What's the cleanest way code-wise to return a statically sized array from an object?

Question

I have a situation where a class holds a vector of constant size 5. I need the data from the vector as an array of size 5 as our std::vector implementation doesn't appear to use contiguous memory (please don't argue about that, I know it should and we've checked it to death). The contiguous memory block is required so we can uuencode/uudecode the memory block easily (those turn an arbitrary memory block into a string).

I'm finding it difficult to return a statically sized array. I can do it by reference or I can do it by wrapping a statically sized array in a structure - but both are a little agitating. By reference requires the calling code to declare the array first and pass it in to the code, and the other option requires that I make an extra structure just for this purpose.

class A {
    public:
        /*return type*/ GetVectorAsArray(/*params*/) { /* implementation */ }

    private:
        std::vector<X> m_vec;
};

So, assuming I need to call GetVectorAsArray, what's the cleanest way to return the 5 values in m_vec to the calling code? You can put whatever you want in return type or params and implementation.

I know you've said not to argue about it, but a vector by definition should be holding elements in contiguous memory. What STL implementation are you using so I know to avoid it in the future? — Chad
– Chad, Commented Sep 8, 2011 at 19:28
Maybe padding is causing the issue with contiguous memory. For example, maybe sizeof(X) == 7. I think that objects are usually 8-aligned or something like that. — Daniel
– Daniel, Commented Sep 8, 2011 at 19:35
@w00te LynxOS uses GNU toolchain and gcc's implementation of vector is fine, at least in my LynxOS 4.0.0 (gcc 2.95.3, vector is stored as three pointers: start, finish, end_of_storage) — Cubbi
– Cubbi, Commented Sep 8, 2011 at 19:43

EddieBytes · Accepted Answer · 2011-09-08 19:47:14Z

6

boost::array is intended for arrays of fixed size, you can always use that. It uses contiguous memory.

Later edit: as correctly pointed out by commenters, std::array is available in the new standard.

edited Sep 8, 2011 at 19:47

answered Sep 8, 2011 at 19:33

EddieBytes

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5 Comments

K-ballo Over a year ago

Note that its now part of C++ Standard Library as of TR1

Kerrek SB Over a year ago

Nowadays that's also available as std::array.

EddieBytes Over a year ago

it is indeed, just making sure there's an answer for people not having access to the new standard.

GManNickG Over a year ago

And if neither is available, just a normal C-style array is fine.

fredoverflow Over a year ago

@GMan: You can't return C-style arrays by value. The OP didn't specify whether he wanted a copy or the original...

Andy Finkenstadt · Accepted Answer · 2011-09-08 19:33:24Z

1

template <typename X, size_t N>
class A {
private:
  X m_vec[N];
public:
  const X* GetVectorAsArray() const { return m_vec; }
  size_t GetVectorLength() const { return N; }
};

Usage:

A<int, 5> vec5_int;

answered Sep 8, 2011 at 19:33

Andy Finkenstadt

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