Filesystem V3 Design

| Filesystem Version 3 Design

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Contents
Introduction Problem Solution Details Other changes Acknowledgements

Caution: This page documents thinking early in the V3 development process, and is intended to serve historical purposes. It is not updated to reflect the current state of the library.

## Introduction

During the review of Boost.Filesystem.V2 (Internationalization), Peter Dimov suggested that the `basic_path` class template was unwieldy, and that a single path type that accommodated multiple character types and encodings would be more flexible. Although I wasn't willing to stop development at that time to explore how this idea might be implemented, or to break from the pattern for Internationalization used the C++ standard library, I've often thought about Peter's suggestion. With the advent of C++0x `char16_t` and `char32_t` character types, the `basic_path` class template approach becomes even more unwieldy, so it is time to revisit the problem in light of Peter's suggestion.

## Problem

With Filesystem.V2, a path argument to a user defined function that is to accommodate multiple character types and encodings must be written as a template. Do-the-right-thing overloads or template metaprogramming must be employed to allow arguments to be written as string literals. Here's what it looks like:

>     template<class Path>
>     void foo( const Path & p );

>     inline void foo( const path & p )
>     {
>       return foo<path>( p );
>     }
>     inline void foo( const wpath & p )
>     {
>       return foo<wpath>( p );
>     }

That's really ugly for such a simple need, and there would be a combinatorial explosion if the function took multiple Path arguments and each could be either narrow or wide. It gets even worse if the C++0x `char16_t` and `char32_t` types are to be supported.

## Solution

Overview:

- A single, non-template, `class path`.
- Each member function is a template accommodating the various applicable character types, including user-defined character types.
- Hold the path internally in a string of the type used by the operating system API; `std::string` for POSIX, `std::wstring` for Windows.

The signatures presented in Problem collapse to simply:

>     void foo( const path & p );

That's a signification reduction in code complexity. Specification becomes simpler, too. I believe it will be far easier to teach, and result in much more flexible user code.

Other benefits:

- All the polymorphism still occurs at compile time.
- Efficiency is increased, in that conversions of the encoding, if required, only occur once at the time of creation, not each time the path is used.
- The size of the implementation code drops approximately in half and becomes much more readable.

Possible problems:

- The combination of member function templates and implicit constructors can result in unclear error messages when the user makes simple commonplace coding errors. This should be much less of a problem with C++ concepts, but in the meantime work continues to restrict over aggressive templates via enable_if/disable_if.

## Details

Encoding Conversions
Host system | char string path arguments | wide string path arguments
Systems with `char` as the native API path character type (i.e. POSIX-like systems) | No conversion. | Conversion occurs, performed by the current path locale's `codecvt` facet.
Systems with `wchar_t` as the native API path character type (i.e. Windows-like systems). | Conversion occurs, performed by the current path locale's `codecvt` facet. | No conversion.

When a class path function argument type matches the operating system's API argument type for paths, no conversion is performed rather than conversion to a specified encoding such as one of the Unicode encodings. This avoids unintended consequences, etc.

## Other changes

Uniform hybrid error handling: The hybrid error handling idiom has been consistently applied to all applicable functions.

## Acknowledgements

Peter Dimov suggested the idea of a single path class that could cope with multiple character types and encodings. Walter Landry contributed both the design and implementation of the copy_any, copy_directory, copy_symlink, and read_symlink functions.

© Copyright Beman Dawes, 2008

Use, modification, and distribution are subject to the Boost Software License, Version 1.0. See www.boost.org/LICENSE_1_0.txt