c++ Compiler could not generate operator= for class

Question

This code:

struct TToken
{
    UnicodeString &DBIDs;
    std::vector<TToken> T;
    TToken(UnicodeString &DI) : DBIDs(DI) {}
    void Append(void) {T.push_back(TToken(DBIDs));}
};

UnicodeString DI="25,40,55";
TToken T(DI);
if (T.DBIDs==DI) // if I comment out this comparison it works fine
....

generates the above compiler error (I'm using C++Builder 10.1 Berlin).

If I change the UnicodeString reference to a pointer, it all works as expected. Does anyone know why the reference generates the error?

By the way, is there an easier way of making DBIDs available to every created vector element instead of passing a pointer/reference all the way down the line? A static member is no good, and I did look at template expressions, but couldn't get an acceptable answer.