Getting opencv error in c++ - c++

Question

I'm try to get the error of opencv! say I have this program:
#include <opencv2\core\core.hpp>
#include <opencv2\highgui\highgui.hpp>
int main (){
cv::Mat frame;
cv::VideoCapture cap(1); // I don't have a second videoinput device!
int key = 0;
while(key !=27){
cap >> frame;
cv::imshow("frame",frame);
key = cv::waitKey(10);
}
cap.release();
return 0;
}
when I run this program I get in the console this message :
OpenCV Error: Assertion failed (size.width>0 && size.height>0) in unknown functi
on, file ..\..\..\opencv\modules\highgui\src\window.cpp, line 261
My question is how can I get this message and save it in a string for every error that I get!
and if it'S possible escaping the program crash!
thanks in advance!

Answer 1

It uses C++ exceptions. See here in the doc for more.
try
{
... // call OpenCV
}
catch( cv::Exception& e )
{
const char* err_msg = e.what();
std::cout << "exception caught: " << err_msg << std::endl;
}
A CV_Assert in the OpenCV code is a macro which calls the OpenCV function error. That function can be seen here. It will always print the error text on stderr unless you don't have the customErrorCallback set. You do that via cvRedirectError, see here.

Answer 2

You have to check whether OpenCV function calls in your code is successfully executed or not. Then you can understand the exact problem. Here is the modified code.
int main (){
cv::Mat frame;
cv::VideoCapture cap(1); // I don't have a second videoinput device!
if ( !cap.isOpened() ) // if not success, exit program
{
cout << "Cannot open the video cam" << endl;
return -1;
}
int key = 0;
while(key !=27){
bool bSuccess = cap.read(frame); // read a new frame from video
if (!bSuccess) //if not success, break loop
{
cout << "Cannot read the frame from video cam" << endl;
break;
}
cv::imshow("frame",frame);
key = cv::waitKey(10);
}
cap.release();
return 0;
}