Non-copy vector

[rainyl]

• Feature: Unsafe version of imencode() that does not copy opencv_dart#304

[Levi-Lesches]

Details: Modify all the vecXXX_cpp2c functions to pass the .data() and .size() directly. For example, this function:

dartcv/dartcv/core/vec.hpp

Lines 16 to 22 in 7bd81ec

	inline VecPoint vecpoint_cpp2c(std::vector<cv::Point> v) {
	auto* ptr = (CvPoint*)calloc(v.size(), sizeof(CvPoint));
	for (int i = 0; i < v.size(); i++) {
	ptr[i] = CvPoint{v[i].x, v[i].y};
	}
	return VecPoint{.ptr = ptr, .length = v.size()};
	}

could be rewritten to be:

inline VecPoint vecpoint_cpp2c(std::vector<cv::Point> v) {
  return VecPoint {.ptr = v.data(), .length = v.size() };
}

Most important thing here is testing. Research shows that vector data should not be changed or overwritten or de-allocated so long as the vector is not modified. That means this would be very bad:

vector<cv::Point> vec;
// fill in vec here
auto result = vecpoint_cpp2c(vec);
vec.push(newPoint);

// THIS SHOULD NO LONGER BE USED!
// Adding new elements can change the internal memory structure of the vector,
// and invalidate the .data() pointer
return result; 

So the main hazard in this PR would be to guarantee that data is never changed after a _cpp2c() function is called.

[rainyl]

@Levi-Lesches How's it going? did you start working on this, if not I will submit a PR to work together.

[Levi-Lesches]

Nope, our winter break started today :)

Happy to start sometime soon but if you have the time go ahead

[Levi-Lesches]

Awesome, thanks!

[rainyl]

😄, it works but still needs testing and optimizatios for production usage.

Fix it or let me know if you find any bugs.