如何在Ubuntu 18.04中正确安装clang并使用C++标准库？我已经安装了一些其他帖子中建议的库，但没有成功。我还安装了两个gcc编译器（7,9），并使用update-alternative命令在它们之间进行切换。同时安装了Clang-6、Clang-9和llvm-9。
我安装的一些库：
sudo apt install libstdc++-9-dev
sudo apt install libc++-9-dev
sudo apt install g++-7-multilib
sudo apt install g++-9-multilib
我认为还有更多，但结果仍然不变。
#include <iostream>
int main()
{
std::cout << "Hello World\n";
}
使用编译器
clang++-9 hello_world.cpp
clang++ hello_world.cpp
clang++-9 -stdlib=libstdc++ hello_world.cpp
clang++ -stdlib=libstdc++ hello_world.cpp
所有的产品
hello_world.cpp:2:10: fatal error: 'iostream' file not found
我找到了很多关于这个话题的帖子，但大部分都是关于Windows、macOS或8年前的内容。还有这篇博客文章。我可能漏掉了一些非常明显的东西，希望有人能指点我一下正确的方向。
我用以下方式安装了clang，但不记得确切的顺序：
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt install clang libclang-dev
sudo apt-get install libclang-9-dev
sudo apt-get install clang-9
sudo apt-get install clang++-9
正如评论中建议的那样，尝试使用clang++ -v hello_world.cpp进行编译，输出结果如下：
$ clang++ hello_world.cpp -v
clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Selected multilib: .;@m64
"/usr/lib/llvm-6.0/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name hello_world.cpp -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -resource-dir /usr/lib/llvm-6.0/lib/clang/6.0.0 -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++ -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/x86_64-linux-gnu -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/backward -internal-isystem /usr/include/clang/6.0.0/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-6.0/lib/clang/6.0.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir /mnt/My_Linux_Files/C++/test2/src -ferror-limit 19 -fmessage-length 80 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o /tmp/hello_world-0f22e3.o -x c++ hello_world.cpp
clang -cc1 version 6.0.0 based upon LLVM 6.0.0 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/backward"
ignoring nonexistent directory "/include"
ignoring duplicate directory "/usr/include/clang/6.0.0/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++
/usr/include/clang/6.0.0/include
/usr/local/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
hello_world.cpp:2:10: fatal error: 'iostream' file not found
#include <iostream>
^~~~~~~~~~
1 error generated.
使用clang++-9：
$ clang++-9 hello_world.cpp -v
clang version 9.0.0-2~ubuntu18.04.2 (tags/RELEASE_900/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Selected multilib: .;@m64
"/usr/lib/llvm-9/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name hello_world.cpp -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -v -resource-dir /usr/lib/llvm-9/lib/clang/9.0.0 -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++ -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/x86_64-linux-gnu -internal-isystem /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/backward -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-9/lib/clang/9.0.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir /mnt/My_Linux_Files/C++/test2/src -ferror-limit 19 -fmessage-length 0 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o /tmp/hello_world-9f4c00.o -x c++ hello_world.cpp
clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++/backward"
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../include/c++
/usr/local/include
/usr/lib/llvm-9/lib/clang/9.0.0/include
/usr/include/x86_64-linux-gnu
/usr/include
End of search list.
hello_world.cpp:2:10: fatal error: 'iostream' file not found
#include <iostream>
^~~~~~~~~~
1 error generated.
这行代码：
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
我有些困惑，clang是否认为已安装了gcc-10？因为我不认为已经安装了：
$ find /usr/ -iname "*gcc-10*"
/usr/share/gcc-10
/usr/share/doc/gcc-10-base
/usr/share/doc/libgcc-10-dev
$
解决方案
正如评论所建议的，安装libstdc++-10解决了我的问题。
clang和clang-9软件包吗？ - dyplibstdc++-9包...你是如何安装它的？ - dypubuntu-toolchain-r/test的 Clang 可能被硬编码为使用 GCC 10 的包含路径，因此你需要安装 GCC 和 libstdc++ 10 才能使其正常工作。 - Alan Birtles