variable length array in c++11

Question

I knew there have been several questions about variable length array (VLA). I thought the general conclusion is that VLA is not supported. But the following code will compile just fine. Anyone can explain it?

#include <iostream>
#include <vector>

using namespace std;

void func(vector<int> v){
    int arr[v.size()] = {0};
    int b = 4;
    return;
}

void func2(int n)
{
    int arr[n];
}

int main()
{
  vector<int> v = {1,2,3};
  func(v);
  func2(10);
  int len = 8;
  int arr[len] = {0};
  return 0;
}

It is accepted as a GCC extension (or a Clang one). But VLAs are not in C++17 — Basile Starynkevitch
– Basile Starynkevitch, Commented Jul 7, 2018 at 20:17
Just don't do it. It's dangerous and not standard. Some compilers may allow it since they already have that C99 functionality. — Ivaylo Valchev
– Ivaylo Valchev, Commented Jul 7, 2018 at 20:18
You cannot prove the correctness of a C++ program "by experiment". The language simply doesn't allow that. A program is correct if it follows the language's rules, and a correct program can be compiled and run. Nothing can be said about the converse. — Kerrek SB
– Kerrek SB, Commented Jul 7, 2018 at 21:10
See Does “int size = 10;” yield a constant expression? for more details — Shafik Yaghmour
– Shafik Yaghmour, Commented Jul 8, 2018 at 1:13

Ebrahim · Accepted Answer · 2018-07-08 16:04:07Z

2

It shows error message "variable-sized object may not be initialized" using compiler g++ 4.8.4 and C++11.

Some compilers partially support this kind of array definition as an extension. See this link. However, it is not in C++ standard.

When you allocate an array on stack memory you must state its length at compile-time. If you want to have a variable length array, you must allocate it on the heap memory.

edited Jul 8, 2018 at 16:04

answered Jul 8, 2018 at 4:16

Ebrahim

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2 Comments

aschepler Over a year ago

"heap memory is used instead of stack memory" - are you sure?

Ebrahim Over a year ago

I was wrong. In some compilers it is supported with some limitations and the memory is allocated in stack memory. I edited the answer thanks to your question.

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