OpenCV/C++ functions - c++

Question

I came across functions used like this:
void func(int, void*);
int main()
{
func(0,0);
}
void func(int, void*)
{
//content
}
And I have no idea, what does it mean. But for some reason, if I copied content from func straight into main, it doesn't work.
I found it used with OpenCV, but no explanation of it.
So I would appreciate, if someone could explain to me what does it mean or why is it used.
So, in my case:
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
using namespace cv;
using namespace std;
Mat src; Mat src_gray;
int thresh = 100;
/// Function header
void thresh_callback(int, void* );
/** #function main */
int main( int argc, char** argv )
{
/// Load source image and convert it to gray
src = imread( "test.png", 1 );
/// Convert image to gray and blur it
cvtColor( src, src_gray, CV_BGR2GRAY );
blur( src_gray, src_gray, Size(3,3) );
/// Create Window
char* source_window = "Source";
namedWindow( source_window, CV_WINDOW_AUTOSIZE );
imshow( source_window, src );
thresh_callback( 0, 0 );
waitKey(0);
return(0);
}
/** #function thresh_callback */
void thresh_callback(int, void* )
{
Mat threshold_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
/// Detect edges using Threshold
threshold( src_gray, threshold_output, thresh, 255, THRESH_BINARY );
/// Find contours
findContours( threshold_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
/// Approximate contours to polygons + get bounding rects
vector<vector<Point> > contours_poly( contours.size() );
vector<Rect> boundRect( contours.size() );
for( int i = 0; i < contours.size(); i++ ) {
approxPolyDP( Mat(contours[i]), contours_poly[i], 3, true );
boundRect[i] = boundingRect( Mat(contours_poly[i]) );
}
/// Draw bonding rects
Mat drawing = Mat::zeros( threshold_output.size(), CV_8UC3 );;
for( int i = 0; i< contours.size(); i++ ) {
rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), Scalar::all(255), 2, 8, 0 );
}
/// Show in a window
namedWindow( "Contours", CV_WINDOW_AUTOSIZE );
imshow( "Contours", drawing );
}
This is code, that I get from the internet. It works well, it detects edges and draw rectangle around them and i can understand it, except declaration of function thresh_callback.
So, my main question is, why it doesn't work without it?
And what does parameters without names mean or why are they there?

Answer 1

1.
void func(int, void*);
declares that there will be a void function called func with two parameters, the first being an int and the second being a void*.
2.
void func(int, void*)
{
//content
}
This is the definition of the function declared in 1. So, if you call func, this function will be executed.
3.
int main()
{
func(0,0);
}
Here you execute func and pass 0 as the first parameter and 0 as the second parameter as well.
4.
If you paste the content of func into main, then it will not work, since the content of func assumes the existence of the two parameters, which do not exist in main.