The C++ community is gradually moving towards the STL paradigm—containers, iterators, function objects, algorithms and adaptors—which provides great advantages in terms of commonality of expression, reduction in developer effort, and greater robustness, maintainability and reuse. However, apart from the standard library and one or two exceptions (Boost , STLSoft ), there is relatively little STL-compliant code, which is, in part, due to the complexities involved in its development.
In this article I'll demonstrate how one kind of enumeration API—the UNIX
opendir()/readdir() API—can be mapped into an STL-compliant sequence-like class providing Input Iterators. I examine the implementation of
readdir_sequence, from UNIXSTL, the UNIX-specific sub-project of STLSoft ().
Before getting into the details, consider the advantages of an STL-compliant approach. Let's say you need to load the names of the sub-directories of
/home/matty/ into a vector of strings. Using the raw
opendir() API, your code might look something like that shown in Listing 1.
It's not an enormous amount of code, to be sure, but it's still quite a bit for something so conceptually straightforward. Let's look at the
readdir_sequence-based version, in Listing 2. It's clear in comparing the two code snippets that
readdir_sequence represents a big win over the version using the
opendir() API. There are several advantages:
closedir(), is automatically handled, via Resource Acquisition Is Initialisation, which improves robustness. Indeed, the first example is not exception-safe, since the
std::stringconstructor and the
std::vector<>::push_back()method may throw exceptions; the second version is exception-safe.
The one flaw in the design is that the value_type is
struct dirent const*, which means you have to explicitly enumerate the entries, rather than use algorithms or iterator-based constructors (see Sidebar).
Let's look now how it's implemented. Listing 3 shows the definition of the readdir_sequence class. It provides
end() methods, which return iterators of member type
const_iterator, since only non-mutating access to the entries is provided. Hence,
readdir_sequence provides a read-only view on a directory.
It also provides an
empty() method, which tests
end(). I've deliberately omitted a
size() member, since the size could only be obtained by conducting an enumeration over the range, which is a costly operation. Not providing this method is an unequivocal documentation of this fact . If one really wants to calculate the range size, it can be done via
For convenience to client code, a
get_directory() method is provided which returns the search directory, having ensured, in the constructor, that it has a trailing path name separator ('/'). This means that if you need to express the returned values in absolute form, you can do so simply, as in:
dirNames.push_back(dir.get_directory() + (*b)->d_name);
The two parameters to the constructor are the directory to search and the flags, which control the search. I'll look at these shortly when I discuss the
Since all the member variables of this class are fully-fledged value types, there is no need to proscribe or provide explicit implementations of either the copy constructor or the copy assignment operator. The compiler provided ones will work quite nicely and safely.
Another notable aspect to the class is that the type of the string member,
m_directory, depends on whether the symbol
PATH_MAX is defined . It if is, then the operating environment has a fixed maximum path limit, and the class uses the STLSoft string
basic_static_string, which has an internal array of the given size, so there's no memory allocation involved. If
PATH_MAX is not defined, then the operating environment does not have a fixed maximum path limit, so the STLSoft string
basic_simple_string is used instead . It's a small efficiency, certainly, but I'm funny like that.
A further saving comes from the use of the
m_scratch member. Since the directory is constant for the life of the sequence/iterator, and it's contents are not revealed directly outside the iterator instance, you are able to reuse it for constructing the full path names of the entries, in order to call
stat() on them. The
m_dirLen member remembers the original length of the directory alone, so it can be truncated to that length (which includes path name separator) ready for each entry name to be appended to it.
opendir() API provides a two-step process to directory enumeration (in contrast to, say, Win32's
FindFirstFile()), so the sequence class starts the enumeration—in its
begin() method—and hands the DIR pointer over to the iterator to walk through the matched items.
Listing 4 shows the definition of the
readdir_sequence::const_iterator class, which is where all the action happens. Because
opendir() affords only single-pass manipulation, only the Input Iterator concept  is supported. Iterator copy construction and copy assignment semantics are supported by the use of a reference-counted shared handle, of type
readdir_sequence::const_iterator::rds_shared_handle , to support iterator instance copying. Multiple concurrent enumerations may be conducted by calling
begin() but since we're dealing with file systems, whose contents may change at any time as a result of other processes, subsequent ranges obtained from
begin() may not contain the same elements.
opendir() API returns all the entries in a particular directory, irrespective of whether they are files or directories, and includes the dots directories—"." and "..". Specifying files without directories, or vice versa, as the second argument to the
readdir_sequence constructor causes only the matching entries to be returned, which is a nice convenience. If neither are specified, it defaults to returning both, because this conforms to the API's behaviour and also because it is more efficient, as you'll see shortly. Since most directory enumeration—in my experience, at least—is not concerned with the dots directories, they are elided from the enumeration range by default. Specifying the
readdir_sequence::includeDots flag causes them to be included in the range.
Entry filtering is performed within
operator++(). Detection of files and/or directories is done by calling
stat(), but this is only called when one type or the other is to be returned; hence no unnecessary costs ensue . If the entry does not match, the
for loop is not broken, and the next entry is retrieved. Dot elision is similarly done by detecting whether the entry is "." or ".." . When
NULL the enumeration is complete, and the iterator enters a state whereby a comparison with that returned by
end() would evaluate to true, terminating the client iteration loop (assuming it's written correctly!).
In my next article, I'll describe the mapping of the UNIX
glob() API, which supports a more refined STL Iterator model , and presents a number of different challenges to providing a simple and efficient sequence class. If you're interested, you can download the STLSoft libraries here.
std::basic_string, since you can use algorithms for them. It is compatible with the IOStreams, while having no knowledge of them whatsoever. But I'm not given to attempting to sell its use to others; we can settle on it being an internal STLSoft implementation component.
readdir_sequenceis not written to be thread-safe, so
rds_shared_handledoes not use atomic integer operations. If you want thread-safety, you must handle that in client code.
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