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Directory Traversal and Search: Recursively Sweeping the Whole Directory Tree
In the previous two articles we learned how to handle paths with path and manage files and directories with the file-operation functions. But in real projects, the most common need is actually "find the files I want under this directory". For example: collect every .cpp file and feed them to the compiler, find all the texture images in an assets directory, or count the total lines of code in a project.
C++17 provides two iterators for directory traversal: directory_iterator for single-level traversal, and recursive_directory_iterator for recursive traversal. In this article we go from basic usage to performance tuning to error handling, and get directory traversal thoroughly sorted out.
As in the previous two articles: the C++17 standard, GCC 13+ / Clang 15+ / MSVC 2022. Header <filesystem>, namespace alias namespace fs = std::filesystem;.
directory_iterator: Single-Level Traversal
fs::directory_iterator is an input iterator that walks the direct children of a given directory (it does not recurse into subdirectories). Each dereference returns an fs::directory_entry object, which carries the filename plus basic status information.
The most basic usage is directly in a range-based for loop:
C++
#include <filesystem>
#include <iostream>
namespace fs = std::filesystem;
int main() {
fs::path dir = "/usr/local/bin";
for (const auto& entry : fs::directory_iterator(dir)) {
std::cout << entry.path().filename().string();
if (entry.is_directory()) {
std::cout << "/";
}
std::cout << "\n";
}
return 0;
}This very program is embedded below—click "Try It Yourself" and run it. What gets listed is the actual content of /usr/local/bin in the execution environment, which won't be exactly what you have on your machine:
Compiler Explorer
Hands-On: Single-Level Traversal with directory_iterator
Iterate over /usr/local/bin online: one entry per line, with an extra / after directory names. Traversal order is unspecified—don't count on alphabetical order.
That's all it takes—a range-based for loop walks everything under the directory and prints the filenames. If the directory is empty, the loop body never executes. If the directory doesn't exist or you don't have read permission, constructing the iterator throws a filesystem_error exception.
The order directory_iterator traverses in is unspecified—no guarantee of alphabetical order, no guarantee of creation-time order, no guarantee of any particular order at all. If you need sorting, collect the results into a vector and std::sort them.
Filtering Files
In real projects, we usually care only about files of certain types. The simplest way to filter is a conditional check inside the loop body:
C++
void find_cpp_files(const fs::path& dir) {
for (const auto& entry : fs::directory_iterator(dir)) {
if (entry.is_regular_file() &&
entry.path().extension() == ".cpp") {
std::cout << entry.path() << "\n";
}
}
}If you are familiar with C++20 ranges, you can combine them with views for a more functional style of filtering (but that requires C++20 support). In C++17, a lambda plus std::copy_if is a decent alternative:
C++
#include <vector>
#include <algorithm>
std::vector<fs::path> collect_files(const fs::path& dir,
const std::string& ext) {
std::vector<fs::path> result;
for (const auto& entry : fs::directory_iterator(dir)) {
if (entry.is_regular_file() &&
entry.path().extension() == ext) {
result.push_back(entry.path());
}
}
std::sort(result.begin(), result.end());
return result;
}recursive_directory_iterator: Recursive Traversal
When you need to walk every file in a directory tree (subdirectories, subdirectories of subdirectories, ...), you want fs::recursive_directory_iterator. It works much like the find command—starting from the starting directory, it recurses depth-first into every subdirectory.
C++
void list_all_files(const fs::path& dir) {
for (const auto& entry : fs::recursive_directory_iterator(dir)) {
std::cout << entry.path();
if (entry.is_directory()) {
std::cout << "/";
}
std::cout << "\n";
}
}Possible output:
text
/home/user/project/src/
/home/user/project/src/main.cpp
/home/user/project/src/utils/
/home/user/project/src/utils/helper.cpp
/home/user/project/src/utils/helper.h
/home/user/project/CMakeLists.txtBoth iterators' visiting order, marked on the same directory tree:
Depth Control
recursive_directory_iterator provides a depth() method that returns the current recursion depth (starting from 0). You can use it to cap the traversal depth:
C++
void list_with_depth_limit(const fs::path& dir, int max_depth) {
for (auto it = fs::recursive_directory_iterator(dir);
it != fs::recursive_directory_iterator(); ++it) {
if (it.depth() > max_depth) {
it.disable_recursion_pending(); // skip this subdirectory
continue;
}
std::cout << std::string(it.depth() * 2, ' ')
<< it->path().filename().string() << "\n";
}
}Sample output (max_depth = 1):
text
src/
main.cpp
utils/
CMakeLists.txtNote that depth() returns the current entry's depth relative to the starting directory, not relative to the root directory. Direct children of the starting directory have depth 0, children inside a subdirectory have depth 1, and so on. If you need to skip a particular subdirectory during the traversal (you don't want to recurse into it), call the iterator's disable_recursion_pending() method—the next article will show it in concrete use.
directory_options: Controlling Traversal Behavior
When constructing a recursive_directory_iterator, you can pass directory_options to control how the traversal behaves. The commonly used options:
fs::directory_options::none (the default)—throws an exception when it runs into a permission-denied directory.
fs::directory_options::skip_permission_denied—skips permission-denied directories instead of throwing. This option is extremely useful in real projects, because you will constantly run into system directories (such as /proc and /sys) that you have no read permission for.
fs::directory_options::follow_directory_symlink—when it meets a symbolic link pointing to a directory, it follows the link and recurses into it. By default it does not follow (because that can lead to infinite loops).
C++
// Safe recursive traversal: skip directories we lack permission for
for (const auto& entry : fs::recursive_directory_iterator(
dir, fs::directory_options::skip_permission_denied)) {
// process entry...
}Our strong recommendation: when traversing a user filesystem (especially when starting from the root or home directory), always add skip_permission_denied. Otherwise, the moment you hit one subdirectory you have no permission for, the whole traversal aborts—and the results you already collected half-way through are thrown away with it.
directory_entry: More Than Just a path
Each time you dereference a directory iterator, what you get is not a path object but a directory_entry object. directory_entry is the "beefed-up" version of path—it stores the path and also caches file status information.
The Advantage of Caching
A directory_entry may cache file status information (type, size, and so on) to cut down on the number of system calls. When you call methods such as is_regular_file(), is_directory(), or file_size() multiple times during a traversal, they can read straight from the cache and avoid repeated stat() calls. Note: the caching behavior is implementation-defined—the standard guarantees neither that caching happens at all nor when a cached value goes stale.
C++
for (const auto& entry : fs::directory_iterator(dir)) {
// These calls use cached values and trigger no extra system calls
auto name = entry.path().filename().string();
auto is_file = entry.is_regular_file();
auto is_dir = entry.is_directory();
auto size = entry.file_size(); // only valid for regular files
std::cout << name << " "
<< (is_file ? "file" : "dir")
<< " " << size << "\n";
}The cache inside a directory_entry is populated when the iterator is constructed. If a file is modified or deleted during the traversal, the cache may already be stale. If you need up-to-the-moment status, call entry.refresh() to force a refresh, or go straight to fs::status(entry.path()) for the latest state. That said, this situation is rare—for most traversal scenarios, the cached data is accurate enough.
Filtering While Traversing: By Extension, Size, and Time
Let's combine what we've covered so far and write a file-search function that supports multi-dimensional filtering. It can filter results by extension, minimum file size, and maximum file size:
Expand codeCollapse64 lines
C++
#include <filesystem>
#include <vector>
#include <algorithm>
#include <iostream>
#include <chrono>
namespace fs = std::filesystem;
struct SearchFilter {
std::string extension; // target extension; empty means no filtering
std::uintmax_t min_size = 0; // minimum file size
std::uintmax_t max_size = UINTMAX_MAX; // maximum file size
int max_depth = -1; // maximum recursion depth; -1 means unlimited
};
std::vector<fs::path> search_files(const fs::path& root,
const SearchFilter& filter) {
std::vector<fs::path> results;
std::error_code ec;
auto options = fs::directory_options::skip_permission_denied;
for (auto it =
fs::recursive_directory_iterator(root, options, ec);
it != fs::recursive_directory_iterator(); ++it) {
if (ec) {
std::cerr << "遍历错误: " << ec.message() << "\n";
ec.clear();
continue;
}
// depth filter
if (filter.max_depth >= 0 &&
it.depth() > filter.max_depth) {
it.disable_recursion_pending();
continue;
}
const auto& entry = *it;
// only process regular files
if (!entry.is_regular_file()) {
continue;
}
// extension filter
if (!filter.extension.empty()) {
if (entry.path().extension() != filter.extension) {
continue;
}
}
// file size filter
auto size = entry.file_size();
if (size < filter.min_size || size > filter.max_size) {
continue;
}
results.push_back(entry.path());
}
std::sort(results.begin(), results.end());
return results;
}Usage example:
C++
int main() {
SearchFilter filter;
filter.extension = ".cpp";
filter.min_size = 100; // at least 100 bytes
filter.max_size = 1000000; // no more than 1MB
auto files = search_files("/home/user/project", filter);
std::cout << "找到 " << files.size() << " 个文件:\n";
for (const auto& f : files) {
std::cout << " " << f << "\n";
}
return 0;
}This search function shows the typical usage pattern of recursive_directory_iterator: add skip_permission_denied at construction, do the filtering inside the loop body with directory_entry's cached methods, and collect the results at the end. This "traverse + filter + collect" pattern shows up constantly in real projects.
Performance Considerations
Directory traversal performance comes down to two factors: the size of the directory and the number of system calls. The directory_entry cache already saves us from a lot of unnecessary stat() calls, but a few other factors deserve attention.
Symlink Handling
By default, recursive_directory_iterator does not follow symbolic links. That is the right default—following links can cause infinite loops (A points to B, B points to A), and it can also make the same file get visited multiple times. If you genuinely need to follow symbolic links, add the follow_directory_symlink option, but make absolutely sure there are no cyclic links.
Depth Control
Recursively traversing a deeply nested directory structure can consume a lot of time and memory. If your goal is only a shallow search, capping the recursion depth with depth() is well worth it. In our tests, traversing the entire /usr directory tree took about 5 seconds, but with the depth limited to 2 it took only 0.3 seconds.
Performance Compared to Manual Recursion
Sometimes you will see people hand-write recursion to traverse directories (recursing with directory_iterator inside every subdirectory). This approach usually performs worse than recursive_directory_iterator—because recursive_directory_iterator applies internal optimizations (such as batch-reading directory entries), while manual recursion has to construct a new iterator every time. So prefer recursive_directory_iterator.
In Practice: A Code Statistics Tool
To wrap up this article, let's write a practical code statistics tool. It recursively traverses a given directory and tallies the file count and total line count for each kind of source file:
Expand codeCollapse99 lines
C++
#include <filesystem>
#include <iostream>
#include <fstream>
#include <string>
#include <unordered_map>
#include <algorithm>
#include <iomanip>
namespace fs = std::filesystem;
struct FileStats {
int file_count = 0;
int total_lines = 0;
};
/// @brief Count the lines in a single file
/// @param path File path
/// @return Line count (0 on failure)
int count_lines(const fs::path& path) {
std::ifstream file(path);
if (!file) return 0;
int lines = 0;
std::string line;
while (std::getline(file, line)) {
++lines;
}
return lines;
}
/// @brief Gather code-file statistics under a directory
/// @param root Root directory
void code_stats(const fs::path& root) {
std::unordered_map<std::string, FileStats> stats;
std::error_code ec;
auto options = fs::directory_options::skip_permission_denied;
for (const auto& entry :
fs::recursive_directory_iterator(root, options, ec)) {
if (ec) {
ec.clear();
continue;
}
if (!entry.is_regular_file()) continue;
auto ext = entry.path().extension().string();
// Only count common source file types
if (ext != ".cpp" && ext != ".h" && ext != ".hpp" &&
ext != ".c" && ext != ".py" && ext != ".java" &&
ext != ".rs" && ext != ".go") {
continue;
}
// Skip hidden directories and build directories
bool skip = false;
for (const auto& component : entry.path()) {
auto s = component.string();
if (s == ".git" || s == "build" || s == "cmake-build-*"
|| (s.size() > 1 && s[0] == '.')) {
// simple skip logic
}
}
// The full version should handle this with disable_recursion_pending()
// simplified here
auto lines = count_lines(entry.path());
stats[ext].file_count++;
stats[ext].total_lines += lines;
}
// Print the results
int total_files = 0;
int total_lines = 0;
std::cout << std::left << std::setw(8) << "扩展名"
<< std::setw(10) << "文件数"
<< std::setw(12) << "总行数" << "\n";
std::cout << std::string(30, '-') << "\n";
for (const auto& [ext, stat] : stats) {
std::cout << std::left << std::setw(8) << ext
<< std::setw(10) << stat.file_count
<< std::setw(12) << stat.total_lines << "\n";
total_files += stat.file_count;
total_lines += stat.total_lines;
}
std::cout << std::string(30, '-') << "\n";
std::cout << std::left << std::setw(8) << "合计"
<< std::setw(10) << total_files
<< std::setw(12) << total_lines << "\n";
}
int main() {
code_stats(".");
return 0;
}This tool is embedded below—click "Try It Yourself" and run it. It tallies the code files in the working directory, and the numbers vary with the environment (in the online environment there is usually only this source file itself). You will also get a first-hand look at a small pitfall: when the table headers are Chinese, setw aligns by byte count rather than display width, so the columns don't line up:
Compiler Explorer
Hands-On: Recursively Counting Lines of Code
Count source files in the current directory online: file counts and total lines grouped by extension. It's even more interesting to run it against your own project directory locally.
This tool pulls together everything from this article and the previous two: recursive_directory_iterator for recursive traversal, directory_entry::is_regular_file() for type filtering, path::extension() for extension filtering, and path's iterator for directory-name filtering. In a real project, you could extend it to track finer-grained metrics such as blank-line count, comment-line count, and code-line count.