C++17 std::fs

[maflcko]

C++17 std::fs would be useful for a stronger rename method (#20435) and dropping boost fs in favor of the stdlib (#20744).

However, we currently use bionic to compile gitian binaries, which uses gcc-7 by default. If we are ok switching to gcc-8 on bionic for our gitian builds and everyone compiling from source on bionic, we could start using std::fs

[vasild]

If this would require just one more package to the apt-get install ... line which users already execute, then I don't see why not go forward with this.

apt-get install foo bar ... gcc-8
export CC=gcc8 CXX=g++8
... compile as usual ...

[kiminuo]

@vasild I believe, you need to add -lstdc++fs for gcc-8 to make it work.

It seems it is important where the flag is placed (at least on my Ubuntu 18.04 & gcc-8):

// This is simple "main.cpp" file to test compiling of a program with std::filesystem on gcc-8.

// Ubuntu 18.04
// works:         g++-8 --std=c++17 main.cpp -lstdc++fs 
// does not work: g++-8 --std=c++17 -lstdc++fs main.cpp 

#include <string>
#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main()
{
    std::string path = "C:\\cpp_filesystem";
    for (auto & p : fs::directory_iterator(path))
        std::cout << p << std::endl;
}

It would be great if somebody could verify my claim though.

Some resources:

• https://stackoverflow.com/questions/53201991/how-to-use-stdfilesystem-on-gcc-8
  • See comment:
    

[promag]

I've verified @kiminuo claim:

docker run --rm -it ubuntu:bionic
apt update && apt install gcc-8 g++-8
# build above example

Edit: @kiminuo however it shouldn't matter here, as we compile and link separately.

[promag]

Concept ACK.

[kiminuo]

@promag I was unlucky to go through link phase when doing:

./autogen.sh && ./configure CC=gcc-8 CXX="g++-8" LDFLAGS="-lstdc++fs" --with-incompatible-bdb && make -j10

on kiminuo:feature/2020-06-replace-boost-filesystem-with-cpp17-filesystem git branch (#19245).

It would be great help resolving this.

Note: It has helped me to add manually in the generated makefile -lstdc++fs as follows:

bitcoin-cli$(EXEEXT): $(bitcoin_cli_OBJECTS) $(bitcoin_cli_DEPENDENCIES) $(EXTRA_bitcoin_cli_DEPENDENCIES) 
	@rm -f bitcoin-cli$(EXEEXT)
	$(AM_V_CXXLD)$(bitcoin_cli_LINK) $(bitcoin_cli_OBJECTS) $(bitcoin_cli_LDADD) $(LIBS) -lstdc++fs

[promag]

@kiminuo oh it also happens just linking.

[practicalswift]

Concept ACK

[vasild]

Concept ACK

I gather gcc 9 does not require -lstdc++fs. Is it as available as gcc 8 on the relevant platforms?

[kiminuo]

@vasild

I gather gcc 9 does not require -lstdc++fs. Is it as available as gcc 8 on the relevant platforms?

gcc-9 is not supported on Ubuntu 18.04 as far as I know. Ubuntu 20.04 (focal) has gcc-9 package (https://packages.ubuntu.com/search?suite=focal&arch=any&searchon=names&keywords=gcc-9). So answer is probably no.

[fanquake]

I will put together some changes for this 📁 .

[jonatack]

Concept ACK

[maflcko]

We've had reports of people unable to compile because of outdated compilers that can't handle the C++17 in our master branch. Once the 22.0 release is out, we might get more complaints about that. I think bumping the compiler versions even higher for the 22.0 release might be a bit too aggressive and rushed.

Assigned 23.0 for now, so that we can collect some feedback after the 22.0 release.

[maflcko]

Any further discussion can be done in the pull