Can't find C++ standard library

[BhJaipal]

everytime I try to compile C++ with clang, I get this

$ clang++-18 alloc.cpp
alloc.cpp:1:10: fatal error: 'iostream' file not found
    1 | #include <iostream>
      |          ^~~~~~~~~~
1 error generated.

why can't it open C++ headers itself?
I have to give header path myself

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Jaipal001 (BhJaipal)

## everytime I try to compile C++ with clang, I get this

$ clang++-18 alloc.cpp
alloc.cpp:1:10: fatal error: 'iostream' file not found
    1 | #include <iostream>
      |          ^~~~~~~~~~
1 error generated.

why can't it open C++ headers itself?
I have to give header path myself

[AaronBallman]

We need some more details to be able to help determine what's going on. What platform are you on? How did you install Clang? Where on your system are the standard library headers?

One thing you can try is passing -v on your command line invocation. That will spit out information about where header search paths are starting from. Additionally, you can pass -### to spit out the -cc1 invocation that has the full paths being passed from the Clang driver to the Clang frontend.

[BhJaipal]

We need some more details to be able to help determine what's going on. What platform are you on? How did you install Clang? Where on your system are the standard library headers?

One thing you can try is passing -v on your command line invocation. That will spit out information about where header search paths are starting from. Additionally, you can pass -### to spit out the -cc1 invocation that has the full paths being passed from the Clang driver to the Clang frontend.

I use Pop OS, Ubuntu derivative
I install clang 18 by adding ppa of llvm clang
Headers are as in normal c++ headers location
/usr/include/c++/11
/usr/include/x86_64-linux-gnu/c++/11

[AaronBallman]

What output do you get from clang -v?

[BhJaipal]

Ubuntu clang version 18.1.8 (++20240731024944+3b5b5c1ec4a3-1exp120240731145000.144)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/12
Candidate multilib: .;@ m64
Selected multilib: .;@ m64

[tru]

Maybe you have libstdc++ headers for GGC11 installed but not 12? Clang will select the highest version of gcc by default if I remember correctly.

[MaskRay]

A common issue on Debian/Ubuntu is that when gcc-18 is (automatically) installed, g++-18 is not. clang will select the gcc 18 directory without libstdc++ headers.

(While it could be frustrating, I don't think it's clangDriver's responsibility to probe more files. Many people don't like clang stating too many files. This is error-prone anyway. Distributions could help in this scenario: when clang is installed, they can detect the case and install g++-18 as well.)

[BhJaipal]

A common issue on Debian/Ubuntu is that when gcc-18 is (automatically) installed, g++-18 is not. clang will select the gcc 18 directory without libstdc++ headers.

(While it could be frustrating, I don't think it's clangDriver's responsibility to probe more files. Many people don't like clang stating too many files. This is error-prone anyway. Distributions could help in this scenario: when clang is installed, they can detect the case and install g++-18 as well.)

Ok, clangd for intellisence
g++ for compiling

[AaronBallman]

A common issue on Debian/Ubuntu is that when gcc-18 is (automatically) installed, g++-18 is not. clang will select the gcc 18 directory without libstdc++ headers.
(While it could be frustrating, I don't think it's clangDriver's responsibility to probe more files. Many people don't like clang stating too many files. This is error-prone anyway. Distributions could help in this scenario: when clang is installed, they can detect the case and install g++-18 as well.)

Ok, clangd for intellisence g++ for compiling

So you have g++-18 installed and clang is still not finding the correct directory? Or do you not have g++-18 installed?