How Does the `is_base_of` Trait Determine Base Class Relationships, Even with Private Inheritance?

Is_Base_Of Trait Implementation

The is_base_of trait provides a means to determine if one class is derived from another, even in the presence of private or protected inheritance. Its implementation relies on a clever use of function templates and user-defined conversion sequences.

How it Works

The is_base_of template takes two template parameters, B and D, representing the potential base and derived classes, respectively. It defines two static member functions:

• check(D*, T): Accepts a pointer to the derived class and a template parameter T.
• check(B*, int): Accepts a pointer to the base class and an integer.

The value static member of is_base_of is then initialized using a sizeof comparison, which essentially checks if the size of the result of invoking check with a Host object is equal to the size of a "yes" result (i.e., an array size of 1).

Private Base Inheritance

The implementation is able to handle private base inheritance because the Host class provides a no-op operator B*() const that returns nullptr. This allows the compiler to substitute a D* for B* in the check(B*, int) function, even though B is a private base of D.

Importance of Const Operator

The operator B*() being declared const is crucial because it ensures that the compiler selects a candidate conversion function that takes the implicit object argument this as a non-const reference. This subtle difference allows the is_base_of trait to differentiate between public and private inheritance by checking if the conversion to the base type is allowed.

Template Check Function

The decision to use a template function check(D*, T) instead of a non-template function check(B*, int) relies on overload resolution rules. When searching for conversion functions, if two functions are equally viable, the return type of the function decides which is better. By using a template with a return type of yes, it allows the compiler to select the correct version of check based on the existence of a user-defined conversion from D* to B*.

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