Float-Array conversion

Question

I got a vector like this:

vector<Vec3f> myCoolVector

that gets filled with arrays like [1.0, 2.0, 3.1] and [4.2, 2.1, 7.7], ....

I'd like to convert the myCoolVector back to a 2d-array of floats to send it back to Swift as a simple multidimensional Float Array (like this: [[Float]]).

But what I get returned (to Swift) is a variable with this crazy type:
Optional<UnsafeMutablePointer<Optional<UnsafeMutablePointer<Float>>>>
...instead of just this type: [[Float]].

Please have a look at my code:

(1) Code - C++:

+(float **)myCoolFunction {
    vector<Vec3f> myCoolVector

    // here fillMyCoolVector() fills the myCoolVector - NICE

    float **floatArrayBackToSwift;
    floatArrayBackToSwift = new float*[myCoolVector.size()];

    for( size_t i = 0; i < myCoolVector.size(); i++ )
    {
        ary[i] = new float[3];
        ary[i][0] = myCoolVector[i][0];
        ary[i][1] = myCoolVector[i][1];
        ary[i][2] = myCoolVector[i][2];
    }
    return floatArrayBackToSwift;
}

(2) Code - Swift:

let floatArray = MyBridge.myCoolFunction() // should be [[1.0, 2.0, 3.1], [4.2, 2.1, 7.7], ...]
print(floatArray)

My question: How can I send the myCoolVector as a simple 2d-Array like [[Float]] back to Swift?

Any help would be very appreciated!

Don't know if this has anything to do with your issue, but `float**` is not a two-dimensional array. In addition, the way you created your "2d array" is not guaranteed to have contiguous data elements. Does Swift require contiguous data? — PaulMcKenzie, Mar 25 '18 at 00:28
It may seem like a "crazy" type, but that's ***exactly*** what `float **` means in C++. All `T *` for any type `T` (note: this doesn't apply to references in C++) in C/C++ are just like `Optional`, in that they're a sum type whose set of valid values is composed of `nullptr` plus the set of all possible values of `T`. And pointers aren't *really* actually arrays. They happen to point to arrays, but there's no guarantee that a `float **` points to an "array" of floats (a contiguous set of `float` values in memory). There could be nothing there at all. — Alexander, Mar 25 '18 at 00:29
I just like to send the vector back to Swift as [[Float]] type. How is this possible?? @Alexander — , Mar 25 '18 at 00:49
I understand, and I don't know, hence why I didn't answer your question. Merely explaining why you see the "crazy" behaviour that you see — Alexander, Mar 25 '18 at 01:02
@JaneDeverly Did you read my comment? A `float**` is *not* a 2 dimensional array, it is merely a pointer to a pointer, nothing more, nothing less. I am a Swift know-nothing, so I'll ask again -- what is the format of a 2-dimensional array that Swift accepts? The way you created your "array" is far different than say [this example, which creates a contiguous array](https://stackoverflow.com/questions/21943621/how-to-create-a-contiguous-2d-array-in-c/21944048#21944048). If Swift expects a bonafide 2-d array (not one created in the way you created it), then your C++ example will not work. — PaulMcKenzie, Mar 25 '18 at 02:15

score 0 · Answer 1 · answered Mar 25 '18 at 03:43

0

Your myCoolFunction returns float **, pointer to pointer to float. And any pointer types in C/C++ cannot be automatically converted into Swift Array, neither any other types in C/C++.

If you really want the result in Array<Array<Float>> (aka [[Float]]), you need to write some Swift code to convert Optional<UnsafeMutablePointer<Optional<UnsafeMutablePointer<Float>>>> to Array<Array<Float>>.

Something like this:

class MyWrappter {
    static func myCoolFunction(count: Int) -> [[Float]] {
        let ptrPtrFloat: UnsafeMutablePointer<UnsafeMutablePointer<Float>?>! = MyBridge.myCoolFunction()

        let bpPtrFloat = UnsafeBufferPointer(start: ptrPtrFloat, count: count)
        return bpPtrFloat.map {ptrFloat in
            [ptrFloat![0], ptrFloat![1], ptrFloat![2]]
        }
    }
}

Remember, your myCoolFunction does not return any info about the size of the vector<Vec3f>, so you may need to pass it to Swift side, with some other method.

Swift is not good at optimizing nested arrays. Better consider adopting simd.float3 or something like that.

answered Mar 25 '18 at 03:43

OOPer

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@JaneDeverly, I'm showing you a full code (only `import Foundation` omitted), which compiles perfectly in my Xcode 9.2. I suspect you have modified other parts of my code. – OOPer Mar 25 '18 at 09:15
I'm sorry, looks like a caching bug in Xcode - restarted and WORKS. However - Did I get it right? It's not possible to get the `count` of the array from the `MyBridge.myCoolFunction()`-call? – Mar 25 '18 at 09:24
@JaneDeverly, right. Your `MyBridge.myCoolFunction()` just returns a pointer and does not contain an info about how many elements the pointer is pointing. – OOPer Mar 25 '18 at 09:29
Ok - thats pretty bad! Can I add some kind of `try-catch` routine to avoid the crash at `[ptrFloat![0], ptrFloat![1], ptrFloat![2]]` if the count is above the real length? @OOPer – Mar 25 '18 at 09:34
@JaneDeverly, with your code, the inner arrays have always 3 elements, so you have no need to check with try-catch. And the outer array, once converted to a pointer, has no info where runtime check can use. So, try-catch may never work. Sometimes it crashes immediately, other times it runs with wrong data. You may need to write another method to return count or add a parameter to assign count. – OOPer Mar 25 '18 at 09:40
@JaneDeverly, I have been missing to note one thing. Your C++ code is actually written in Objective-C++, so, if rewrite your C++ code to use Objective-C types like `NSArray *`, the Swift side code can be simpler. – OOPer Mar 25 '18 at 11:42

Float-Array conversion

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