getting the length of array provided array is being accessed through pointer

Question

For ex. I have a function

void len(int *A){ // or we have its reference

// can I find the length of array here or
// we just need to pass the length along as the parameter

}

You can't. `A` is a pointer, not an array. Use `std::vector` instead of passing length explicitly unless you really have to. Also take a look at [`std::span`](https://en.cppreference.com/w/cpp/container/span) - it will be available in C++20. — Evg, Nov 01 '20 at 07:38

score 2 · Accepted Answer · answered Nov 01 '20 at 07:40

2

can I find the length of array here or we just need to pass the length along as the parameter

You can't find the length once the array has decayed into a pointer so one option is to supply the length as a parameter as you said yourself.

Another option is to not let it decay into a pointer:

template<size_t N>
void len(int(&A)[N]) {
    // N is the length
}

Or you could use a std::vector<int>:

void len(std::vector<int>& A) {
    // A.size() returns the length
}

answered Nov 01 '20 at 07:40

Ted Lyngmo

Theoretically you can’t, but in some cases where a sentinel is present it is possible. Of course it is important that the sentinel is unique in that array. – Michi Nov 01 '20 at 07:42
1

@Michi Yes, that's a workaround. – Ted Lyngmo Nov 01 '20 at 07:44

1 Answers1