Question

This question has been bothering me for a while.

If I do int* a = new int[n], for example, I only have an pointer that points to the beginning of array a, but how does C/C++ know about n? I know if I want to pass this array to another function, then I have to pass the length of the array with it, so I guess C/C++ does not really know how long this array is.

I know we can infer the end of a character array char* by looking for the NUL terminator. But is there a similar mechanism for other arrays, like int? Meanwhile, char can be more than a character — you can also treat it as an integer type. Then how does C++ know where this array ends then?

This question starts to bother me even more when I am developing embedded Python (If you are not familiar with embedded python, you may ignore this paragraph and just answer the above questions. I will still appreciate it). In Python there is a “ByteArray”, and the only way to convert this “ByteArray” to C/C++ is to use PyString_AsString() to convert it to char*. But if this ByteArray has 0 in it, then C/C++ would think that char* array stops early. This is not the worst part. The worst part is, say I do a

char* arr = PyString_AsString(something)
void* pt = calloc(1, 1000); 

if st happens to start with 0, then C/C++ will almost guarantee to wipe out everything in arr, since it thinks arr ends right after a NULL appears. Then it might just wipe out everything in arr by allocating a a trunk of memory to pt.

Thank you very much for your time! I really appreciate it.

Answer 1

C/C++ doesn’t; it’s the allocator (the little piece of code that implements malloc(), free(), etc.) that knows how long it is. C/C++ is welcome to wee all over itself, free of the constraints of having to worry about the length.

Also, PyString_AsStringAndSize().