Variable Length array forbidden in ISO C++ error in sublime text 3

Question

I recently changed my C++ compiler, and I am experiencing this strange error that says ISO C++ forbids variable length array. I distinctly remember that this error was not encountered by my previous compiler. Here is my code snippet that I wrote just to check the reliability of this new compiler.

#include <iostream>
using namespace std;

int main()
{
    int n;
    cin>>n;
    int a[n];
    for(int i=0;i<n;i++)
        cin>>a[i];
    for(int i=0;i<n;i++)
        cout<<a[i]<<" ";
    return 0;
}


In function 'int main()':
test.cpp:8:9: error: ISO C++ forbids variable length array 'a' [-Wvla]
int a[n]={0};

You see even when user is taking input in 'n', the compiler states that the array is having variable length. Suggestions regarding other compiler version are also welcome!

It's exactly what the error says. Standard C++ forbids arrays with lengths that aren't known at compile time. Some compilers have extensions that allow VLAs, but they usually aren't a good idea. — eesiraed, Jan 04 '20 at 06:04
[Variable-length arrays](https://en.wikipedia.org/wiki/Variable-length_array) are not part of C++. Some compilers might add it as a non-standard extension. Use [`std::vector`](https://en.cppreference.com/w/cpp/container/vector) instead. — Some programmer dude, Jan 04 '20 at 06:05

Ted Lyngmo · Accepted Answer · 2020-01-04T06:20:40.920

2

Replace the VLA with a std::vector:

#include <iostream>
#include <vector>

int main()
{
    int n;
    std::cin>>n;
    std::vector<int> a(n);  // was VLA: int a[n];
    for(int i=0;i<n;i++)
        std::cin>>a[i];
    for(int i=0;i<n;i++)
        std::cout<<a[i]<<" ";
}

edited Jan 04 '20 at 06:20

answered Jan 04 '20 at 06:06

Ted Lyngmo

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1

[std::vector::vector (constructor)](https://en.cppreference.com/w/cpp/container/vector/vector) – David C. Rankin Jan 04 '20 at 06:12

Adrian Mole · Answer 2 · 2020-01-04T06:17:04.947

Your new compiler is correct: variable-length arrays are not allowed in standard C++! (GNU g++ allows them as an extension). See here: Why aren't variable-length arrays part of the C++ standard?

You should modify your code to use the std::vector type, instead:

#include <iostream>
#include <vector> // Definitions for the std::vector types
//using namespace std; // This can cause problems - better only "using" specifics:
using std::cin;
using std::cout;

int main()
{
    size_t n; // You CAN use "int" but "size_t" is more accurate here
    cin >> n;
//  int a[n]; // Forbidden in standard C++
    std::vector<int> a(n); // Declare "a" as a vector with "n" elements
    for (size_t i = 0; i < n; i++)
        cin >> a[i]; // You can still use the "[i]" syntax to access the elements
    for (size_t i = 0; i < n; i++)
        cout << a[i] << " ";
    return 0;
}

Feel free to ask for further clarification and/or explanation.

score 0 · Answer 3 · answered Jan 04 '20 at 06:07

The code is not valid in standard C++. As per C++ standard, array size must be a constant expression. Variable lenghth arrays are not allowed in C++ because C++ provides std::vector for that. C standard allows VLA(variable length arrays) while C++ does not.

Variable Length array forbidden in ISO C++ error in sublime text 3

3 Answers3