```cpp
BlockEmergeData::~BlockEmergeData()
{
        for (cb_with_param_t cb: callbacks) {
                void *cb_param = cb.second;
                delete cb_param;
        }
}
```
My approach here was to delete the resource when the object managing the resource lifetime is itself deleted. I changed the data struct to a class to define this destructor, and it compiles, but I get a warning that deleting a `void*` is undefined.