C++ :: Getting Error - Incomplete Type Is Not Allowed

When declare and assign an instance of a user-defined struct in a function. And the struct (theStruct) is not declared in the same header file as the function (theFunction). Like this:

files:
"A.h": declares the struct in a class (theClass)
"A.cpp": implements the struct
"B.h": declares the function
"B.cpp": implements the function, error here

I think making the instance (inst) a reference might solve this. But the instance is assigned to a return value from a function (returnFunc). Like:

void theFunction() {
...
theClass::theStruct inst = returnFunc(...);
//returnFunc() returns an instance of theStruct
//the error is at 'inst'
...
}

What do you think?