Why Should I Avoid Inheriting from `std::string` in C ?

Dangers of Inheriting from std::string

While using inheritance can provide code reusability, it's important to understand the specific requirements for classes to act as base classes. One crucial consideration highlighted by Effective C is that a class should have a virtual destructor if it intends to behave polymorphically.

However, std::string does not possess a virtual destructor, which raises concerns about inheriting from it. The question arises whether the absence of a virtual destructor is the sole reason for avoiding inheritance from std::string.

In C , inheritance is primarily used for two purposes:

• Private inheritance: For mixins and aspect-oriented programming using templates.
• Public inheritance: Specifically for polymorphic scenarios.

Non-polymorphic inheritances have no practical value in C and can be avoided. Free functions provide a more suitable alternative, allowing you to extend classes without forcing clients to switch to your custom string class.

The Slicing Problem

In C , classes are value types, unlike reference types in other OO languages. This leads to the slicing problem, where passed-by-value objects are truncated to the base class size, leading to inconsistencies. This is demonstrated in the code snippet below:

int StringToNumber(std::string copyMeByValue)
{
    // ... Your code to convert 'copyMeByValue' to a number.
}

If you pass a derived object of std::string to this function, the slicing problem will occur, resulting in unexpected behavior. Since non-derived functions are tightly coupled with base class members, these inconsistencies can arise.

Conclusion

Avoid inheriting from std::string unless you explicitly need polymorphic behavior. For code reusability purposes, prefer non-member functions and members in classes that are not intended for inheritance. By adhering to this practice, you can ensure code maintainability and prevent potential issues associated with inheritance mismatches.

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