Compiler-generated data and data structures

Introduction

The compiler generates several data when compiling a program, this article describes its layout.

Classes layout

One of the most important data in an object pascal program are classes. A class is a pointer to the actucal data of a given instance. The first entry in the data of an object is a pointer to its virtual method table (VMT). This name is historic because this table contains nowadays more information than only the pointers to the virtual methods. It contains information about the

VMT layout

The layout of the VMT generated by FPC (Aug. 2006) is shown in the following table.

entry offset

explanation
vmtInstanceSize	0
vmtParent	sizeof(ptrint)*2;

      vmtClassName            = vmtParent+sizeof(pointer);

|-

      vmtDynamicTable         = vmtParent+sizeof(pointer)*2;

|-

      vmtMethodTable          = vmtParent+sizeof(pointer)*3;

|-

      vmtFieldTable           = vmtParent+sizeof(pointer)*4;

|-

      vmtTypeInfo             = vmtParent+sizeof(pointer)*5;

|-

      vmtInitTable            = vmtParent+sizeof(pointer)*6;

|-

      vmtAutoTable            = vmtParent+sizeof(pointer)*7;

|-

      vmtIntfTable            = vmtParent+sizeof(pointer)*8;

|-

      vmtMsgStrPtr            = vmtParent+sizeof(pointer)*9;

|-

      { methods }
      vmtMethodStart          = vmtParent+sizeof(pointer)*10;

|-

      vmtDestroy              = vmtMethodStart;

|-

      vmtNewInstance          = vmtMethodStart+sizeof(pointer);

|-

      vmtFreeInstance         = vmtMethodStart+sizeof(pointer)*2;

|-

      vmtSafeCallException    = vmtMethodStart+sizeof(pointer)*3;

|-

      vmtDefaultHandler       = vmtMethodStart+sizeof(pointer)*4;

|-

      vmtAfterConstruction    = vmtMethodStart+sizeof(pointer)*5;

|-

      vmtBeforeDestruction    = vmtMethodStart+sizeof(pointer)*6;

|-

      vmtDefaultHandlerStr    = vmtMethodStart+sizeof(pointer)*7;