Interface delegation

From Lazarus wiki
Jump to navigationJump to search

This page describes FPC implementation of interface delegation, and its diffrences with Delphi.

Structure of implemented interfaces

RTTI members

Whenever a class is declared as implementing one or more interfaces, its implemented interfaces are stored in table which is accessible at runtime with TObject.GetInterfaceTable function. This function returns a pointer to a TInterfaceTable record, which contains number of elements followed by one or more TInterfaceEntry records:

  TInterfaceTable = record
    EntryCount: ptruint;
    Entries   : array [0..0] of TInterfaceEntry;

Each implemented interface is described by TInterfaceEntry (aligment details omitted, see rtl/inc/ for full declaration):

  TInterfaceEntry = record
    IID     : pguid;        { this is nil for Corba interfaces, non-nil for COM }
    VTable  : Pointer;
    IOffset : ptruint;      { meaning depends on IType field, see below }
    IIDStr  : pshortstring; { never nil. For COM interfaces, this is upper(GuidToString(IID^)) }
    IType   : TInterfaceEntryType;
 TInterfaceEntryType = (
   etStandard,             { Implemented directly by the class }
                           {   IOffset is offset from start of object instance to hidden field storing interface VMT }
   etVirtualMethodResult,  { Delegated to property of type interface, which is accessed by virtual method }
                           {   IOffset is offset from class VMT to slot storing the accessor function }
   etStaticMethodResult,   { Delegated to property of type interface, which is accessed by static method }
                           {   IOffset is direct address of the accessor method }
   etFieldValue,           { Delegated to property of type interface, which is accessed by field }
                           {   IOffset is offset from start of object instance to accessor field }
   etVirtualMethodClass,   { Same as etVirtualMethodResult, but accessor property has class type }
   etStaticMethodClass,    { Same as etStaticMethodResult, but accessor property has class type }
   etFieldValueClass       { Same as etFieldValue, but accessor property has class type }

Delphi uses different TInterfaceEntry structure. Since Delphi does not support Corba interfaces, it does not have IIDStr counterpart. More important, interfaces delegated to class-type properties are stored by Delphi as 'directly implemented', and there's no easy way for runtime to determine whether they are accessed by field or by method.

Instance changes

For interfaces implemented directly by class, each instance of the class gets a hidden field containing a pointer to VMT of the interface. Thus, every directly implemented interface increases the class instance size by sizeof(Pointer) bytes. An offset to this field is stored by compiler in IOffset field of the corresponding TInterfaceEntry. At runtime, the TObject.InitInstance procedure initializes these hidden fields with proper VMT pointers.

FPC specifics: we use a special value of FPC_EMPTYINTF as a value for class interface table whenever compiler can determine that neither the class itself nor any of its ancestors implement any interfaces. FPC_EMPTYINTF is simply a pointer to interface table with zero entries. Its difference to a nil pointer is that TObject.InitInstance immediately exits when it encounters FPC_EMPTYINTF. This way, we yield higher constuction speed for the 'normal' (non-interfaced) objects.

Delphi specifics: a hidden field is also added for interfaces delegated to class-type properties.

GetInterface function and the 'as' operator

Using 'as' operator in text of the program typically ends up in a call to TObject.GetInterface method, unless the class overrides the standard behavior, e.g. by providing a customized QueryInterface method. For Corba interfaces, TObject.GetInterfaceByStr method is called instead, specifying the string identifier of the interface rather than its GUID.

Typecasting class to interface

Calls to interface methods