Determine the size of a C++ array programmatically?


Question

This question was inspired by a similar question: How does delete[] “know” the size of the operand array?

My question is a little different: Is there any way to determine the size of a C++ array programmatically? And if not, why? Every function I've seen that takes an array also requires an integer parameter to give it the size. But as the linked question pointed out, delete[] must know the size of the memory to be deallocated.

Consider this C++ code:

int* arr = new int[256];
printf("Size of arr: %d\n", sizeof(arr));

This prints "Size of arr: 4", which is just the size of the pointer. It would be nice to have some function which prints 256, but I don't think one exists in C++. (Again, part of the question is why it doesn't exist.)

Clarification: I know that if I declared the array on the stack instead of the heap (i.e. "int arr[256];") that the sizeof operator would return 1024 (array length * sizeof(int)).

1
61
3/3/2019 8:21:33 PM

Accepted Answer

delete [] does know the size that was allocated. However, that knowledge resides in the runtime or in the operating system's memory manager, meaning that it is not available to the compiler during compilation. And sizeof() is not a real function, it is actually evaluated to a constant by the compiler, which is something it cannot do for dynamically allocated arrays, whose size is not known during compilation.

Also, consider this example:


int *arr = new int[256];
int *p = &arr[100];
printf("Size: %d\n", sizeof(p));

How would the compiler know what the size of p is? The root of the problem is that arrays in C and C++ are not first-class objects. They decay to pointers, and there is no way for the compiler or the program itself to know whether a pointer points to the beginning of a chunk of memory allocated by new, or to a single object, or to some place in the middle of a chunk of memory allocated by new.

One reason for this is that C and C++ leave memory management to the programmer and to the operating system, which is also why they do not have garbage collection. Implementation of new and delete is not part of the C++ standard, because C++ is meant to be used on a variety of platforms, which may manage their memory in very different ways. It may be possible to let C++ keep track of all the allocated arrays and their sizes if you are writing a word processor for a windows box running on the latest Intel CPU, but it may be completely infeasible when you are writing an embedded system running on a DSP.

65
5/25/2016 10:31:42 AM

No, there is no way to do that in Standard C++.

There is no really good reason why not that I'm aware of. Probably, the size was considered an implementation detail, and best not exposed. Note that when you say malloc(1000), there is no guarantee that the block returned is 1000 bytes --- only that it's at least 1000 bytes. Most likely it's about 1020 (1K minus 4 bytes for overhead). In that case, the "1020" size is the important one for the run-time library to remember. And of course, that would change between implementations.

Which is why the Standards committee added std:vector<>, which does keep track of it exact size.


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