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 VLA, the main reasons for the exclusion of Variable-length Arrays from the C++ standard, and alternatives to VLAs in C++.

Table of Contents:

• What are Variable-length arrays (VLAs)?
• How to Create Variable Length Arrays in C++
• Limitations of VLA in C++ Standard
  • Memory Management Concerns
  • Complexity and Compatibility
  • Lack of Type Safety
• Alternatives to VLAs in C++
  • std::vector
  • std::array
  • Dynamic Arrays with new
  • std::unique_ptr or std::shared_ptr
  • Custom Wrapper Classes
• Conclusion

What are Variable-length arrays (VLAs)?

Variable-length arrays 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 flexible memory allocation. Also, the size of the variable length array is determined at runtime based on the input given by the user.

How to Create Variable Length Arrays in C++

To create a Variable Length Array (VLA) in C++, declare an array with a size that is set by a variable at runtime, like int arr[n]; after reading n from the user. Note that this only works with some compilers, as it is not part of the C++ standard.

Example:

Output:

This program takes a number n and stores n alphabetic characters in a std::vector<char>. It then prints all the entered characters using a loop.

Limitations of VLA in C++ Standard

Below are some of the main reasons why the Variable-length Arrays are not in the C++ standard.

1. Memory Management Concerns

When variable-length arrays 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 a stack overflow, which gives errors or undefined behaviour, and another problem is that VLA cannot handle and debug heap-based dynamic allocation. Also, C++ provides safer memory management practices, which encourage 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 behaviour.

2. Complexity and Compatibility 

The use of variable length arrays (VLA) 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 an 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 pass it to a function that can accept a fixed-size array printArray(int (&arr) [5])).

Alternatives To VLA in C++

Below are a few alternatives to variable-length arrays 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 with 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, initialises 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 C++ array type, is used instead of VLAs to integrate with STL algorithms. The setValues function initialises the array elements to multiples of 10, and the std::array type with the size 5 sets its values and prints both the elements and size of the C++ array types 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, initialising 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 behaviour in the program. Also, the main function shows the use of the DynamicArray object, adding 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 C++ 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 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++ code.

Below are articles designed to help you understand the basic structure and syntax of C++.

C++ assignment operators – Shows how custom classes use assignment.

Substring in C++ – Demonstrates real-world examples of substring usage.

Pass objects to functions in C++ – Shows use of const for read-only references.

Getline in C++ – Covers delimiter options and stream handling.

Array decay or array to pointer conversion in C++ – Shows impact on array traversal and sizeof.

Read binary files in C++ – Demonstrates reading structs and raw memory.

ifstream get in C++ – Demonstrates file parsing at the character level.

istream readsome in C++ – Useful for non-blocking I/O scenarios.

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