layxmp.txt

开放源码的编译器open watcom 1.6.0版的源代码

Examples of layouts for class objects
=====================================

- related documents:
    - LAYOUT.TXT -- discusses particular layouts

- history:
    - 91/11/28 -- J.W.Welch         -- initial version


- construction displacements are not shown

- Borland and Microsoft layouts match when virtual bases are not present,
  except, possibly, for the location of the VFPTR

- speed: Borland wins after construction; slightly slower during


Example(1) : single virtual class
---------------------------------

  class V1                                                  { int m1; };
  class B : public virtual V1                               { int mb; };
  class A : public B                                        { int ma; };

  Microsoft:            Borland:
  ---------             -------

  B:    VBPTR           B:  addr(V)
        mb                  mb
  A:    ma              A:  ma
  V1:   m1              V1: m1

- space: Borland wins always (no virtual table, pointer smaller or same size)


Example(2) : two virtual classes
--------------------------------

  class V1                                                  { int m1; };
  class V2                                                  { int m2; };
  class B : public virtual V1, public virtual V2            { int mb; };
  class A : public B                                        { int ma; };


  Microsoft:            Borland:

  B:    VBPTR           B:  addr(V1)
        mb                  addr(V2)
  A:    ma                  mb
  V1:   m1              A:  ma
  V2:   m2              V1: m1
                        V2: m2

- space: Borland wins in large-data model
         Microsoft wins in small-data model after 3 objects allocated


Example(3) : three virtual classes, two levels
----------------------------------------------

  class V1                                                  { int m1; };
  class V2                                                  { int m2; };
  class B : public virtual V1, public virtual V2            { int mb; };
  class A : public B                                        { int ma; };
  class C : public virtual A                                { int mc; };


  Microsoft:            Borland:

  C:    VBPTR-C         C:  addr(B)
        mc                  addr(V1)
                            addr(V2)
                            mc
  B:    VBPTR-B         B:  addr(V1)
        mb                  addr(V2)
  A:    ma                  mb
  V1:   m1              A:  ma
  V2:   m2              V1: m1
                        V2: m2

- space: Microsoft wins after 2 objects allocated