Consider a template class like this:

```
template <int opt1 = 1,
          bool opt2 = false,
          bool opt3 = true,
          int opt4 = 50>
class X  { };

```

I try to change just one parameter, but it seems that C++ can't do that. Or am I wrong? How to achieve something like this:

```
X<opt2 = true> x;

```