compiling 01_hellow.cpp

[rinka-meltiron]

g++ -o 01-hellow -std=c++20 01-hellow.cpp on my Linux machine did not work.
The issue with gcc is:
$ g++ -o 01-hellow -std=c++20 01-hellow.cpp
01-hellow.cpp:3:1: error: ‘import’ does not name a type
3 | import std;
| ^~~~~~
01-hellow.cpp:3:1: note: C++20 ‘import’ only available with ‘-fmodules-ts’, which is not yet enabled with ‘-std=c++20’
01-hellow.cpp: In function ‘int main():
01-hellow.cpp:7:5: error: ‘cout’ was not declared in this scope
7 | cout << "Hello, World!" << '\n';
| ^~~~
01-hellow.cpp:1:1: note: ‘std::cout’ is defined in header ‘<iostream>’; did you forget to ‘#include <iostream>’?
+++ |+#include <iostream>
1 | // 01-hellow.cpp : prints a line of text to the console

and with clang is:
$ clang++ -o 01-hellow -std=c++20 -stdlib=libc++ 01-hellow.cpp
01-hellow.cpp:3:8: fatal error: module 'std' not found
import std;
~~~~~~~^~~
1 error generated

I suggest updating the code for people who use linux, vim (or similar editors). A lot of us use the command line.

A quick answer of course is:
// 01-hellow.cpp : prints a line of text to the console

#include <iostream>
using namespace std;

int main() {
cout << "Hello, World!" << '\n';
}

Thanks,

[cpp-tutor]

Just to clarify, the C++ code in the "modules" directory (which uses import) is not (yet) compatible with g++. Please use Clang under Linux with the -fmodules flag.

All of the code in the "headers" directory (which uses #include) should compile with either g++ or Clang under Linux with the flags you described above. Should you discover any that do not, please add to this issue.

[allen-c]

clang++ -std=c++23 01-hellow.cpp -o 01-hellow
01-hellow.cpp:3:8: fatal error: module 'std' not found
3 | import std;
| ~~~~~~~^~~
1 error generated.

clang++ --version
clang version 19.0.0git (git@github.com:llvm/llvm-project.git cbcdf126ccc774c063b5d5140c1393ff5305dded)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin

How to solve the compile error?

[cpp-tutor]

Please try adding: -stdlib=libc++

[Krannich479]

Hi @cpp-tutor,
thank you for your assistance! Unfortunately I face the same issue even though I use clang. Running
clang++ -fmodules -std=c++2b -stdlib=libc++ -o 01-hellow 01-hellow.cpp
(for 'Working draft for ISO C++ 2023 DIS' standard) or
clang++ -fmodules -std=c++20 -stdlib=libc++ -o 01-hellow 01-hellow.cpp
both result in

01-hellow.cpp:3:1: error: unknown type name 'import'
import std;
^
01-hellow.cpp:4:17: warning: using directive refers to implicitly-defined namespace 'std'
using namespace std;
                ^
01-hellow.cpp:7:5: error: use of undeclared identifier 'println'
    println("Hello, World!");
    ^
1 warning and 2 errors generated.

Additional info:

% clang -v
Apple clang version 13.1.6 (clang-1316.0.21.2.5)
Target: x86_64-apple-darwin21.6.0
Thread model: posix

% sw_vers
ProductName:	macOS Monterey
ProductVersion:	12.5.1
BuildVersion:	21G83

[cpp-tutor]

Hi Thomas, I notice that you are using quite an old version of clang, as I recall "import std;" didn't work for me until clang 14.Also please be aware that "println()" is C++23 (look for header "print" in /usr/include/c++[...]), and I'm not even sure if is available with "import" yet. I can't recommend Compiler Explorer because "-stdlib=libc++" is not available, so my best suggestion is to download and compile the latest clang/LLVM, and enable sub-project "libcxx". (This does take a long time.) Alternatively, you can clone the "1.0 C++20" branch of the repo, and use the source code from the "headers" directory. Kind regards, Richard

[cpp-tutor]

Still likely an issue for many in 2025, but with a solution for clang-18 and newer, see this page (summary below):

Firstly, compile the std module into the current directory (in a similar style as for VS2022 C++ Modules):

clang++ -std=c++23 -stdlib=libc++ \
    -Wno-reserved-identifier -Wno-reserved-module-identifier \
    --precompile -o std.pcm /usr/share/libc++/v1/std.cppm

Then, reference this new file and libc++ to compile a single source file to an executable (in this case test.cpp -> test):

clang++ -std=c++23 -stdlib=libc++ \
    -fmodule-file=std=std.pcm -o test std.pcm test.cpp

Paths shown above should be correct as for most Linuxes (in particular, Debian Bookworm, with clang++-19 used instead and packages clang-19 and libc++-19-dev installed). Closing this issue as resolved.