fix some samples colliding with std::beta

also add comments explaining the change

samples/cpp/cloning_gui.cpp

@@ -39,7 +39,12 @@
 #include <iostream>
 #include <stdlib.h>
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+using std::cin;
+using std::cout;
+using std::endl;
+using std::string;
+
 using namespace cv;
 
 Mat img0, img1, img2, res, res1, final, final1, blend;

samples/cpp/tutorial_code/HighGUI/BasicLinearTransformsTrackbar.cpp

@@ -8,6 +8,7 @@
 #include "opencv2/imgcodecs.hpp"
 #include "opencv2/highgui.hpp"
 
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
 using namespace cv;
 
 /** Global Variables */

samples/cpp/tutorial_code/ImgProc/BasicLinearTransforms.cpp

@@ -8,7 +8,10 @@
 #include "opencv2/highgui.hpp"
 #include <iostream>
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+using std::cin;
+using std::cout;
+using std::endl;
 using namespace cv;
 
 /**

samples/cpp/tutorial_code/ImgProc/changing_contrast_brightness_image/changing_contrast_brightness_image.cpp

@@ -2,7 +2,7 @@
 #include "opencv2/imgcodecs.hpp"
 #include "opencv2/highgui.hpp"
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
 using namespace cv;
 
 namespace

samples/cpp/tutorial_code/core/AddingImages/AddingImages.cpp

@@ -8,7 +8,11 @@
 #include <iostream>
 
 using namespace cv;
-using namespace std;
+
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+using std::cin;
+using std::cout;
+using std::endl;
 
 /**
  * @function main

samples/cpp/tutorial_code/photo/seamless_cloning/cloning_gui.cpp

@@ -38,7 +38,12 @@
 #include <iostream>
 #include <stdlib.h>
 
-using namespace std;
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+using std::cin;
+using std::cout;
+using std::endl;
+using std::string;
+
 using namespace cv;
 
 Mat img0, img1, img2, res, res1, final, final1, blend;