How similar are Boost filesystem and the standard

问题:

I need a filesystem library for use with a C++11-capable compiler or a C++14-capable one - so it can't be be from C++17.

Now, I know that the filesystem library going into C++17 is based based on Boost::Filesystem; but - are they similar enough for me to use the Boost library and then seamlessly switch to the standard version at a later time, without changing more than, say, a using statement? Or are there (minor/significant) differences between the two? I know that for the case of variant, the Boost and the standard library versions differ quite a bit.

回答1:

There are a number of differences. Some were, I believe, Boost changes that were never propagated. For example, there is no path.filename_is_dot() query (as discussed below, it would be less useful in std::filesystem anyway).

There was also a good bit of late-breaking news on this front:

1. Support for non-POSIX-like filesystems:
   • Specify whether a string is OS-native or POSIX-like (or let the implementation decide, which is (still) the default)
   • An implementation may define additional file types (beyond regular, directory, socket, etc.)
   • An implementation may define file_size for a directory or device file
2. filename(), normalization, and relative/absolute conversions redefined (examples for POSIX):
   • path("foo/.").lexically_normal()=="foo/" (is the opposite in Boost)
   • path("foo/").filename()=="" (is path(".") in Boost)
   • remove_filename() leaves the trailing slash and is thus idempotent (it assigns parent_path() in Boost)
   • path(".profile").extension()=="" (is the whole name in Boost)
   • path decompositions and combinations can preserve things like alternate data stream names that are normally invisible
   • path("foo")/"/bar"=="/bar" (is path("foo/bar") in Boost), which allows composing relative file names with others (absolute or relative) and replaces Boost's absolute()
   • Boost's system_complete() (which takes only one argument) is renamed to absolute()
   • canonical() thus takes only one argument (fixed in a DR)
   • lexically_relative() handles .. and root elements correctly
   • permissions() takes more arguments (Boost combines them into a bitmask)

Note that Boost.Filesystem v4 is under development and is supposed to be C++17-compatible (but therefore incompatible in many respects with v3).

回答2:

Caveat: This answer does not reflect several last-minute changes before C++17 was finalized. See @DavisHerring's answer.

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The Boost filesystem inserter and extractor use & as the escape character for " and &.

The standard will use std::quoted (which uses \ by default) to escape ", which in turn use \\ to escape \, see this reference.

Demo

It is likely the only one difference between them.

The reason for that difference can be found at N3399