Question

Not sure if the answer would be the same for Java, C# and C++, so I categorized all of them. Answer for all languages would be nice.

All days I’ve been thinking, that if I allocate array all the cells would be in one, contiguous space. So if there isn’t enough memory in one piece in system there will be raised out of memory exception.

Is it all right, what I said? Or is there possibility, that allocated array would be paginated?

Answer 1

C++ arrays are contiguous, meaning that the memory has consecutive addresses, i.e. it’s contiguous in virtual address space. It need not be contiguous in physical address space, since modern processors (or their memory subsystems) have a big map that associates virtual pages with physical pages. Processes running in user mode never see physical addresses of their arrays.

I think in practice most or all Java implementations are the same. But the programmer never sees an actual address of an array element, just a reference to the array and the means to index it. So in theory, a Java implementation could fracture arrays and hide that fact in the [] operator, although JNI code can still view the array in the C++ style, at which point a contiguous block would be needed. This is assuming there’s nothing in the JVM spec about the layout of arrays, which jarnbjo tells me there isn’t.

I don’t know C#, but I expect the situation is pretty similar to Java – you can imagine that an implementation might use the [] operator to hide the fact that an array isn’t contiguous in virtual address space. The pretense would fail as soon as someone obtained a pointer into it. [Edit: Polynomial says that arrays in C# can be discontiguous until someone pins them, which makes sense since you know you have to pin objects before passing them into low-level code that uses addresses.]

Note that if you allocate an array of some large object type, then in C++ the array actually is that many large structures laid end-to-end, so the required size of the contiguous allocation depends on the size of the object. In Java, an array of objects is “really” an array of references. So that’s a smaller contiguous block than the C++ array. For native types they’re the same.