Why Should You Avoid Inheriting from `std::string`?

Why It's Ill-Advised to Inherit from std::string

In Effective C , the emphasis is placed on the fact that a virtual destructor is crucial for a class's ability to exhibit polymorphic behavior. The absence of a virtual destructor in std::string, therefore, effectively prohibits deriving from it. This leads us to the question: What additional criteria must a class fulfill to qualify as a suitable base class?

Eligibility for Base Class Status

Contrary to the assumption implied in the question, a class does not need to support polymorphism to serve as a base class. Inheritance in C serves two primary purposes:

1. Private Inheritance: This technique is leveraged for innovative practices like mixins and template-based aspect-oriented programming.
2. Public Inheritance: This form of inheritance is strictly reserved for establishing polymorphic scenarios.

std::string as a Base Class

The reason goes beyond the lack of a virtual destructor. std::string was never intended to function as a base class, polymorphic or otherwise. This can be attributed to the properties of classes in C .

Unlike other object-oriented languages where classes are reference types, C treats classes as value types. As a result, when a base class object is copied, only its members are copied, not those of any derived classes. This phenomenon, known as the slicing problem, can lead to inconsistencies and erroneous behavior.

Thus, rather than implementing inheritance for method extension, it is considered best practice in C to utilize non-friend, non-member functions or composition. Inheritance should be reserved for template metaprogramming or establishing polymorphic behavior.

Preventing Non-Polymorphic Inheritance

In instances where a base class is intended solely for code reusability, there is no effective way to prevent the creation of pointers to derived classes. However, the use of private inheritance, restricting the access of derived class methods, and avoiding public base class methods can discourage their usage.

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