C++17 std::fs #20460

1. 
   MarcoFalke commented at 10:58 am on November 23, 2020: member

   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

2. MarcoFalke added the label Brainstorming on Nov 23, 2020
3. MarcoFalke added the label Build system on Nov 23, 2020
4. 
   vasild commented at 11:25 am on November 23, 2020: member

   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.

   0apt-get install foo bar ... gcc-8
   1export CC=gcc8 CXX=g++8
   2... compile as usual ...
   

5. 
   kiminuo commented at 11:35 am on November 23, 2020: contributor

   @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):

    0// This is simple "main.cpp" file to test compiling of a program with std::filesystem on gcc-8.
    1
    2// Ubuntu 18.04
    3// works:         g++-8 --std=c++17 main.cpp -lstdc++fs 
    4// does not work: g++-8 --std=c++17 -lstdc++fs main.cpp 
    5
    6#include <string>
    7#include <iostream>
    8#include <filesystem>
    9namespace fs = std::filesystem;
   10
   11int main()
   12{
   13    std::string path = "C:\\cpp_filesystem";
   14    for (auto & p : fs::directory_iterator(path))
   15        std::cout << p << std::endl;
   16}
   

   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:
6. 
   promag commented at 11:44 am on November 23, 2020: member

   I’ve verified @kiminuo claim:

   0docker run --rm -it ubuntu:bionic
   1apt update && apt install gcc-8 g++-8
   2# build above example
   

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

7. 
   promag commented at 11:45 am on November 23, 2020: member

   Concept ACK.
8. 
   kiminuo commented at 12:51 pm on November 23, 2020: contributor

   @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:

   0bitcoin-cli$(EXEEXT): $(bitcoin_cli_OBJECTS) $(bitcoin_cli_DEPENDENCIES) $(EXTRA_bitcoin_cli_DEPENDENCIES)	[@rm](/bitcoin-bitcoin/contributor/rm/) -f bitcoin-cli$(EXEEXT)
   1	$(AM_V_CXXLD)$(bitcoin_cli_LINK) $(bitcoin_cli_OBJECTS) $(bitcoin_cli_LDADD) $(LIBS) -lstdc++fs
   

9. 
   promag commented at 1:00 pm on November 23, 2020: member

   @kiminuo oh it also happens just linking.
10. 
    practicalswift commented at 2:54 pm on November 23, 2020: contributor

    Concept ACK
11. 
    vasild commented at 1:54 pm on November 24, 2020: member

    Concept ACK

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

12. 
    kiminuo commented at 2:05 pm on December 15, 2020: contributor

    @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.

13. 
    fanquake commented at 7:23 am on December 17, 2020: member

    I will put together some changes for this 📁 .
14. 
    jonatack commented at 9:42 pm on March 2, 2021: member

    Concept ACK
15. MarcoFalke added this to the milestone 23.0 on Mar 3, 2021
16. 
    MarcoFalke commented at 7:27 am on March 3, 2021: member

    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.

17. 
    MarcoFalke commented at 1:43 pm on October 21, 2021: member

    Any further discussion can be done in the pull
18. MarcoFalke closed this on Oct 21, 2021

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19. DrahtBot locked this on Oct 30, 2022

Contributors
MarcoFalke vasild kiminuo promag practicalswift fanquake jonatack

Labels
Brainstorming Build system

Milestone
23.0