GFact | Why doesn’t C++ have Variable Length Arrays library?

While studying DSA, we all have encountered Arrays and their fixed-size property. But have you ever thought about Why C language allows us the Variable-Length Array but C++ doesn’t? And what should we do in C++ if we want a Variable-Length Array?

Well, we will talk about the above in detail in the following post but first, we should know about the term Variable-length Array.

What does a Variable-Length Array mean?

Variable-Length Array term is used for runtime sized or variable sized arrays. The size of such arrays is defined at run-time. 

Variably modified types must be declared at either block scope or function prototype scope and include: 

• variable-length arrays and 
• pointers to variable-length arrays. 

Variable-Length Array is a feature where we can allocate an auto array (on stack) of variable size. It can be used in a typedef statement. 

Example to show Variable-Length Array

Below program compiles and runs fine in C language:

#include <stdio.h>

void fun(int n)
{
   // Creating a Variable-Length Array
   // based on the size passed as Input to this function
   int arr[n];

   // ......
}  

int main()
{
  // Passing a size for creation of
  // Variable Length Array at runtime
  fun(6);
}

Does C or C++ Support Variable-Length Array?

C Language supports Variable-Length Array from C99 standard. 

Note: In C99 or C11 standards, there is feature called flexible array members, which works same as the above. 

C++ standard ( till C++11 ) doesn’t support Variable-Length Array. The C++11 standard mentions array size as a constant expression. So the above program may not be a valid C++ program. 

The program may work in GCC compiler, because GCC compiler provides an extension to support them.

As a side note, the latest C++14 mentions array size as a simple expression (not a constant expression) and hence can be used for creating a Variable-Length Array.

Why doesn’t C++ support Variable-Length Array?

Variable-Length Array (VLAs) are not part of the C++ standard because they were not included in the original C++98 standard. The C++ language is based on C, and VLAs were not part of the C standard at the time. Additionally, the C++ standardization committee has chosen to focus on other features and improvements, such as templates and the Standard Template Library (STL), rather than adding VLAs to the language. Some C++ compilers support VLAs as an extension to the language, but they are not a standard feature.

What are the alternatives for Variable-Length Array in C++?

In C++ STL library, we have Vectors which works as a dynamic array which can be used in the place of Variable-Length Array, in which we can define the length of the vector on the runtime itself .

Below is the code for vectors in C++ :

C++

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int n = 5;
    // Initialize the vector with variable size n
    vector<int> v(n);
    for (int i = 0; i < n; i++) {
        v[i] = i;
    }

    // printing the v vector
    for (int i = 0; i < n; i++) {
        cout << v[i] << " ";
    }
    cout << "\n";
    // Initialize a vector without any size
    vector<int> vec;
    vec.push_back(1);
    vec.push_back(2);
    vec.push_back(3);
    vec.push_back(4);
    vec.push_back(5);
    // ...

    // To iterate over it:
    for (auto it : vec) {
        cout << it << " ";
    }
    cout << endl;
    return 0;
}

Output

0 1 2 3 4 
1 2 3 4 5