Why Should One Refrain from Deriving from std::string
In his book Effective C , Scott Meyers advises against deriving from the std::string class, primarily due to its lack of a virtual destructor. However, this is not the only reason to avoid such inheritance.
Defining Criteria for Inheritance
To serve as an appropriate base class, a class must:
- Enable Polymorphism: Inheritance is intended for polymorphic situations where derived classes exhibit specialized behaviors.
- Avoid Slicing Problems: Value types in C , unlike reference types in Java or C#, compound the slicing problem by potentially copying only a portion of the derived class, leading to data inconsistencies.
std::string as a Base Class
std::string fails to meet these criteria for the following reasons:
- Lack of Virtual Destructor: When a class does not have a virtual destructor, derived classes inherit a non-virtual destructor, which may lead to unexpected behavior while deleting objects through base class pointers.
- Absence of Reusability: std::string was not designed as a base class, primarily focusing on its functionality as a string container. Use of non-friend, non-member functions or composition are more suitable alternatives for adding additional functionality.
Additionally, even in scenarios where inheritance is solely for reusability, C lacks mechanisms to prevent clients from creating objects of a derived class using a base class pointer. This can violate the intended purpose of restricting such usage.
Consequences of Inheritance Violation
Ignoring these guidelines can result in code that is:
- Harder to Understand: Violating the inherent purpose of inheritance leads to confusion and maintenance difficulties.
- Error-Prone: Slicing problems and improper type casts may occur, resulting in runtime errors.
- Less Efficient: Slicing copies introduce unnecessary object copies, affecting performance.
In conclusion, inheritance in C should be reserved for polymorphic situations, and std::string should not be used as a base class due to its design constraints and the slicing problem. Employing alternative methods like non-member functions or composition promotes idiomatic C practices and ensures code clarity and reliability.
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