Can't set variable length with variable

Question

What I'm trying to do right now is to create an array with a length that is defined by a variable. However, when I put the variable in the array length, it gives me a "Variable length array of non-POD element type 'glm::vec2'" error. However, if I replace the variable with an actual number, the error goes away. Why does this happen and how can I fix this?


int numtriangles = sector1.numtriangles;


glm::vec2 tex[test]; //Using a variable generates an error
glm::vec3 vertices[10]; //No error here

Variable length arrays are not supported in C++. It is just a compiler extension and if it did compile on other platforms. — Mahesh, Apr 06 '12 at 15:02
possible duplicate of [Is there a way to initialize an array with non-constant variables? (C++)](http://stackoverflow.com/questions/972705/is-there-a-way-to-initialize-an-array-with-non-constant-variables-c) — Anders Abel, Apr 06 '12 at 15:05

score 7 · Accepted Answer · edited Aug 27 '13 at 18:14

You cannot have variable length arrays(VLA) in standard C++.
Variable length arrays are not allowed by the C++ Standard. In C++ the length of the array needs to be a compile time constant. Some compilers do support VLA as a compiler extension, but using them makes your code non-portable across other compilers.

You can use, std::vector instead of an VLA.

score 4 · Answer 2 · edited May 23 '17 at 10:27

See this question Is there a way to initialize an array with non-constant variables? (C++)

Short answer is no you cannot directly do this. However you can get the same effect with something like

int arraySize = 10;
int * myArray = new int[arraySize];

Now myArray is a pointer to the array and you can access it like an array like myArray[0], etc.

You can also use a vector which will allow you to have a variable length array. My example allows you to create an array with a variable initailizer however myArray will be only 10 items long in my example. If you aren't sure how long the array will ever be use a vector and you can push and pop items off it.

Also keep in mind with my example that since you've dyanmically allocated memory you will need to free that memory when you are done with the array by doing something like

delete[] myArray;

Here is a little sample app to illustrate the point

#include <iostream>
using namespace std;


int _tmain(int argc, _TCHAR* argv[])
{
    int arraySize = 10;
    int * myArray = new int[arraySize];
    myArray[0] = 1;
    cout << myArray[0] << endl;

    delete[] myArray;
}

+1 for bringing up dynamic memory allocation. The preferred method is for newbies to stay away from dynamic memory allocation and use `std::vector` instead. — Thomas Matthews, Apr 06 '12 at 16:00

score 1 · Answer 3 · answered Apr 06 '12 at 15:06

1

use STL. IF you want a variable length array you can use vectors under #include<vector>

Native c++ array donot nave variable length array.

answered Apr 06 '12 at 15:06

IndieProgrammer

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SirDarius · Answer 4 · 2012-04-06T15:32:32.630

0

When you declare an array with a length specifier, only constants are allowed.

Actually it's when the program is compiled that the array length is evaluated.

Note however that it's illegal in C++ to declare int test[]; like the compiler has no way to know how much space to allocate for the variable.

Without a length specifier, there is no actual memory that is reserved for the array, and you have to resort to using pointers and dynamic memory allocation:

int * test = new int[12];
// or
int * test = new int[val]; // variable works here

// and don't forget to free it
delete [] test;

Using int test[12] actually creates an array that is statically initialized once and for all to contain 12 integers at compile time. Do not ever attempt to do delete [] test with a variable declared this way, as it's most certainly going to make your program crash.

To be precise, if the array is declared in a function, it will use space on the program stack, and if declared in a global context, program data memory will be used.

edited Apr 06 '12 at 15:32

answered Apr 06 '12 at 15:13

SirDarius

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For the `n`th time **Pointers and arrays are not the same thing!** Please stop spreading around this nonsense!! – Alok Save Apr 06 '12 at 15:15
@Als "an array can be considered a constant pointer" better ? (from http://www.cplusplus.com/doc/tutorial/pointers/) – SirDarius Apr 06 '12 at 15:23
No, that is just wrong,that particular website is notorious for its blunders. Good Read: [How do I use arrays in C++?](http://stackoverflow.com/questions/4810664/how-do-i-use-arrays-in-c) – Alok Save Apr 06 '12 at 15:27
i'm too old to debate, and it's not a place for it either, anyways. – SirDarius Apr 06 '12 at 15:33

score 0 · Answer 5 · edited Nov 23 '14 at 19:03

C++ doesn't support declare variable length array. So to use a array with a length you may

Assume a big number which is highest possible length of your array. Now declare an array of that size. And use it by assuming that it an array of your desire length.
```
#define MAX_LENGTH 1000000000
glm::vec2 tex[MAX_LENGTH];
```
to iterate it
```
for(i=0; i<test; i++) {
    tex[i];
}
```
Note: memory use will not minimized in this method.
Use pointer and allocate it according your length.
```
glm::vec2 *tex;
tex = new glm::vec2[test];
```
enter code here
```
for(i=0; i<test; i++) {
    tex[i];
}
delete [] tex; // deallocation
```
Note: deallocation of memory twice will occur a error.

Use other data structure which behave like array.

vector<glm::vec2> tex;
for(i=0; i<test; i++){ 
    tex.push_back(input_item);
}
/* test.size() return the current length */

Can't set variable length with variable

5 Answers5

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