Concatenation of multiple absolute std::filesystem::path instances

Question

Why is the result of concatenating the following paths /c/d?

std::filesystem::path{"/a"} / std::filesystem::path{"b"} / std::filesystem::path{"/c/d"}

My mental model so far had the resulting path set to /a/b/c/d. To my surprise, it was simply /c/d. Curious to know where I went wrong here. (And what the right mental model is)

Answer 1

/a is an absolute path, and b is a relative path, so concatenating them will produce /a/b.

But /c/d is also an absolute path, so concatenating it with anything in front of it is basically a no-op, the absolute path will take priority, so the final result is just /c/d.

This is discussed in more detail on cppreference.com:

std::filesystem::operator/(std::filesystem::path)

Concatenates two path components using the preferred directory separator if appropriate (see operator/= for details).

Effectively returns path(lhs) /= rhs.

std::filesystem::path::operator/=

path& operator/=( const path& p );

If p.is_absolute() || (p.has_root_name() && p.root_name() != root_name()), then replaces the current path with p as if by operator=(p) and finishes.

To get the result you want, drop the leading / from /c/d to make it a relative path:

std::filesystem::path{"/a"} / std::filesystem::path{"b"} / std::filesystem::path{"c/d"}

Answer 2

what the right mental model is

a / b means "if b is not absolute, then it's relative to a".

This lets you have a "default" base directory, where the rhs is assumed to be located, if the path is not absolute.