Convert Node Array (variable length) to a const float** to call opencv.calcHist

Context

I'm working currently on https://github.com/piercus/node-opencv (forked from https://github.com/peterbraden/node-opencv), I'm implementing a binder for calcHist function.

Problem

• The C++ function is taking const float** ranges as input (see http://docs.opencv.org/2.4/modules/imgproc/doc/histograms.html#calchist)
• I want to set the size of the input considering
• I do not know how to conditionally set the size of this const float** without breaking the compiler

Workaround

Considering that maximum number of dimensions is 3, i have made a workaround (see full source )

// Wrap Javascript input which is like [[0, 256], [0, 256]]
Local<Array> nodeRanges = Local<Array>::Cast(info[3]->ToObject());

// create a first table
float histRanges[dims][2];

for (unsigned int i = 0; i < dims; i++) {
  Local<Array> nodeRange = Local<Array>::Cast(nodeRanges->Get(i)->ToObject());
  float lower = nodeRange->Get(0)->NumberValue();
  float higher = nodeRange->Get(1)->NumberValue();
  histRanges[i][0] = lower;
  histRanges[i][1] = higher;
}
 
// minimum length is 1 so i can fullfill first range without issue
float first_range[] = { histRanges[0][0], histRanges[0][1] };
float second_range[] = { 0, 0}; // here is my problem, do i really need to do this
float third_range[] = { 0, 0};// same problem here

if(dims >= 2){
  second_range[0] = histRanges[1][0];
  second_range[1] = histRanges[1][1];
}
if(dims >= 3){
  third_range[0] = histRanges[2][0];
  third_range[1] = histRanges[2][1];
}

// now i can create a const float** compatible type 
const float* histRanges1[] = {first_range, second_range, third_range};

[... other stuffs ...]

// const float** is needed here
cv::calcHist(&inputImage, 1, channels, cv::Mat(), outputHist, dims, histSize, histRanges1, uniform);

Question

Is it possible to do it in an elegant way without creating a "zero-filled" object?

I would like to have maximum size of input 32 (instead of 3).

You don't need to copy the contents of histRanges, as the numbers in it are already laid out as float arrays, just like cv::calcHist requires. You only need to create an array of pointers to those arrays.

float histRanges[dims][2];
const float* ranges[dims];

for (unsigned int i = 0; i < dims; i++) {
  Local<Array> nodeRange = Local<Array>::Cast(nodeRanges->Get(i)->ToObject());
  float lower = nodeRange->Get(0)->NumberValue();
  float higher = nodeRange->Get(1)->NumberValue();
  histRanges[i][0] = lower;
  histRanges[i][1] = higher;
  ranges[i] = histRanges[i];
}

cv::calcHist(&inputImage, 1, channels, cv::Mat(), outputHist, dims, histSize, ranges, uniform);