`std::filesystem::path::operator/(/*args*/)` not working as expected - c++

Question

I have a class with an initialiser list in the constructor where one of the fields I'm initialising is a std::filesystem::path but it doesn't seem to be initialising to the expected value.
MyClass::MyClass(
unsigned int deviceSerial,
const std::string& processName
) :
deviceSerial(deviceSerial),
processName(processName),
configFilePath(GetBasePath() / std::to_string(deviceSerial) / ("#" + processName + ".json"))
{
/* Parameter checks */
}
Using the debugger I can see that GetBasePath() is returning exactly what I expect (returns std::filesystem::path with correct path) but the / operator doesn't seem to be having an effect. Once inside the body of the constructor I can see that configFilePath is setup to the result of GetBasePath() without the extra info appended.
I'm using MSVS-2019, I have the C++ language standard set to C++17 and in debug mode I have all optimisations disabled.
I have also tested the following in the body of the class and I still see path as simply the result of GetBasePath() and the extra items are not being appended.
{
auto path = GetBasePath(); // path = "C:/Users/Me/Desktop/Devices"
path /= std::to_string(deviceSerial); // path = "C:/Users/Me/Desktop/Devices"
path /= ("#" + processName + ".json"); // path = "C:/Users/Me/Desktop/Devices"
}
On a slight side note I also tried the above test with += instead of /= and I still see the same results.
Edit
As requested, below is a minimal complete and verifiable example.
#include <Windows.h>
#include <cstdio>
#include <filesystem>
#include <memory>
#include <string>
std::string ExpandPath(const std::string &str) {
auto reqBufferLen = ExpandEnvironmentStrings(str.c_str(), nullptr, 0);
if (reqBufferLen == 0) {
throw std::system_error((int)GetLastError(), std::system_category(),
"ExpandEnvironmentStrings() failed.");
}
auto buffer = std::make_unique<char[]>(reqBufferLen);
auto setBufferLen =
ExpandEnvironmentStrings(str.c_str(), buffer.get(), reqBufferLen);
if (setBufferLen != reqBufferLen - 1) {
throw std::system_error((int)GetLastError(), std::system_category(),
"ExpandEnvironmentStrings() failed.");
}
return std::string{buffer.get(), setBufferLen};
}
int main() {
unsigned int serial = 12345;
std::string procName = "Bake";
std::filesystem::path p(ExpandPath("%USERPROFILE%\\Desktop\\Devices"));
std::printf("Path = %s\n", p.string().c_str());
// p = C:\Users\Me\Desktop\Devices
p /= std::to_string(serial);
std::printf("Path = %s\n", p.string().c_str());
// p = C:\Users\Me\Desktop\Devices
p /= "#" + procName + ".json";
std::printf("Path = %s\n", p.string().c_str());
// p = C:\Users\Me\Desktop\Devices
std::getchar();
}
I've also used this example and tested with `p.append()` and got the same result.

Answer 1

I'd like to give thanks to #rustyx and #Frank for their suggestions, following this advice has led me to discover a bug in the way I create the initial string that gets passed to the path constructor (Also #M.M who found the exact bug while I was typing this answer)
I created a function (that is in use in my class) std::string ExpandPath(const std::string& path) which uses the Windows API to expand any environment variables in a path and return a string. This string is generated from a char* and a count, that count includes the null byte so when creating an string using the constructor variant std::string(char* cstr, size_t len) this includes the null byte in the string itself.
Because I was using the debugger to interrogate the variables it reads C-style strings and stops at the null byte. In my original example I also use printf() as I just happen to prefer this function for output, but again this stops printing at the null byte. If I change the output to use std::cout I can see that the output has the expected path but with an extra space their (the null byte being printed as a space). Using std::cout I see that my paths result as the following with each append:
Path = C:\Users\Me\Desktop\Devices
Path = C:\Users\Me\Desktop\Devices \12345
Path = C:\Users\Me\Desktop\Devices \12345\#Bake.json
Summary:
Bug in my ExpandPath() where
return std::string{buffer.get(), setBufferLen};
Should be
return std::string{buffer.get(), setBufferLen - 1};