Leedehai/Clang-with-libc++.md

## Clang-with-libc++.md

      
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    Clang with libc++


TL;DR: On macOS you are fine, if you didn't tweak its default compiler toolchain. If you did tweak it (e.g. use
Clang downloaded from its official source or with a package manager), or if your OS is Linux, read below.


The C++ standard library has many well-maintained implementations, chief of them
LLVM's libc++ and GNU's libstdc++.
The standard library, including the headers and the binary itself, usually come with
the downloaded compiler package: libc++ is with Clang
and libstdc++ is with GCC.

If specified with -stdlib=libc++, the Clang compiler
will compile with its headers and link with the library binary. Otherwise, the
system default is used. The easiest way to verify is:

Compile a simple program with clang++ your_program.cc -o your_program -stdlib=libc++ -v
to check the headers are picked up from Clang's installation directory (where
libc++'s headers are at), and
Execute that program to ensure the program loader can discover the libc++
library binary (absense of libc++ won't trigger a link time error, because this
is a dynamic library).

On macOS

The Apple Clang compiler coming with the developer toolchain or XCode
has and uses libc++, and it is the system default. Therefore, you can rest
assured it works.
However, if you use another version of Clang (usually downloaded from the
official LLVM release website), you need to ensure

libc++ libray headers and binary is present (it should came with your Clang
installation).
libc++ libray binary can be picked up by program loader by compiling and
running a test program if not, append the library binary's directory to environment
variable DYLD_LIBRARY_PATH) (note: otool -L [test_program] only shows paths of
libraries need to load, but not whether they are present).
In Clang's compile and link commands, specify -stdlib=libc++^[1].
Some libc++ installations may also require you to specify -lc++abi in the link
command (TODO: how to verify this library is the one that came from the downloaded
Clang, not the system-provided libc++)?

On Linux

The system default is libstdc++. Therefore, to ensure the headers and library
binary used by Clang are from libc++, you need to manually make sure:

libc++ libray headers and binary is present (it should came with your Clang
installation).
libc++ libray binary can be picked up by program loader by compiling and
running a test program, or let ldd [test_program] show paths of libraries need
to load and whether they are present (if not, append the library binary's
directory to environment variable LD_LIBRARY_PATH).
In Clang's compile and link commands, specify -stdlib=libc++^[1]. Some
libc++ installations may also require you to specify -lc++abi in the link command.


^[1] For link commands, if you use ld/lld/gold instead of
letting Clang do it for you, be sure to specify the path to the libc++ binary.

Replacing libstdc++ with libc++ was motivated by the former's slow support
of C++17 std::filesystem: even
in the one that came with GCC 7.5, released in November 2019, it was still
experimental.
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