filesystem (C++17)

filesystem (C++17)

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Prerequisites

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1. What is a filesystem?

<filesystem> provides a standard, portable way to work with files and directories.

Before C++17:

• OS-specific APIs (POSIX / Windows)
• Boost.Filesystem

Now:

• Built into the standard library
• Cross-platform

#include <filesystem>

namespace fs = std::filesystem;

Component	Description
fs::path	represents file path
fs::directory_iterator	iterate directory
fs::exists	check existence
fs::create_directory	create folder
fs::remove	delete file
fs::file_size	get file size

Use it when:

• file/directory manipulation
• cross-platform file handling
• writing tools, scripts, utilities
• logging, file scanning

2. filesystem functions

fs::path

Represents a file system path.

fs::path p = "folder/file.txt";

p.filename();        // "file.txt"
p.extension();       // ".txt"
p.stem();            // "file"
p.parent_path();     // "folder"

fs::path p = "folder";
p /= "file.txt";   // folder/file.txt

/= is preferred over string concatenation

Checking Files

if (fs::exists("file.txt"))
    std::cout << "File exists\n";

Type Check

fs::is_regular_file(p);
fs::is_directory(p);

Directory Operations

Create Directory

fs::create_directory("new_folder");

Create Nested

fs::create_directories("a/b/c");

Remove

fs::remove("file.txt");       // file
fs::remove_all("folder");     // recursive

Iterating Directory

for (const auto& entry : fs::directory_iterator("folder"))
    std::cout << entry.path() << "\n";

folder/
 ├── a.txt
 ├── b.txt
 └── sub/
      └── c.txt

Result:
folder/a.txt
folder/b.txt
folder/sub

Recursive Iteration

for (const auto& entry : fs::recursive_directory_iterator("folder"))
    std::cout << entry.path() << "\n";

folder/
 ├── a.txt
 ├── b.txt
 └── sub/
      └── c.txt

Result:
folder/a.txt
folder/b.txt
folder/sub
folder/sub/c.txt

File Size & Info

auto size = fs::file_size("file.txt");

Last Modified Time

auto time = fs::last_write_time("file.txt");

Copy / Move Files

fs::copy("source.txt", "dest.txt");

Move / Rename

fs::rename("old.txt", "new.txt");

Error Handling

try
{
    fs::remove("file.txt");
}
catch (const fs::filesystem_error& e)
{
    std::cout << e.what();
}

Non-throwing Version

std::error_code ec;
fs::remove("file.txt", ec);

if (ec)
    std::cout << ec.message();

3. Performance Considerations

System Calls

Many operations call the OS:

• exists
• file_size
• directory_iterator

These are relatively expensive

• Avoid repeated exists() calls
• Cache results if possible
• Prefer batch operations