CODEWITHSUNDEEP
c++opencvmatrix
hfs
hfs

Reputation: 31

OpenCV 3: How to pass cv::Mat as an optional argument

I would like to write a function in C++ that takes a matrix as an optional argument. If the user ends up passing the matrix when calling the function, it will be modified by the function, so I'm implementing a pass by reference.

According to the accepted answer for this previous question, this can be done by making the default value an empty Mat, such as:

void foo(..., cv::Mat &matrix = cv::Mat()) {
    // code

    return;
}

However, when I try to compile this in OpenCV 3.2, I get the «invalid initialization of non-const reference of type ‘cv::Mat&’ from an rvalue of type ‘cv::Mat’» error.

Has this feature changed since OpenCV 2.4, or could the issue be somewhere else?

Upvotes: 3

Views: 2573

Answers (2)

Madre Chigado
Madre Chigado

Reputation: 41

This is because C++ does not allow you to initialize a non-const reference with an rvalue.

So you can either:

1) make the reference const -- wouldn't suit you since you want to change the content of matrix in the function

or

2) have a cv::Mat init variable stored globally in your module:

static const cv::Mat g_mat = cv::Mat();

void foo(..., cv::Mat &matrix = g_mat) {

Upvotes: 4

Riccardo Giubilato
Riccardo Giubilato

Reputation: 110

I might be wrong because I'm a beginner in those matters but here's what I think. You want to pass by ref an optional argument. The issue is that a reference with the symbol & is a reference to an l-value, which is an object that exists. C++ does not allow what you have written because matrix is an r-value, that is a temporary object that has not an actual memory address associated to it. The only way you can pass cv::Mat &matrix = cv::Mat() is by const-ing it,const cv::Mat &matrix = cv::Mat() which of course is not what you need.

The simplest solution IMHO is to just overload your function definition so that you have

void foo(...args); void foo(...args, cv::Mat &matrix);

Hope it helps

Upvotes: 8

Related Questions