I got the C++ code as follows:

```
 string cachefile = filename + ".cache";
 ifstream cache(cachefile.c_str(), ios::binary);

```

As I did not find it in a C++ reference, what are these codes doing (like `ifstream` etc)? And what is the cache file please? Why should it be created? What are the advantages and how to interpret it? Above all, what is the function of the above codes?