The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
template<typename T, T Z = T{}>
class RefMem {
private:
T zero;
public:
RefMem() : zero{Z} {
}
};
int null = 0;
int main()
{
RefMem<int> rm1, rm2;
rm1 = rm2; // OK
RefMem<int&> rm3; // ERROR: invalid default value for N
RefMem<int&, 0> rm4; // ERROR: invalid default value for N
extern int null;
RefMem<int&,null> rm5, rm6;
rm5 = rm6; // ERROR: operator= is deleted due to reference member
}