Why aren’t Variable Length Arrays Part of the C++ Standard?

Variable length arrays(VLAs) are arrays whose size can be determined at runtime in C++ programming. The VLAs are available in C99, but they are not a part of the C++ standard library due to their various disadvantages. In this article, we will discuss VLAs, the main reasons for the exclusion of Variable-length Arrays from the C++ standard, and alternatives to VLAs for C++.

Table of Contents:

• What are Variable-length arrays (VLAs)?
• Reasons for Exclusion of Variable-length Arrays from the C++ Standard
  • 1. Memory Management Concerns
  • 2. Complexity and Compatibility
  • 3. Lack of Type Safety
• Alternatives for Variable-Length Arrays in C++
  • 1. std::vector
  • 2. std::array
  • 3. Dynamic Arrays with new
  • 4. std::unique_ptr or std::shared_ptr
  • 5. Custom Wrapper Classes
• Conclusion

What are Variable-length arrays (VLAs)?

Variable length arrays(VLAs) are arrays whose size is known at runtime rather than at compile time, which helps in more flexible memory allocation. However, VLAs are not part of the C++ standard, they are mainly found in C99.

Example to show VLA in C:

Output:

The above code shows the use of variable length arrays (VLAs) with size n in C for the flexible memory allocation. Also, the size of the variable length array is determined at runtime based on the input given by the user.

Reasons for Exclusion of Variable-length Arrays from the C++ Standard

Below are some of the main reasons for the exclusion of Variable-length Arrays from the C++ standard.

1. Memory Management Concerns

When variable length arrays (VLAs) are used to allocate memory on the stack, which has a limited size, then memory management concerns occur. If a VLA is too large, it can lead to stack overflow, which gives errors or undefined behavior, and another problem is that VLAs cannot handle and debug heap-based dynamic allocation. Also, C++ provides safer memory management practices, which encourages the use of dynamic memory allocation.

Example:

Output:

The code helps you to understand how the VLAs are used in C++ to allocate a large array size on the stack, which can cause a stack overflow due to excessive memory usage.

Note: A stack overflow is typically not catchable in C++ and may lead to undefined behavior.

2. Complexity and Compatibility 

The use of variable length arrays(VLAs) in C++ will give complexity in the code language as it needs changes to existing features, and then this complexity leads to compatibility issues with the existing C++ code.

Example:

Output:

The code shows how a VLA is used in C++, causing a compilation error and incompatibility issue when it is passed to a template function that expects a fixed-size array.

3. Lack of Type Safety

Variable length arrays(VLAs) in C ++ lead to a lack of type safety, which is a very important aspect of the C++ programming language. As the VLAs are not a part of the C++ standard, they can create vagueness in the type handling, especially when you are using the template functions.

Example:

Output:

The code shows how a VLA in C++ is used, which gives a type-safety error and shows undefined behavior, as it tries to declare an array with a user-defined size and then passed to a function that can accept a fixed-size array printArray(int (&arr) [5])).

Alternatives for Variable-Length Arrays in C++

Below are a few alternatives to variable-length arrays VLAs that you can use in C++ programming.

1. std::vector

The std::vector, which is a part of the C++ standard library, must be used in place of VLAs. Also, the std::vector provides dynamic array capabilities with automatic memory management, and you can also use it over the raw pointers.

Example:

Output:

The code shows how the std::vector is used to create a dynamic array with size 5 instead of VLAs for automatic memory management, initializes the elements to multiples of 10, and prints it to the console.

2. std::array

You can use the std::array, which is a fixed-size array when the size of the array is known at compile time, as it will provide better type safety and integration with the STL algorithms.

Example:

Output:

The code shows how the std::array, which is a fixed-size array, is used instead of VLAs to integrate with STL algorithms. The setValues function initializes the array elements to multiples of 10, and the std::array with the size 5 sets its values and prints both the elements and size of the array to the console.

3. Dynamic Arrays with new

You should create the dynamic arrays with the new operator instead of using the variable length arrays but also manage memory manually. 

Example:

Output:

The code shows how the new operator is used for the dynamic memory allocations with size 5, instead of VLAs, and also it initializes the elements of the array to multiples of 10 and prints them to the console.

4. std::unique_ptr or std::shared_ptr

You can use smart pointers such as std::unique_ptr or std::shared_ptr instead of VLAs because they will help you to manage the memory and also the dynamic arrays automatically without any risk of memory leaks.

Example:

Output:

The code shows how a smart pointer, std::unique, is used to create and manage a dynamic array of size 5 automatically, initializing its elements to multiples of 10 and printing them to the console.

5. Custom Wrapper Classes

You should use the custom wrapper class in C++ when you need to implement a particular behavior or feature in your program, as it encapsulates dynamic arrays and manages memory.

Example:

Output:

The code shows how the custom wrapper is used to handle the dynamic arrays and to implement a particular behavior in the program. Also, the main function shows the use of the DynamicArray object, adding the multiples of 5 and then printing the array elements with the current size to the console.

Conclusion

As we have discussed above, the variable length arrays(VLAs) provide the memory allocation by allowing the array size to be determined at runtime. However, VLAs are not a part of the C++ standard library because they have some limitations that cause so many issues in the programs. So, by understanding the VLAs, the reasons why they aren’t part of the C++ standard library, and the alternatives to VLAs, you can write efficient and type-safe C++ codes.

Why aren’t Variable-Length Arrays Part of the C++ Standard – FAQs