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instead of :

void matchTemplate (Mat image, Mat templ, Mat result, int method);

you probably wanted :

matchTemplate(image, templ, result, method);

the former is not doing anything, it's a (useless)declaration, not a function call.

so your result Mat is empty, and imshow does not like it.

also please throw in a waitKey(0) between imshow() and return, else you won't see anything