Iterator Lab                

In this lab, we will look at iterating over a non-container. We could have just as well called this the “Virtual Container Lab”, but the multiple meanings of virtual would confuse things.                 

The files for this lab are in ~cs253/Lab/Iterator. Copy them to a temporary directory.                 

Create a file called recit12.txt, and turn it in for credit.                 

A Non-Container                

If someone asked you to name the continents, you might say:

• Africa
• Asia
• North America
• South America
• Australia
• Antarctica

(Then, an enjoyable argument about Europe would break out.)                 

Did you actually have that list of continents written down, and you iterated over them? Probably not. Instead, you generated that list, as needed. Similarly, in C++, we sometimes want to iterate over “containers” that don’t really hold anything, but instead have dynamically-generated contents.                 

Directory Iteration with Linux System Calls                

Consider this program, dir-simple.cc:                 

#include <iostream>     // for cout
#include <string>       // for "…"s
#include <dirent.h>     // for opendir, readdir, closedir

using namespace std;

int main() {
    DIR *dp = opendir(".");
    while (dirent *d = readdir(dp))
        if (d->d_name != "."s && d->d_name != ".."s)
            cout << "Filename: " << d->d_name << '\n';
    closedir(dp);
    return 0;
}

Note that d->d_name is a C string, but "…"s is a C++ string, so != comparison works.                 

It displays all the files in the current directory (“.”) except for the current directory itself (“.”) and the parent directory (“..”). It does so with the opendir() / readdir() / closedir() functions.                 

Every time readdir() is called, it reads another entry from the directory, in a system-dependent manner. It returns a pointer to a struct that contains the d_name field, a C-style string. If readdir() runs out of names in the directory, it returns a null pointer.                 

This program is certainly not in our usual C++ style, but it works. The opendir() / readdir() / closedir() functions are not part of any C++ standard, though they are POSIX, which is quite good. Until C++17, which is still in the future, as far as this class is concerned, there was no standard C++ way to read a directory, so you take what you can get.                 

Directory Iteration in the C++ Manner                

dir-object.cc is more in C++ style:                 

#include <iostream>
#include "Directory.h"

using std::cout;

int main() {
    Directory dir(".");
    for (auto name : dir)
        cout << "Filename: " << name << '\n';
}

We have a “container”, of type Directory, initialized with “.”, the current directory. We then iterate over that container using a typical for-each loop.                 

Of course, there’s complexity hidden in Directory.h and Directory.cc. Unsurprisingly, they use the opendir() / readdir() / closedir() functions.                 

Understanding the code                

As a group, discuss and understand Directory.h and Directory.cc.                 

Points to consider:

• Why doesn't the Directory ctor just read everything into a vector?
• What about iterator post-increment?
• What about iterator ==?

Exercises                

1. What happens when the Directory ctor is given a non-existant directory name? Try it. Fix it, commenting the changes with “Fix #1”.
2. It’s cumbersome for the . and .. filtering code to be in operator++. Make it a separate, private method called .wanted(), commenting the changes with “Fix #2”.
3. Add a second, optional, argument to the Directory ctor that specifies a string to match. For example, Directory foo(".", "cat") would yield catalog, tomcat, and vacate, but not zulu. Do the pattern-matching in your new .wanted() method. If the string is not supplied to the constructor, then return all files except for . and ... Commenting the changes with “Fix #3”.
4. Copy your Directory.h and Directory.cc to recit12.txt.