clang 6 not supporting unordered_map::merge?

Question

With this trivial example, I get a compilation error:

#include <unordered_map>

int main() {
    std::unordered_map<int, int> a, b;
    a.merge(b);
}

Error:

$ clang++ -std=c++17 merge.cpp
merge.cpp:5:4: error: no member named 'merge' in 'std::__1::unordered_map<int, int, std::__1::hash<int>, std::__1::equal_to<int>, std::__1::allocator<std::__1::pair<const int, int> > >'
        a.merge(b);
        ~ ^
1 error generated.

Versions:

$ clang++ --version
clang version 6.0.0 (tags/RELEASE_600/final)
Target: x86_64-apple-darwin17.5.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin

According to cppreference , this should be legal since C++17. GCC 7 is happy to compile it.

Answer 1

I had same issue. I was outraged, why I can't compile application which use unordered_map::merge() using macOS 10.14.6

unordered_map::merge() was added in c++17 with proposal P0083R3 "Splicing Maps and Sets (Revision 5) ", see github blame .

Documentation presents common compilers' support for new C++ features . Find row "Splicing Maps and Sets" and check you compiler version.

"Splicing Maps and Sets" compilers' support:

+-----------------------+---------------------+------+
| Compiler              | Version             | Link |
+-----------------------+---------------------+------+
| GCC libstdc++         |                 7.1 | [1]  |
| Clang libc++          |                 8.0 | [2]  |
| MSVC Standard Library | 19.12 (VS 2017 15.5)| [3]  |
| Apple Clang           |                   - | [4]  |
+-----------------------+---------------------+------+

1. https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html
2. https://libcxx.llvm.org/cxx1z_status.html
3. https://docs.microsoft.com/cpp/overview/visual-cpp-language-conformance?view=vs-2019
4. Apple LLVM have special implementation of clang compiler, see description below

if you request a clang version, you get something like this:

➜ ~ clang++ --version

Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Apple's clang version number has nothing to with the official's one. Some users try to map apple's and official clang versions here . I think, it's not always possible.

Also I couldn't find exhaustive table of c++ standard support status for different Apple Clang versions, unlike another compilers (share it in comments if you have). All we have is official xcode release notes .

But mac OS users can still take advantage of all the new features of c++17. You should only... install original llvm. Detailed guide read in article "Installing LLVM/Clang on OSX" by Phillip Johnston .

Installing llvm using brew is much safer:

llvm is keg-only, which means it was not symlinked into /usr/local, because macOS already provides this software and installing another version in parallel can cause all kinds of trouble.

Install llvm:

// optional "--with-toolchain" from article is deprecated
brew install llvm

Check status of llvm

➜ ~ brew info llvm
llvm: stable 8.0.1 (bottled), HEAD [keg-only]
Next-gen compiler infrastructure
https://llvm.org/
/usr/local/Cellar/llvm/8.0.1 (6,807 files, 3.3GB)
  Poured from bottle on 2019-09-14 at 14:19:29
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/llvm.rb
==> Dependencies
Build: cmake ✔
Required: libffi ✔, swig ✔
==> Requirements
Build: xcode ✔
==> Options
--HEAD
    Install HEAD version
==> Caveats
To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH run:
  echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.zshrc

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/usr/local/opt/llvm/lib"
  export CPPFLAGS="-I/usr/local/opt/llvm/include"

Check clang version:

➜ ~ /usr/local/Cellar/llvm/8.0.1/bin/clang++ --version
clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-apple-darwin18.7.0
Thread model: posix

Configure project

Now you can define compiler, I use CMake and add lines in CMakeLists.txt:

set(CMAKE_C_COMPILER "/usr/local/Cellar/llvm/8.0.1/bin/clang")
set(CMAKE_CXX_COMPILER "/usr/local/Cellar/llvm/8.0.1/bin/clang++")
set(CMAKE_CXX_STANDARD 17)

or pass cmake command's options:

➜ ~ cmake \
  -D CMAKE_C_COMPILER="/usr/local/Cellar/llvm/8.0.1/bin/clang" \
  -D CMAKE_CXX_COMPILER "/usr/local/Cellar/llvm/8.0.1/bin/clang++" \
  /path/to/CMakeLists.txt

or define env variables:

➜ ~ export CC=/usr/local/Cellar/llvm/8.0.1/bin/clang
➜ ~ export CXX=/usr/local/Cellar/llvm/8.0.1/bin/clang++
➜ ~ cmake /path/to/CMakeLists.txt