struct VirtualInheritanceTestSetup

Setup the required information for a test hierarchy including virtual inheritance. Virtual

inheritance in C++ isn't just inheritance with virtual functions, but rather:

class Derived : public virtual Bar { ... }

The "virtual" in this case means that the base class is accessed indirectly, allowing diamond

inheritance to be resolved. This indirect operation is expressed as an expression in the

DWARF inheritance information rather than the constant offset used by normal inheritance.

This class sets up an inheritance hierarchy with three steps of inheritance between four classes:

class Derived : public IntermediateDerived { // Non-virtual.

int derived_i = 1;

};

class IntermediateDerived : public virtual IntermediateBase { // Virtual

int intermediate_derived_i = 2;

};

class IntermediateBase : public Base { // Non-virtual

int intermediate_base_i = 3;

};

class Base {

int base_i = 4;

};

The binary structure looks like this:

Value

+----------------------------------+------------+

Derived -> | derived_i (4 bytes) | 1 |

+----------------------------------+------------+

IntermediateDerived -> |

<vtable

_ptr> (8 bytes) | kVtablePtr |

| intermediate_derived_i (4 bytes) | 2 |

+----------------------------------+------------+

IntermediateBase -> | intermediate_base_i (4 bytes) | 3 |

+----------------------------------+------------+

Base -> | base_i (4 bytes) | 4 |

+----------------------------------+------------+

Note that this is actually backwards than most compilers will generated (they will normally

put "Base" at the beginning of "IntermediateBase") but doing it this way allows us to have an

offset for each step of inheritance which is better for testing.

+---------------------------------------------------------------------+

kVirtualDataAddress -> |

<offset

of "IntermediateBase" from "IntermediateDerived"> (8 bytes) |

|

<some

other value> (8 bytes) |

|

<some

other value> (8 bytes) |

+---------------------------------------------------------------------+

vtable_ptr -> |

<vtable

entries> |

| ... |

The vtable_ptr referenced in the structure is 24 bytes after the offset needed (this is taken

from what GCC generated for a test). This offset is retrieved and added to the

IntermediateDerived pointer to get the address of IntermediateBase (so the offset should be 4).

This uses GCC's style of expressions. See also ResolveCollectionTest.VirtualInheritance which

tests Clang's version of virtual inheritance.