Is there a max array length limit in C++?


Question

Is there a max length for an array in C++?

Is it a C++ limit or does it depend on my machine? Is it tweakable? Does it depend on the type the array is made of?

Can I break that limit somehow or do I have to search for a better way of storing information? And what should be the simplest way?

What I have to do is storing long long int on an array, I'm working in a Linux environment. My question is: what do I have to do if I need to store an array of N long long integers with N > 10 digits?

I need this because I'm writing some cryptographic algorithm (as for example the p-Pollard) for school, and hit this wall of integers and length of arrays representation.

1
163
12/18/2009 10:26:45 PM

Accepted Answer

There are two limits, both not enforced by C++ but rather by the hardware.

The first limit (should never be reached) is set by the restrictions of the size type used to describe an index in the array (and the size thereof). It is given by the maximum value the system's std::size_t can take. This data type should always be the largest integer type of a system.

The other limit is a physical memory limit. The larger your objects in the array are, the sooner this limit is reached because memory is full. For example, a vector<int> of a given size n typically takes about four times as much memory as an array of type vector<char> (minus a small constant value). Therefore, a vector<char> may contain more items than a vector<int> before memory is full. The same counts for the native C-style arrays int[] and char[].

Additionally, this upper limit may be influenced by the type of allocator used to construct the vector because an allocator is free to manage memory any way it wants. A very odd but nontheless conceivable allocator could pool memory in such a way that identical instances of an object share resources. This way, you could insert a lot of identical objects into a container that would otherwise use up all the available memory.

Apart from that, C++ doesn't enforce any limits.

155
5/5/2014 11:04:52 PM

Nobody mentioned the limit on the size of the stack frame.

There are two places memory can be allocated:

  • On the heap (dynamically allocated memory).
    The size limit here is a combination of available hardware and the OS's ability to simulate space by using other devices to temporarily store unused data (i.e. move pages to hard disk).
  • On the stack (Locally declared variables).
    The size limit here is compiler defined (with possible hardware limits). If you read the compiler documentation you can often tweak this size.

Thus if you allocate an array dynamically (the limit is large and described in detail by other posts.

int* a1 = new int[SIZE];  // SIZE limited only by OS/Hardware

Alternatively if the array is allocated on the stack then you are limited by the size of the stack frame. N.B. vectors and other containers have a small presence in the stack but usually the bulk of the data will be on the heap.

int a2[SIZE]; // SIZE limited by COMPILER to the size of the stack frame

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