Swift 4: Pass multidimensional array to c++ - c++

Question

I have next function in c++:
double* solveSystemOfEquatationWithAugmentedMatrix(double **matrix, size_t rows)
On the swift part I have simple [[Double]] that have to passed
I`ve tried next conversation:
let array = UnsafeMutablePointer<UnsafeMutablePointer<Double>>.allocate(capacity: matrix.count)
The I`ve tried to run over each inner pointer and allocate capacity for each inner pointer, but no luck... there is even no allocate function in each inner row.
What is elegant and fast way to do such conversation and pass to c++?

Answer 1

I believe it would be best to create an ObjectiveC wrapper around your matrix buffer. This way you will have access directly to its memory and still be able to use it in swift.
You will need to use your header to control what access you need to the class but here is an example:
#interface Matrix : NSObject
#property (nonatomic, readonly) int rowCount;
#property (nonatomic, readonly) int columnCount;
- (double)elementAt:(int)row column:(int)column;
- (void)setElementAt:(int)row column:(int)column to:(double)value;
- (instancetype)initWithRowCount:(int)rowCount columnCount:(int)columnCount;
#end
So we have a basic constructor which accepts number of rows and number of columns as an input. You might actually need only one parameter. And the implementation:
#import "Matrix.h"
#interface Matrix ()
#property (nonatomic) double *buffer;
#property (nonatomic) double *solvedBuffer; // Not sure what this is and how you wish to serve it
#property (nonatomic) int rowCount;
#property (nonatomic) int columnCount;
#end
#implementation Matrix
- (void)dealloc {
[self freeBuffer];
}
- (instancetype)initWithRowCount:(int)rowCount columnCount:(int)columnCount {
if((self = [super init])) {
self.rowCount = rowCount;
self.columnCount = columnCount;
[self generateBuffer];
}
return self;
}
- (double)elementAt:(int)row column:(int)column {
return self.buffer[column*self.rowCount + row];
}
- (void)setElementAt:(int)row column:(int)column to:(double)value {
self.buffer[column*self.rowCount + row] = value;
}
- (void)generateBuffer {
if(self.rowCount > 0 && self.columnCount > 0) {
int size = sizeof(CGFloat)*self.rowCount*self.columnCount;
self.buffer = malloc(size);
memset(self.buffer, 0, size); // Set it all to zero
}
}
- (void)freeBuffer {
if(self.buffer) {
free(self.buffer);
self.buffer = nil;
}
}
- (void)solveSystem {
self.solvedBuffer = solveSystemOfEquatationWithAugmentedMatrix((double **)self.buffer, self.rowCount);
}
#end
Nothing special really. We have access to buffer, ability to generate it and need to release it manually when done.
I added the "solve system" method which I am not sure what you expect as an output so you will need to create interface for it on your own.
Anyway, if this is an Xcode project you simply create a new file and select to be ObjectiveC. Xcode will ask you to create a bridging header which you need to confirm. Then find this bridging header and add #import "Matrix.h" or whatever your file names will be and this object will be available to you in Swift. As for the C part you can see all should be pretty straight forward. IF you really need to expose C++ then simply rename your Matrix.m to Matrix.mm and Xcode should do the rest for you.
Please also be careful if your matrix should be column or row major. The access may change to self.buffer[row*self.columnCount + column] in the other case in 2 methods.