Friend declaration error on compiling.

[billbeno]

I am coming at the following error. I am unable to get a fix for it. Kindly help. Thanks in advance.

`In file included from /home/beno/.local/src/ncorr_2D_cpp/build/../include/ncorr.h:12,

             from /home/beno/.local/src/ncorr_2D_cpp/src/ncorr.cpp:8:

/home/beno/.local/src/ncorr_2D_cpp/build/../include/Image2D.h:36:21: error: friend declaration of ‘void imshow(const ncorr::Image2D&, ncorr::Image2D::difference_type)’ specifies default arguments and isn’t a definition [-fpermissive]
36 | friend void imshow(const Image2D&, difference_type delay = -1);
| ^~~~~~

`