Creating dynamic sized array using MSVC C++ compiler

Question

I am using visual studio 2013 express and MSVC compiler.

I get an error on executing the following lines of code.

#include<iostream>
using namespace std;
int main()
{
     int n;
     cin>>n;
     int a[n];
     return 0;
}

It says expression must have a constant value on the line on which I declare array a. I searched and found this c++ array - expression must have a constant value

It says you need to turn on a compiler option to allow it. How do I set that option in Visual studio express?

Use the standard and portable std::vector instead of non-standard and non-portable variable-length arrays. — Some programmer dude
– Some programmer dude, Commented Jun 12, 2019 at 5:53
@Someprogrammerdude But using vector would allocate it on heap instead of stack. Does changing the compiler to gcc allow me to do dynamic size allocation of array? — Raj
– Raj, Commented Jun 12, 2019 at 6:03
Variable-length arrays are a controversial C99 feature that C11 had to make optional again. It is not a C++ feature and MSVC++ does not implement it at all. The non-portable alternative is _malloca(), designed to prevent the runtime mishap this site is named for when you allocate too much. — Hans Passant
– Hans Passant, Commented Jun 12, 2019 at 6:04
Possible duplicate of Why aren't variable-length arrays part of the C++ standard? — Miles Budnek
– Miles Budnek, Commented Jun 12, 2019 at 6:12
@Raj HERE you can find all sorts of information why you shouldn't go after your plan =) — skratchi.at
– skratchi.at, Commented Jun 12, 2019 at 6:15

Fureeish · Accepted Answer · 2019-07-13 23:27:17Z

4

You can use pointers

int*a = new int [n];

You have to delete before going out of your a's scope:

delete[] a;

But better use vector:

vector<int> a(n);

You can also use llvm smallvector which is optimized for small arrays with no heap allocation if the size was small

llvm::SmallVector<int, 5> smallVector;
 for(int i = 0; i < 5; i++) { 
    smallVector.push_back(i); } // No heap allocations have been performed up to this point.
     smallVector.push_back(6); // heap allocation now

But keep in mind the compiler will decide where to allocate. Smallvector

edited Jul 13, 2019 at 23:27

Fureeish

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answered Jun 12, 2019 at 5:54

Oblivion

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2 Comments

Raj Over a year ago

But that would allocate it on heap memory region. I want to allocate it on stack region

Oblivion Over a year ago

No other option. Clang has smal vector that might allocate in stack if n was small.

bahij zeineh · Accepted Answer · 2019-06-13 05:52:06Z

1

try the following:

#include<iostream>
using namespace std;
int main()
{
    int n;
    cin >> n;
    int *a=new int[n];
    delete[] a;
    return 0;
}

the way your doing it it gets allocated on the stack and for that it has to be constant whereas this way its on the heap and it can be whatever value.

i dont think there is a compiler option to change that

edited Jun 13, 2019 at 5:52

answered Jun 12, 2019 at 5:55

bahij zeineh

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2 Comments

Raj Over a year ago

But there are performance issues when allocating it on heap instead of stack.

skratchi.at Over a year ago

Please, never forget to delete the memory!

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