GPT答：

7i24.cn

The error message indicates that you are trying to access a private member variable `cap` of the class `grabImage` in the file `main.cpp`. The declaration of this variable in the header file `grabImage.h` shows that it is declared as private.

To fix this error, you need to either change the access specifier of the `cap` variable to public or provide a public method in the `grabImage` class that allows accessing and modifying this variable.