Ambiguous Conversions

Explicit and implicit conversions from pointers or references to class types can cause ambiguities. Figure 9.11 shows the following:

• The declaration of an object of type D.
  
  
• The effect of applying the address-of operator (&) to that object. Note that the address-of operator always supplies the base address of the object.
  
  
• The effect of explicitly converting the pointer obtained using the address-of operator to the base-class type A. Note that coercing the address of the object to type A* does not always provide the compiler with enough information as to which subobject of type A to select; in this case, two subobjects exist.

The conversion to type A* (pointer to A) is ambiguous because there is no way to discern which subobject of type A is the correct one. Note that you can avoid the ambiguity by explicitly specifying which subobject you mean to use, as follows:

(A *)(B *)&d       // Use B subobject.
(A *)(C *)&d       // Use C subobject.

Figure 9.11   Ambiguous Conversion of Pointers to Base Classes