How to Disambiguate Class Members in Multiple Inheritance with Non-Overlapping Sets of Types?

Disambiguating Class Members in Multiple Inheritance with Non-Overlapping Sets of Types

The challenge arises when inheriting from multiple bases with non-overlapping sets of types. When calling a member function, the compiler expects to identify the specific base class to use, but in such cases, it encounters ambiguity.

While the Contains template determines whether a type exists within a variadic pack, the merge rules for class member lookup in cases where the derived class's declaration set is empty can result in ambiguity. As neither base class has members in the derived class's declaration set, the merge process merges the lookup sets from each base, leading to an ambiguous call.

Possible Solutions

• Explicit Using Declarations: By adding explicit using declarations for each base class's member function, the derived class effectively defines those members, making its declaration set non-empty and bypassing the ambiguous merge rules.

  <code class="cpp">struct Derived : public Base<int, char>, public Base<double, void>
  {
    using Base<int, char>::foo;
    using Base<double, void>::foo;
  };</code>

• Base Collector: Using a template metaprogramming technique, implement a BaseCollector class that collects the members of all base classes and declares them using within the derived class. This approach provides a generic solution without requiring explicit using declarations.

  <code class="cpp">struct Derived : BaseCollector<Base2<int>, Base2<std::string>>
  { };</code>

• Variadic Pack Expansion Using Declaration (C 17): The use of variadic pack expansion can simplify the BaseCollector implementation, making it both shorter and more efficient.

  <code class="cpp">template <typename... Bases>
  struct BaseCollector : Bases...
  {
    using Bases::foo...;
  };</code>

  By employing these techniques, we can disambiguate class member calls in multiple inheritance scenarios with non-overlapping sets of types, allowing for clear and unambiguous code execution.

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