How do I re-enable C++17 removed features in clang?

Question

I have some 3rdparty libraries that are built as part of a large project. Some libraries require C++17, others require features that are removed in C++17 (std::auto_ptr, std::fun_ptr).

For this project, Clang is used (version 11). From searching this mismatch of required versions shouldn't be a problem if I use the clang definition _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES.

However, this has zero effect. I tried this definition in a tiny test sample:

#include <memory>

int main(void)
{
    std::auto_ptr<int> thing(new int(1));
    return 0;
}

Without specifying a C++ std, it compiles fine.

However, if I specify C++17 it fails with -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES

PS E:\> clang++ -std=c++17 -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES .\autoptr.cpp
.\autoptr.cpp:5:10: error: no member named 'auto_ptr' in namespace 'std'
    std::auto_ptr<int> thing(new int(1));
    ~~~~~^
.\autoptr.cpp:5:22: error: expected '(' for function-style cast or type construction
    std::auto_ptr<int> thing(new int(1));
                  ~~~^
.\autoptr.cpp:5:24: error: use of undeclared identifier 'thing'
    std::auto_ptr<int> thing(new int(1));
                       ^
3 errors generated.

Are c++17 removed features no longer available in Clang or do I need extra compiler options?

Clang version:

PS E:\> clang++ -v
clang version 11.1.0
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\LLVM\bin

edit: Interesting, this compiles fine on a a different system I have. No extra define needed.

#@#:~/kraken$ clang++ -v
clang version 10.0.0-4ubuntu1~18.04.2
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

edit2: Upgraded from 11.1.0 to 12.0.1. No change

$ clang++ -v -stdlib=libc++ -std=c++17 -D_LIBCPP_ENABLE_CXX17_REMOVED_FEATURES -D_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR autoptr.cpp
clang version 12.0.1
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\LLVM\bin
clang++: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
 "C:\\Program Files\\LLVM\\bin\\clang++.exe" -cc1 -triple x86_64-pc-windows-msvc19.29.30038 -emit-obj -mrelax-all -mincremental-linker-compatible --mrelax-relocations -disable-free -disable-llvm-verifier -discard-value-names -main-file-name autoptr.cpp -mrelocation-model pic -pic-level 2 -mframe-pointer=none -fmath-errno -fno-rounding-math -mconstructor-aliases -munwind-tables -target-cpu x86-64 -tune-cpu generic -v -resource-dir "C:\\Program Files\\LLVM\\lib\\clang\\12.0.1" -D _LIBCPP_ENABLE_CXX17_REMOVED_FEATURES -D _LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR -internal-isystem "C:\\Program Files\\LLVM\\lib\\clang\\12.0.1\\include" -internal-isystem "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Professional\\VC\\Tools\\MSVC\\14.29.30037\\include" -internal-isystem "C:\\Program Files (x86)\\Microsoft Visual Studio\\2019\\Professional\\VC\\Tools\\MSVC\\14.29.30037\\atlmfc\\include" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\Include\\10.0.19041.0\\ucrt" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.19041.0\\shared" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.19041.0\\um" -internal-isystem "C:\\Program Files (x86)\\Windows Kits\\10\\include\\10.0.19041.0\\winrt" -std=c++17 -fdeprecated-macro -fdebug-compilation-dir "E:\\" -ferror-limit 19 -fno-use-cxa-atexit -fms-extensions -fms-compatibility -fms-compatibility-version=19.29.30038 -fdelayed-template-parsing -fcxx-exceptions -fexceptions -faddrsig -o "C:\\Users\\JONATH~1.NOB\\AppData\\Local\\Temp\\autoptr-152d67.o" -x c++ autoptr.cpp
clang -cc1 version 12.0.1 based upon LLVM 12.0.1 default target x86_64-pc-windows-msvc
#include "..." search starts here:
#include <...> search starts here:
 C:\Program Files\LLVM\lib\clang\12.0.1\include
 C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30037\include
 C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.29.30037\atlmfc\include
 C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt
 C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\shared
 C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\um
 C:\Program Files (x86)\Windows Kits\10\include\10.0.19041.0\winrt
End of search list.
autoptr.cpp:5:10: error: no member named 'auto_ptr' in namespace 'std'
    std::auto_ptr<int> thing(new int(1));
    ~~~~~^
autoptr.cpp:5:22: error: expected '(' for function-style cast or type construction
    std::auto_ptr<int> thing(new int(1));
                  ~~~^
autoptr.cpp:5:24: error: use of undeclared identifier 'thing'
    std::auto_ptr<int> thing(new int(1));
                       ^
3 errors generated.

This output shows the problem. LLVMs libcxx wasn't being used. using -D_HAS_AUTO_PTR_ETC=1 fixes the issue for me.

Answer 1

Check which c++ stdlib is being used with clang's -v flag. I had assumed the llvm's libcxx was being used as the -stdlib=libcxx was being ignored. In fact, msvc's stdlib was being picked up. With msvc use:

clang++ -std=c++17 -D_HAS_AUTO_PTR_ETC=1 file.cpp