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I have a small program, written in C++ that contains a class with a large array. The class looks like this:

class Test
{
public:
    Test();
    ...
private:
    int myarray[45000000];
};

Now, this array is read in from a file. I would like to do this with the constructor directly, instead of bothering to call any extra functions. The array only needs to be read in once, and afterwards will not change anymore. It has the exact size specified.

My constructor looks like this:

Test()
{
   memset(myarray, 0, sizeof(myarray));
   FILE* fstr = fopen("myfile.dat", "rb");
   size_t success= fread(myarray, sizeof(myarray), 1, fstr);
   fclose(fstr);
}

Using Visual Studio 2012 Ultimate: When trying to start a program that uses this class, it crashes with an "APPCRASH" as soon as the class is created, and when trying to debug it (which I have next to no knowledge of), tells me that the error is a Stack overflow.

The mystery of this all is that in my previous version, where myarray was a static variable, and I had to call a static function to set it, everything went just fine. But trying to convert this to a constructor, try as I might, all my attempts fail.

So what am I doing wrong here?

Mark Anderson
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    You're class is too big for the stack. Try allocating your array in the heap rather than on the stack. – Nick Feb 28 '13 at 13:40

3 Answers3

4

Even if your int were the minimum size of 2 bytes, your array would use about 86MB of memory. A typical maximum stack size is 1MB. If the storage for your Test object is allocated on the stack, you'll easily overflow. You'll need to either dynamically allocate your array or not load all of it into memory at once. Better yet, use a standard container that uses dynamic allocation for its elements, like `std::vector.

Joseph Mansfield
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3

This declaration in your class:

int myarray[45000000];

is trying to allocate 45,000,000 * 4 bytes per int (assuming 32-bit) = 180MB of memory. No way your stack is going to support that. You need to redesign the app to change how you load your file.

David W
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so you presumably do this in your main (or anywhere else)

int main ()
{
  Test t; // Hello StackOverflow
}

What you need is allocate it on the heap:

int main ()
{
  Test* t = new Test;
  delete t;
}

It didn't crash with static variable because static variables are NOT allocated on the stack

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    Better still, change `myarray` into a dynamic array (preferably `std::vector`, so you don't need to handle any memory management yourself). Then users of `Test` won't have any deathtraps to avoid. – Mike Seymour Feb 28 '13 at 13:49
  • @MikeSeymour, I agree. But the question was how to avoid SO in this particular case. –  Feb 28 '13 at 13:57
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    Indeed, that was the question. And in my opinion, the best answer is to redesign the class so it doesn't cause SO, rather than requiring users of the class to use it in unconventional ways. – Mike Seymour Feb 28 '13 at 14:00
  • Thank you very much for your explanation, this indeed worked. I tried to follow @MikeSeymour's advice, but I just can't get fread to work with the vector class, so I guess I will have to stick by this method for the time being. – Mark Anderson Feb 28 '13 at 18:40
  • @MarkAnderson: For a vector, `myarray.resize(45000000); fread(&myarray[0], myarray.size(), 1, fstr);` should do the trick. – Mike Seymour Feb 28 '13 at 18:43