Reason: the compiler doesn’t know the concrete implementation of struct or class,

Analysis: it usually occurs in the following situations: suppose we have a class some defined in some. H, implemented in some. CPP, and we need to use the method of this some in other. CPP, so we can declare a class some in other. H, and declare the method to be used. This will lead to the above problems,

Solution: include some. H in other.cpp, so the compiler will find the specific definition of class some according to the header file, and the problem will be solved.

Details:

1. Forget to define the header file

2. There is no reference header file