[Dwarf-discuss] Proposal: Add support for "property" with getter/setter (based on Pascal properties)

Martin lists@mfriebe.de
Tue May 7 12:10:48 GMT 2024


# Proposal to implement "properties" (based on Pascal)

## Background

Pascal has a property construct, that allows a "variable like" 
identifier, which can either point to a field (member variable) or a 
getter/setter function.
```
     TFoo = class
       FField: integer;
       function GetProp: integer;
       procedure SetProp(AVal: Integer);
       property MyProp: integer read GetProp write SetProp;
       property MyOtherProp: integer read FField;
     end;
```
* Properties can exist in a structure, interface, or on a global level.
* Properties can be read/write, read only, write only.
* Properties can have one or more array like indexes (of any type).
```
     function GetProp(AIdx:word; AIdx2: boolean): integer;
     procedure SetProp(AIdx:word; AIdx2: boolean; AVal: Integer);
     property MyProp[AIdx:word; AIdx2: boolean]: integer read GetProp 
write SetProp;
```
* Properties can share a method, and provide an index (constant) to 
identify which property called the method.
```
     function GetProp(AIndex: Integer): integer;
     procedure SetProp(AIndex: Integer; AValue: integer);
     property MyProp1: integer index 1 read GetProp write SetProp;
     property MyProp2: integer index 2 read GetProp write SetProp;
```
* Properties can have a "default" flag, indicating that the array [] 
access can omit the property name. I.e. accessing "Object[n]" is a 
shortcut for the default property. (default for non-array properties is 
being considered for future addition)
* Properties can have "default" and "stored" values for streaming 
(constant or via function).
* Properties can be elevated to a higher visibility (private/public) in 
inherited classes.

There may be partial overlaps with properties in Objective-C and C#

## Proposed Changes

### `DW_TAG_Property` or `DW_TAG_Property_Pascal`

This tag can occur anywhere where `DW_TAG_MEMBER` can occur. It can also 
occur on a `global scope`.

It supports (at least) the following existing attributes:
* DW_AT_Name
* DW_AT_Type
* DW_AT_Accessibility
* DW_AT_external
* DW_AT_virtuality
* DW_AT_start_scope
* DW_AT_decl_column, DW_AT_decl_file and DW_AT_decl_line.
* ... others from the var/member tags, except for actual value or 
location of value

It will support the following new attributes
* DW_AT_Default_Property flag
   Specify this is a default property
* DW_TAG_Property_Setter
* DW_TAG_Property_Reader
* DW_TAG_Property_Default
* DW_TAG_Property_Stored

### `DW_TAG_Property_[Setter|Getter|...]`

Specifies how the property is accessed for read/write and other access.

#### Reference to a field/var/function.

* DW_AT_Property_Forward  reference/constant
   A reference to an existing the field or function.
   * This could be a constant for the reader/default/stored accessors.
   * This could be a string to support the current 
`AT_APPLE_property_setter` extension

* DW_AT_Property_Object  reference/expression/constant
   The object on which the value is stored (value for 
`DW_OP_push_object_address`). This can be omitted for inherited classes, 
if it computes to the same address as the current class.

* DW_TAG_Member, DW_TAG_Variable, DW_TAG_subprogram.
   For inlining the accessor. E.g. subprogram for C#

The referenced element must have the same `DW_AT_Type` as the property. 
Except for "stored" which should be boolean.

#### function parameters

* A getter should
   * take one parameter, which is the _this value of the object. For 
global properties no parameters are given
* A setter should
   * take _this as the first parameter (except for global properties)
   * the `value` as the 2nd parameter.

For shared/indexed properties the order of parameters should default to
* _this
* indexes from array-like access
* the `value` (setter only)

For the **shared getter/setter index** the value of the argument must be 
specified. For this the list of arguments to a getter/setter can be 
specified.
* DW_TAG_Property_Argument_List
* containing a list of DW_TAG_Property_Argument
* each having the following attributes.
   * DW_AT_Property_Argument_Number
     zero-based number of the function parameter for which a value is given
   * DW_AT_Property_Argument_Value  reference or constant or expression 
(returning value)
     The value that should be passed

For the position of the `value` in the parameter list
* DW_AT_Property_Value_Argument_Number const/expression

The values for _this, array-index and value (if not specified) are 
passed to the function remaining parameters.


#### Other attributes for getter/setter/default/stored

The following attributes should be allowed in 
DW_TAG_Property_[Setter|Getter|...] to overwrite value given in the 
DW_TAG_Property
* DW_AT_Accessibility
* DW_AT_external
* DW_AT_virtuality
* DW_AT_start_scope

### Other usage forms of DW_TAG_Property

To change accessibility (private/public) of a property in an inherited 
class, DW_TAG_Property will be specified with just a name and accessibility.
```
     DW_TAG_PROPERTY
       DW_AT_NAME
       DW_AT_ACCESSIBILITY
```

In more generic terms, a property that has no getter or setter, and is 
not "abstract" is modifying an inherited property. Either accessibility, 
or addifng "default" or "stored". (This could alternatively be done by 
specifing a new attribute DW_AT_Property_Inherhit)

### Possible compatibility to existing AT_APPLE_property extension

If a property getter/setter (but not stored/default) only needs 
DW_AT_Property_Forward then instead of having each of them in a 
`DW_TAG_Property_[Setter|Getter]` there could be attributes in 
`DW_TAG_Property`: `DW_AT_Property_Forward_Setter` and 
`DW_AT_Property_Forward_Getter`

## References

FreePascal property doc 
https://www.freepascal.org/docs-html/ref/refse27.html
Oxygene https://docs.elementscompiler.com/Oxygene/Members/Properties/
APPLE extension for Objective-C 
https://github.com/llvm/llvm-project/blob/main/llvm/docs/SourceLevelDebugging.rst#debugging-information-format-1



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