seqlist.h:54:25: error: out-of-line definition of 'Union' does not match any declaration in 'seqList<elemType>'

The error message you're encountering suggests that there's a mismatch between the inline declaration (likely in the header file `seqlist.h`) and the actual implementation (line 54) of a function called "Union" or possibly a constructor/destructor for the `seqList` template class. Here's what it means:

1. **Constructor/Destructor Misalignment**: If `Union` refers to a destructor, like the one shown in your first reference[^1], the compiler is looking for an inline definition of `~seqList()` that matches the declaration in the header file. Ensure that the body of the destructor defined in the source (.cpp) file corresponds to the signature declared in `seqlist.h`.

// In seqlist.cpp
template <class elemType>
seqList<elemType>::~seqList() {
    delete[] data;
}

2. **Function Declaration vs Definition**: If `Union` is actually a different member function (e.g., a member function other than the destructor), make sure you've correctly declared it in the header (`seqlist.h`) and then implemented it in the source file.

For example, if `length()` is meant to be inline:

**In seqlist.h**

// ...
inline int length() const; // Declare the function as inline
// ...

**In seqlist.cpp**

// ...
template <class elemType>
int seqList<elemType>::length() const { return currentLength; }
// ...