Managing resources

Suppose you are writing a function that opens a file, reads from it, and closes it afterward. You are developing on a procedural platform (like most operating system APIs are) offering a set of functions to perform these tasks. Note that all “operating system” functions in this example are deliberately fictional but resemble their real-world counterparts. The functions interesting to us in that API are:

// opens the file called "name", returns a pointer
// to a file descriptor for that file (nullptr on failure)
FILE *open_file(const char *name);
// returns the number of bytes read from the file into
// buf. Preconditions: file is non-null and valid, buf
// points to a buffer of at least capacity bytes, and
// capacity >= 0
int read_from(FILE *file, char *buf, int capacity);
// closes file. Precondition: file is non-null and valid,
void close_file(FILE *file);

Suppose your code needs to process the data read from the file, but that...