Member Avatar for ItecKid

Hello,

How to call a function which takes dynamic array argument? That is to say, if I have:

void fctn (int *a[], int n)
{
*a = &n;
//do stuff with a.
}

How to call this in main? Must I have another array declared in main to call it with?

Thanks for the help.

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  • Latest Post Latest Post by thelamb

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Generated

The declaration used in the first post is asking for an array of int pointers (the compiler treats that parameter as a pointer-to-pointer). Passing or assigning the address of a local (for example doing *a = &n inside the function) will create a dangling pointer when the function returns — that is unsafe. For details on how array parameters behave and decay to pointers see Array-to-pointer conversion.

As hinted, if the function really needs "an array of int pointers" the caller must provide an array (or pointer) of int* values and ensure each pointer is valid for the lifetime required:

void fctn(int** ptrs, std::size_t n) {
    for (std::size_t i = 0; i < n; ++i)
        if (ptrs[i]) /* use *ptrs[i] safely */;
}

int main() {
    const std::size_t N = 3;
    int** ptrs = new int*[N];
    for (std::size_t i = 0; i < N; ++i) ptrs[i] = new int(static_cast<int>(i + 1));
    fctn(ptrs, N);
    for (std::size_t i = 0; i < N; ++i) delete ptrs[i];
    delete[] ptrs;
}

If the goal is a single contiguous dynamic array of ints, prefer a signature that takes int* (or better, std::vector<int>&). As suggested, a contiguous block is simpler and safer:

void use_contig(std::vector<int>& v) {
    for (auto &x : v) x += 1;
}

int main() {
    std::vector<int> v(5);
    use_contig(v);
}

If the function must allocate memory for the caller, either return the pointer or accept a reference to the pointer (int*&) — that avoids assigning the address of a local. Summary: pick the right model (array-of-pointers vs contiguous block vs factory function), manage ownership explicitly, and avoid using addresses of local variables.

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Member Avatar for Narue

Just make sure you match the expected type. In this case, int *a[] is equivalent to int **a , so I would expect the function to be called something like this:

int main()
{
  int **a;

  // ...

  fctn ( a, SIZE );
}
Member Avatar for thelamb 
int main()
{
    int* a = new int[2];
    func(a);
    delete[] a;
}

void func( int* a )
{
    // Do something with a
}
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