Passing in and modifying an array in a function

Question

I have been looking at other posts and trying to get this working for a bit, but can't seem to manage it.

Basically I want to pass a "char myArray[10]" though into a function, have the function assign the values and then hand it back. It generally looks like this at the moment:

int MyClass::GetArray(char array[10])
{   
    char p[10];
    ... // a value is assigned to p
    memcpy(&array, &p, sizeof(p)); // Here array ends up being 0x3232323232323232 <Error reading characters of string.>

    return 0;
}

Called with:

    char array[10];
    myclass.GetArray(array);

So, I assume I need to pass the array through as a reference to the array[10] created before calling the function. But for that I am unsure how to create a pointer to a fixed array without making it either a general char* pointer or a pointer to an array of chars.

Secondly is the memcpy error (in the code comments above). I'm not sure if that is related or not though.

Use std::array in place of built-in arrays. It has a size function (you don't know the size of the parameter in your example) and it supports assignment. — Qaz
– Qaz, Commented Jul 12, 2014 at 21:14
In memcpy, pass in array instead of &array. It'll decay to a pointer to the first element. Same for p. Why are you copying the memory anyway? (Actually, it could only (easily) make sense if you were passing a reference to the array, but that's what you're asking about, so nevermind~) — user123
– user123, Commented Jul 12, 2014 at 21:14

Some programmer dude · Accepted Answer · 2014-07-12 21:19:05Z

1

Then thing is that when you pass an array to a function, it decays to a pointer. So when you use the address-of operator & on array in the function, you're taking the address of the pointer, meaning you get a pointer to a pointer.

That, by the way, leads to undefined behavior.

Other than that it's all okay, you don't have to pass the array (or rather, pointer) by reference. It's just not very... C++-ish. :)

answered Jul 12, 2014 at 21:19

Some programmer dude

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3 Comments

user1539405 Over a year ago

Okie. Removing the refernce(&) as you and Mohammad suggested works perfectly. But it sounds like you might have a better suggestion on how to do this? :) @chris Yeah, I didn't realise that the compiler ignored that value. For my program though everything will be 10 bytes and the char array[10] will be the only array passed through :)

Some programmer dude Over a year ago

@user1539405 I would suggest using std::array even if it's the only array you use. The formal parameter to the function would then need to passed by reference, e.g. std::array<char, 10>& array. Using a standard contain like std::array would keep you from making mistakes like (old C-style) arrays decaying to pointers.

user1539405 Over a year ago

All right, will update it to an std array. And report back when I somehow screw it up ;)

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