template_tricks

template_tricks

```
#include <string.h>
#include <memory>  // for std::unique_ptr
#include <mutex>  // for std::mutex
#include <iterator>
 
template <class T, class elem_type = class T::value_type>
class ring_iterator;  // forward declaration
 
template <class T, size_t N>
class ring {
public:
    typedef size_t size_type;
    typedef T value_type;
private:  // don't change order of definitions
    size_type cap_;  // the capacity of the array
    size_type size_;  // the number of elements in the array
    uint8_t mask_;
    std::unique_ptr<T[]> array_{nullptr};
    size_type head_;  // read/consume/front
    size_type tail_;  // write/produce/back
    bool empty_;  // true if empty
    std::mutex mutex_{};
public:
    ring(uint8_t m): cap_{N}, size_{}, mask_{m}, array_{std::unique_ptr<T[]>(new T[cap_])}, head_{}, tail_{}, empty_{true} { memset(array_.get(), 0, (size_type)cap_ * sizeof(T)); }
    friend class ring_iterator<T, class T::value_type>;  // here's the friending
};
 
template <class T, class elem_type>
class ring_iterator {
public:
    typedef size_t size_type;
    typedef T value_type;
private:
    T *ring_;
    size_type offset_;
public:
    ring_iterator(T *r, size_type o) : ring_{r}, offset_{o} {
    }
};
 
int main(int argc, char* argv[]) {
 
    uint8_t t = 0;  // usually it's a bit mask
    ring<uint8_t, 8> ring(t);  // this line causes the trouble
  
    //...
 
    return 0;
}
```

https://www.learncpp.com/cpp-tutorial/134-template-non-type-parameters/