Why do boost::filesystem::path and std::filesystem::path lack operator+?

问题

Consider the following assertions about path decomposition, where each local variable e.g. stem has the obvious initialization e.g. auto stem = path.stem() —

assert(root_path == root_name / root_directory);
assert(path == root_name / root_directory / relative_path);
assert(path == root_path / relative_path);

assert(path == parent_path / filename);
assert(filename == stem + extension);

This all works, except for the last line — because fs::path does not define an operator+. It has operator+=, but no operator+.

What's the story here?

---

I've determined that I can make this code compile by adding my own operator+. Is there any reason not to do this? (Notice this is in my own namespace; I'm not reopening namespace std.)

fs::path operator+(fs::path a, const fs::path& b)
{
    a += b;
    return a;
}

My only hypotheses on the subject are:

• Maybe the designers worried that operator+ would be too easily confused with std::string's operator+. But that seems silly, since it does exactly the same thing semantically (so why care if it gets conflated?). And it also seems that the designers didn't care about newbies' confusion when they designed path.append("x") to do something semantically different from str.append("x") and path.concat("x") to do something semantically the same as str.append("x").

• Maybe path's implicit conversion operator string_type() const would cause some variety of p + q to become ambiguous. But I've been unable to come up with any such case.

回答1:

This was entered as a defect against the filesystem library, and because of the interface complexity and the ability to work around by converting to strings and back to paths, it was judged to not be a defect. Read all about it here: https://timsong-cpp.github.io/lwg-issues/2668

来源：https://stackoverflow.com/questions/46137433/why-do-boostfilesystempath-and-stdfilesystempath-lack-operator