## How to Disambiguate Class Member Calls in Multiple Inheritance with Overlapping Type Sets?

Disambiguation of Class Member in Multiple Inheritance with Overlapping Sets

In C , multiple inheritance with overlapping sets of types can lead to ambiguity when calling member functions with template parameters. To understand why, let's examine a base class template:

<code class="cpp">template <typename ... Types>
class Base {
public:
    template <typename T>
    typename std::enable_if<Contains<T, Types ...>::value>::type
    foo() {
        std::cout << "Base::foo()\n";
    }
};

The foo() member can only be called when the specified template parameter is present in the template parameter list of Base. Now, suppose we define a derived class Derived that inherits from multiple instances of Base with non-overlapping sets of types:

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

When calling Derived().foo(), the compiler might not be able to determine which base class to use because the template parameter int is present in both base classes. This ambiguity would lead to a compiler error.

Possible Solutions

1. Explicit Base Specification: The ambiguity can be resolved by explicitly specifying the base class to use, e.g., Derived().Base::foo(). However, this solution requires the caller to know the specific base class, which can be undesirable.
2. Using Declarations: In C , using declarations can bring members of base classes into the namespace of the derived class. By adding using Base::foo; and using Base::foo; to the derived class declaration, the ambiguity can be disambiguated. However, this solution requires the user of the derived class to include these using declarations, which can be cumbersome and repetitive for large type lists.
3. Base Collector Class Template: A more elegant solution is to employ a collector class template that combines the declarations of all base classes and exposes the members through using declarations. For example:

<code class="cpp">template <typename... Bases>
struct BaseCollector;

template <typename Base>
struct BaseCollector<Base> : Base
{
    using Base::foo;
};

template <typename Base, typename... Bases>
struct BaseCollector<Base, Bases...>:
    Base,
    BaseCollector<Bases...>
{
    using Base::foo;
    using BaseCollector<Bases...>::foo;
};

struct Derived: public BaseCollector<Base<int, char>, Base<double, void>>
{};</code>

This approach allows the derived class to access the correct base class implementation without any need for explicit using declarations or base class specification.

Understanding the ambiguity and implementing effective solutions in multiple inheritance with overlapping sets helps ensure clear and correct code execution.

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