A virtual table is generally treated as an array of function pointers, although compilers are free to put data pointers (in MI and VI scenarios, or to typeinfos), integers (for fixups), or sentinel elements (such as NULL pointers) into it as well. The layout is generally compiler-specific (or ABI-specific where multiple C++ compilers share an ABI), but stable provided the classes being compiled have stable interfaces (otherwise you'd have to recompile your code all the time, and that's a drag). There are also additional tables that are needed to handle corner cases involving virtual and multiple inheritance, and to make sure that virtual calls during derived class construction behave as the Standard says they should under those circumstances (those are what the VTTs and construction tables in the output below are for).
As to the specific case of GCC 4.x: the -fdump-class-hierarchy switch indeed acts as described (and then some). I tested it on Coliru using the sample code below:
struct Base
{
virtual ~Base() {}
virtual void f() = 0;
};
struct OtherBase
{
virtual ~OtherBase() {}
virtual void g() {}
};
struct Derived: public Base
{
virtual ~Derived() {}
virtual void f() {}
};
struct MultiplyDerived: public Base, public OtherBase
{
virtual ~MultiplyDerived() {}
virtual void f() {}
virtual void g() {}
};
struct OtherDerived: public Base
{
virtual ~OtherDerived() {}
virtual void f() {}
};
struct DiamondDerived: public Derived, public OtherDerived
{
virtual ~DiamondDerived() {}
virtual void f() {}
};
struct VirtuallyDerived: virtual public Base
{
virtual ~VirtuallyDerived() {}
virtual void f() {}
};
struct OtherVirtuallyDerived: virtual public Base
{
virtual ~OtherVirtuallyDerived() {}
virtual void f() {}
};
struct VirtuallyDiamondDerived: public VirtuallyDerived, public OtherVirtuallyDerived
{
virtual ~VirtuallyDiamondDerived() {}
virtual void f() {}
};
struct DoublyVirtuallyDiamondDerived: virtual public VirtuallyDerived, virtual public OtherVirtuallyDerived
{
virtual ~DoublyVirtuallyDiamondDerived() {}
virtual void f() {}
};
struct MixedVirtuallyDerived: virtual public Base, public OtherBase
{
virtual ~MixedVirtuallyDerived() {}
};
struct MixedVirtuallyDiamondDerived: public VirtuallyDerived, public MixedVirtuallyDerived
{
virtual ~MixedVirtuallyDiamondDerived() {}
virtual void f() {}
virtual void g() {}
};
struct VirtuallyMultiplyDerived: virtual public Base, virtual public OtherBase
{
virtual ~VirtuallyMultiplyDerived() {}
};
struct OtherVirtuallyMultiplyDerived: virtual public Base, virtual public OtherBase
{
virtual ~OtherVirtuallyMultiplyDerived() {}
};
struct MultiplyVirtuallyDiamondDerived: public VirtuallyMultiplyDerived, public OtherVirtuallyMultiplyDerived
{
virtual ~MultiplyVirtuallyDiamondDerived() {}
virtual void f() {}
virtual void g() {}
};
and received from G++ (mangled name guide: TI's are typeinfos, TV's are vtables, and Th's and Tv's are thunks used to make correct virtual calls in the presence of multiple and/or virtual inheritance):
Vtable for Base
Base::_ZTV4Base: 5u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI4Base)
16 0u
24 0u
32 (int (*)(...))__cxa_pure_virtual
Class Base
size=8 align=8
base size=8 base align=8
Base (0x0x7fd42c0355a0) 0 nearly-empty
vptr=((& Base::_ZTV4Base) + 16u)
Vtable for OtherBase
OtherBase::_ZTV9OtherBase: 5u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI9OtherBase)
16 (int (*)(...))OtherBase::~OtherBase
24 (int (*)(...))OtherBase::~OtherBase
32 (int (*)(...))OtherBase::g
Class OtherBase
size=8 align=8
base size=8 base align=8
OtherBase (0x0x7fd42c035600) 0 nearly-empty
vptr=((& OtherBase::_ZTV9OtherBase) + 16u)
Vtable for Derived
Derived::_ZTV7Derived: 5u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI7Derived)
16 (int (*)(...))Derived::~Derived
24 (int (*)(...))Derived::~Derived
32 (int (*)(...))Derived::f
Class Derived
size=8 align=8
base size=8 base align=8
Derived (0x0x7fd42c02d138) 0 nearly-empty
vptr=((& Derived::_ZTV7Derived) + 16u)
Base (0x0x7fd42c035660) 0 nearly-empty
primary-for Derived (0x0x7fd42c02d138)
Vtable for MultiplyDerived
MultiplyDerived::_ZTV15MultiplyDerived: 11u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI15MultiplyDerived)
16 (int (*)(...))MultiplyDerived::~MultiplyDerived
24 (int (*)(...))MultiplyDerived::~MultiplyDerived
32 (int (*)(...))MultiplyDerived::f
40 (int (*)(...))MultiplyDerived::g
48 (int (*)(...))-8
56 (int (*)(...))(& _ZTI15MultiplyDerived)
64 (int (*)(...))MultiplyDerived::_ZThn8_N15MultiplyDerivedD1Ev
72 (int (*)(...))MultiplyDerived::_ZThn8_N15MultiplyDerivedD0Ev
80 (int (*)(...))MultiplyDerived::_ZThn8_N15MultiplyDerived1gEv
Class MultiplyDerived
size=16 align=8
base size=16 base align=8
MultiplyDerived (0x0x7fd42c04aaf0) 0
vptr=((& MultiplyDerived::_ZTV15MultiplyDerived) + 16u)
Base (0x0x7fd42c0356c0) 0 nearly-empty
primary-for MultiplyDerived (0x0x7fd42c04aaf0)
OtherBase (0x0x7fd42c035720) 8 nearly-empty
vptr=((& MultiplyDerived::_ZTV15MultiplyDerived) + 64u)
Vtable for OtherDerived
OtherDerived::_ZTV12OtherDerived: 5u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI12OtherDerived)
16 (int (*)(...))OtherDerived::~OtherDerived
24 (int (*)(...))OtherDerived::~OtherDerived
32 (int (*)(...))OtherDerived::f
Class OtherDerived
size=8 align=8
base size=8 base align=8
OtherDerived (0x0x7fd42c02d1a0) 0 nearly-empty
vptr=((& OtherDerived::_ZTV12OtherDerived) + 16u)
Base (0x0x7fd42c035780) 0 nearly-empty
primary-for OtherDerived (0x0x7fd42c02d1a0)
Vtable for DiamondDerived
DiamondDerived::_ZTV14DiamondDerived: 10u entries
0 (int (*)(...))0
8 (int (*)(...))(& _ZTI14DiamondDerived)
16 (int (*)(...))DiamondDerived::~DiamondDerived
24 (int (*)(...))DiamondDerived::~DiamondDerived
32 (int (*)(...))DiamondDerived::f
40 (int (*)(...))-8
48 (int (*)(...))(& _ZTI14DiamondDerived)
56 (int (*)(...))DiamondDerived::_ZThn8_N14DiamondDerivedD1Ev
64 (int (*)(...))DiamondDerived::_ZThn8_N14DiamondDerivedD0Ev
72 (int (*)(...))DiamondDerived::_ZThn8_N14DiamondDerived1fEv
Class DiamondDerived
size=16 align=8
base size=16 base align=8
DiamondDerived (0x0x7fd42c0625b0) 0
vptr=((& DiamondDerived::_ZTV14DiamondDerived) + 16u)
Derived (0x0x7fd42c02d208) 0 nearly-empty
primary-for DiamondDerived (0x0x7fd42c0625b0)
Base (0x0x7fd42c0357e0) 0 nearly-empty
primary-for Derived (0x0x7fd42c02d208)
OtherDerived (0x0x7fd42c02d270) 8 nearly-empty
vptr=((& DiamondDerived::_ZTV14DiamondDerived) + 56u)
Base (0x0x7fd42c035840) 8 nearly-empty
primary-for OtherDerived (0x0x7fd42c02d270)
Vtable for VirtuallyDerived
VirtuallyDerived::_ZTV16VirtuallyDerived: 8u entries
0 0u
8 0u
16 0u
24 (int (*)(...))0
32 (int (*)(...))(& _ZTI16VirtuallyDerived)
40 (int (*)(...))VirtuallyDerived::~VirtuallyDerived
48 (int (*)(...))VirtuallyDerived::~VirtuallyDerived
56 (int (*)(...))VirtuallyDerived::f
VTT for VirtuallyDerived
VirtuallyDerived::_ZTT16VirtuallyDerived: 2u entries
0 ((& VirtuallyDerived::_ZTV16VirtuallyDerived) + 40u)
8 ((& VirtuallyDerived::_ZTV16VirtuallyDerived) + 40u)
Class VirtuallyDerived
size=8 align=8
base size=8 base align=8
VirtuallyDerived (0x0x7fd42c02d2d8) 0 nearly-empty
vptridx=0u vptr=((& VirtuallyDerived::_ZTV16VirtuallyDerived) + 40u)
Base (0x0x7fd42c0358a0) 0 nearly-empty virtual
primary-for VirtuallyDerived (0x0x7fd42c02d2d8)
vptridx=8u vbaseoffset=-40
Vtable for OtherVirtuallyDerived
OtherVirtuallyDerived::_ZTV21OtherVirtuallyDerived: 8u entries
0 0u
8 0u
16 0u
24 (int (*)(...))0
32 (int (*)(...))(& _ZTI21OtherVirtuallyDerived)
40 (int (*)(...))OtherVirtuallyDerived::~OtherVirtuallyDerived
48 (int (*)(...))OtherVirtuallyDerived::~OtherVirtuallyDerived
56 (int (*)(...))OtherVirtuallyDerived::f
VTT for OtherVirtuallyDerived
OtherVirtuallyDerived::_ZTT21OtherVirtuallyDerived: 2u entries
0 ((& OtherVirtuallyDerived::_ZTV21OtherVirtuallyDerived) + 40u)
8 ((& OtherVirtuallyDerived::_ZTV21OtherVirtuallyDerived) + 40u)
Class OtherVirtuallyDerived
size=8 align=8
base size=8 base align=8
OtherVirtuallyDerived (0x0x7fd42c02d340) 0 nearly-empty
vptridx=0u vptr=((& OtherVirtuallyDerived::_ZTV21OtherVirtuallyDerived) + 40u)
Base (0x0x7fd42c035900) 0 nearly-empty virtual
primary-for OtherVirtuallyDerived (0x0x7fd42c02d340)
vptridx=8u vbaseoffset=-40
Vtable for VirtuallyDiamondDerived
VirtuallyDiamondDerived::_ZTV23VirtuallyDiamondDerived: 16u entries
0 0u
8 0u
16 0u
24 (int (*)(...))0
32 (int (*)(...))(& _ZTI23VirtuallyDiamondDerived)
40 (int (*)(...))VirtuallyDiamondDerived::~VirtuallyDiamondDerived
48 (int (*)(...))VirtuallyDiamondDerived::~VirtuallyDiamondDerived
56 (int (*)(...))VirtuallyDiamondDerived::f
64 18446744073709551608u
72 18446744073709551608u
80 18446744073709551608u
88 (int (*)(...))-8
96 (int (*)(...))(& _ZTI23VirtuallyDiamondDerived)
104 (int (*)(...))VirtuallyDiamondDerived::_ZThn8_N23VirtuallyDiamondDerivedD1Ev
112 (int (*)(...))VirtuallyDiamondDerived::_ZThn8_N23VirtuallyDiamondDerivedD0Ev
120 (int (*)(...))VirtuallyDiamondDerived::_ZThn8_N23VirtuallyDiamondDerived1fEv
Construction vtable for VirtuallyDerived (0x0x7fd42c02d3a8 instance) in VirtuallyDiamondDerived
VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived0_16VirtuallyDerived: 8u entries
0 0u
8 0u
16 0u
24 (int (*)(...))0
32 (int (*)(...))(& _ZTI16VirtuallyDerived)
40 0u
48 0u
56 (int (*)(...))VirtuallyDerived::f
Construction vtable for OtherVirtuallyDerived (0x0x7fd42c02d410 instance) in VirtuallyDiamondDerived
VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived8_21OtherVirtuallyDerived: 15u entries
0 18446744073709551608u
8 0u
16 0u
24 (int (*)(...))0
32 (int (*)(...))(& _ZTI21OtherVirtuallyDerived)
40 0u
48 0u
56 (int (*)(...))OtherVirtuallyDerived::f
64 8u
72 8u
80 (int (*)(...))8
88 (int (*)(...))(& _ZTI21OtherVirtuallyDerived)
96 0u
104 0u
112 (int (*)(...))OtherVirtuallyDerived::_ZTv0_n32_N21OtherVirtuallyDerived1fEv
VTT for VirtuallyDiamondDerived
VirtuallyDiamondDerived::_ZTT23VirtuallyDiamondDerived: 7u entries
0 ((& VirtuallyDiamondDerived::_ZTV23VirtuallyDiamondDerived) + 40u)
8 ((& VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived0_16VirtuallyDerived) + 40u)
16 ((& VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived0_16VirtuallyDerived) + 40u)
24 ((& VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived8_21OtherVirtuallyDerived) + 40u)
32 ((& VirtuallyDiamondDerived::_ZTC23VirtuallyDiamondDerived8_21OtherVirtuallyDerived) + 96u)
40 ((& VirtuallyDiamondDerived::_ZTV23VirtuallyDiamondDerived) + 40u)
48 ((& VirtuallyDiamondDerived::_ZTV23VirtuallyDiamondDerived) + 104u)
Class VirtuallyDiamon 与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
