"mexFunction is missing" Error while obtaining sizes from MEX S-Function in Matlab

Question

First of all thanks a lot for reading this post and being able to help. I have an .cpp custom code and I could create a mexa64 typing:

mex mycpp.cpp

Now I would like to implement this .mexa64 file into Simulink. I have create a mycpp.c and mycpp_wrapper.c with simulink functions and inputs/outputs (as if I was using s-function builder). So Now I open Simulink with S.Function and I type mycpp on it but it gives me this error:

Error while obtaining sizes from MEX S-function 'universaltest' in 'uni_simulink/S-Function'.
Caused by:
Invalid MEX-file '/home/user/Workspace/universaltest.mexa64': Function mexFunction is missing.

how can I solve it? By the way, I can not use S-Function builder because it is using gcc compiler and not g++... I try to solve it with "mex -setup C++" but it still does not work. Does anyone know how I can solve this as well?

Thanks a lot.