Fixing undefined behavior when Accessing Object Names in a std::vector of Interfaces in C++

Learn how to correctly access string views stored in a vector of interface pointers in C++ to avoid `undefined behavior` and improve memory safety.
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Fixing undefined behavior when Accessing Object Names in a std::vector of Interfaces in C++

In C++, storing different objects that inherit from the same interface inside a single vector is a common task. However, if you're working with std::string_view to store object names, you might run into unexpected issues like undefined behavior. In this post, we'll take a closer look at the problem and guide you through a clean and effective solution.

The Problem: Accessing Object Names

Consider you have the following scenario:

You are storing objects of different derived classes inside a std::vector of a base interface type.

Each object has a name stored as a std::string_view, which is initialized before the object instantiation.

On accessing the object's name after storing it in the vector, you encounter undefined behavior.

Example Code

Here's a simplified example to illustrate the issue:

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Expected vs Actual Outcome

When you run the above code, the output is as follows:

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As you can see, while the name appears correctly before storing, it turns out to be undefined afterwards.

Understanding the Cause

The root cause of this behavior lies in the way C++ handles member data. Each name variable in the base (IFoo) and derived class (Foo) is treated as a separate entity.

When you utilize the vector of IFoo*, the reference to name resolves to the member variable in the base class, which is an uninitialized std::string_view.

Only virtual functions exhibit polymorphic behavior, meaning they can be overridden. Data members, on the other hand, do not and will cause confusion in such contexts.

The Solution: Use Virtual Functions

To resolve this issue, you need to employ virtual functions. Here's how you can modify your code to achieve the desired functionality:

Updated Code

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Key Changes Explained

Use of Virtual Method: The name() method is now a virtual function in the base class. This ensures that when you call foo->name() on an interface pointer, the appropriate derived class method is invoked, providing the expected name.

Smart Pointers: The switch to std::unique_ptr<IFoo> along with std::make_unique<Foo>() enhances memory management and improves exception safety. This way, you avoid manual memory management pitfalls.

Reference in Loop: When iterating over the vector, use const auto& foo to avoid creating unnecessary copies, promoting performance.

Conclusion

By adopting virtual functions for accessing object properties rather than direct data members, you can effectively prevent undefined behavior when working with polymorphic types in C++. Additionally, integrating smart pointers bolsters memory safety in your code. Implement these tips in your C++ projects to write more robust and reliable applications.