I have a typedef inside a class from a c++ machine library (dlib) that looks like this:

```
typedef dlib::matrix<double, 64, 1> sample_type;

```

Except the 64 is actually a variable. I'd like the typedef to be visible throughout the class, but I want the second parameter (the 64) be an argument when the constructor is called. How can I achieve this, while keeping the typedef visible everywhere?