCK are replaced by CLASS and END_CLASS respectively.
Variables are only declared in the VAR section . The sections VAR_INPUT, VAR_OUTPUT, VAR_IN_OUT, and VAR_TEMP are not allowed.

According to the IEC-table 48 "Class" (line 4a and 4b), variables are declared in the VAR_EXTERNAL section as well.
A class has no body. A class may define only methods.
A call of an instance of a class is not possible. Only the methods of a class can be called.

For details on the optional keywords FINAL, ABSTRACT, EXTENDS, IMPLEMENTS, see "Declaration of a function block in ST" for their explanation. Just observe that:

EXTENDS derives the class from a different class (this will be the base class).
IMPLEMENTS implements one or more →interfaces in the class.

The USING namespace directive after the name of the class is optional as well. See "Namespaces in ST: usage" for details on this directive.

Example 1: Class with methods
Class CCounter VAR m_iCurrentValue : DINT; (* Default = 0 *) m_bCountUp : BOOL:=TRUE; END_VAR VAR PUBLIC m_iUpperLimit : DINT:=+40; m_iLowerLimit : DINT:=-40; END_VAR METHOD PUBLIC Count : DINT IF (m_bCountUp AND m_iCurrentValue<m_iUpperLimit) THEN m_iCurrentValue:= m_iCurrentValue+1; END_IF; IF (NOT m_bCountUp AND m_iCurrentValue>m_iLowerLimit) THEN m_iCurrentValue:= m_iCurrentValue-1; END_IF; Count := m_iCurrentValue; END_METHOD METHOD public SetDirection VAR_INPUT bCountUp : BOOL; END_VAR m_bCountUp:=bCountUp; END_METHOD END_CLASS

Example 1: Class with methods

Class CCounter

VAR

  m_iCurrentValue : DINT; (* Default = 0 *)

  m_bCountUp : BOOL:=TRUE;

  END_VAR

  VAR PUBLIC

    m_iUpperLimit : DINT:=+40;

    m_iLowerLimit : DINT:=-40;

  END_VAR

  METHOD PUBLIC Count : DINT

    IF (m_bCountUp AND m_iCurrentValue<m_iUpperLimit) THEN

      m_iCurrentValue:= m_iCurrentValue+1;

    END_IF;

    IF (NOT m_bCountUp AND m_iCurrentValue>m_iLowerLimit) THEN

      m_iCurrentValue:= m_iCurrentValue-1;

    END_IF;

  Count := m_iCurrentValue;

  END_METHOD

  METHOD public SetDirection

    VAR_INPUT

      bCountUp : BOOL;

    END_VAR

    m_bCountUp:=bCountUp;

  END_METHOD

END_CLASS

You are able to use the following language elements for ST within a class:

Concept Information

Blanks, tabs or new lines in ST

Comments in ST

Declaration of a method

Declaration of external variables in ST

Declaration of function block instances or class instances in ST

Declaration of internal variables in ST

Declaration of variables based on an interface

Read-only statement

Statement to ignore ST-objects

Statement to suppress warnings

The usage of these language elements makes it possible to use other elements (e.g. declaration of STRING variables within the section VAR ... END_VAR ) as well. Such elements are not listed here.