C++ :: Bad Value Result From Operator Using Objects

I keep getting an undesired value in this code. I've tried several methods, but none are giving me the correct answer. The out put is always zero, when in this case it should be 10!!

Here's the object structure:

template<class T, class _b>
struct quantity {
private: T value;
public:
explicit quantity(T val): value(val) {};
T getValue() { return value; };

[Code] .....

Here's the operation:

int main() {
quantity<int, bool> m(20);
quantity<float, bool> m_f(10);
quantity<double, bool> m_d(NULL);

m_d = m_f;

[Code] .....

Why does it seem that the assignment operator is the harder operator to overload? Maybe it's just my luck, but I seem to always run into issues whenever I work with it. I hardly ever experience errors when overloading any of the other operators.