8

When i run this code in my Devcpp compiler->

#include<bits/stdc++.h>
using namespace std;
int main()
{
    vector<int> vec;
    for(int i=0;i<100000000;i++)
    vec.push_back(i);
}

It works even on run time. But when i run->

#include<bits/stdc++.h>
using namespace std;
int arr[1000000000];
int main()
{
    return 0;
}

It gives me link error.

As long as space is required both arr and vec requires the same space.Then why is it that vec code runs even fine on run time but arr code doesnt even compile.

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8
  • @dvenkatsagar no, that doesn't make sense. Commented Feb 1, 2016 at 19:19
  • @dvenkatsagar That doesn't make sense. Commented Feb 1, 2016 at 19:19
  • @MarcusMüller Jinx. You owe me a soda. :) Commented Feb 1, 2016 at 19:20
  • 2
    What compiler error? It compiles for me with clang on osx. Commented Feb 1, 2016 at 19:20
  • 3
    Why should I not #include <bits/stdc++.h>? Commented Feb 1, 2016 at 19:21

2 Answers 2

Reset to default
10

The issue is with the allocation. In the first case, std::vector default allocator uses dynamic allocation, which in principle can allocate as much memory as you want (bounded of course by the OS and the amount of physical memory) whereas in the second case it uses the memory available for static allocation (technically the array has static storage duration), which in your case is smaller than 1000000000 * sizeof int bytes. See this for a nice answer regarding the various types of allocations in a C program (which also applies for C++).

Btw, avoid #include<bits/stdc++.h>, as it is non-standard. Include only the standard headers you need. One more issue: I don't think you get a compile-time error, you probably get a run-time error. In other words, the code compiles just fine, but fails to run.

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11 Comments

@StevenBurnap I am not sure OP is getting a compile error though, it is probably a run time error. Will edit the answer.
Yeah, what you say makes perfect sense if the OP got a runtime error.
arr is not defined on the stack. It is a global object.
@RSahu Missed that it was defined outside the function, will edit, thanks.
@StevenBurnap Are you sure about that? Isn't the global data segment quite small? In any case, the standard does not mandate anything regarding where/how the allocations are being done. And what do you mean by as long as the index is an integral type? The size of the array has to be an integral type.
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2

It seems that the object

int arr[1000000000];

is too large to fit in the global data of your program for your environment. I don't get a compile time error but I get a link time error in my environment also (cygwin/g++ 4.9.3).

Reducing the size by one tenth work for me. It may work for you also. I don't know how you can determine the maximum size of objects that can fit in global data.

Space available in stack is the smallest in size.
Space available in global data is larger that that.
Space available in heap is the largest of all.

If your object is too large to fit in stack, try to put into global data.
If your object is too large to fit in global data, use heap.

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4 Comments

int arr[1000000000]; is an object? really?
@StahlRat, In the C++ object model, every variable is an object.
Annex B of the C++ standard names 262144 as a guideline for maximum object size. If I understand correctly, this means that for a non-dynamic int array, anything up to 262144 / sizeof(int) elements should be fine with a quality implementation. But of course, "these quantities are only guidelines and do not determine compliance".
@StahlRat: object not in terms of classes, though. They're rather ADTs (Abstract Data Types).

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