c++ - c++20 library support for xcode 12

Question

can i use the c++20 library with xcode 12? (xcode 12 beta 5, with clang version 10.0.0).

so in xcode build settings, instead of

// in xcode build settings, "c++ standard library"
CLANG_CXX_LIBRARY = libc++

maybe use something like (does not work => clang err: invalid library name)

CLANG_CXX_LIBRARY = libc++20 // eg. libc++20 & libc++2a are invalid names

i have already set (works by providing c++20 language support, but does not provide c++20 library support)

// in xcode build settings, "c++ language dialect"
CLANG_CXX_LANGUAGE_STANDARD = c++2a // ok but does not provide c++20 library

im aware that the c++20 library is not yet complete/officially released.

question:

do you know of any (easy) way to use the (preliminary) c++20 library with xcode 12?

thanks

Answer 1

There can be easier ways, or I might have done something redundant (let me know what can be removed), but here's a surefire way.

Build llvm with

cmake -G "Sublime Text 2 - Ninja" -DCMAKE_BUILD_TYPE=Release 
-DLLVM_ENABLE_PROJECTS=“clang;libcxx;libcxxabi” -DCMAKE_CXX_STANDARD=17 -DLLVM_INCLUDE_TESTS=OFF 
-DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_CCACHE_BUILD=ON -DLLVM_CREATE_XCODE_TOOLCHAIN=ON 
-DCMAKE_INSTALL_PREFIX="easy to clean location"
-DLLVM_ENABLE_RTTI=OFF 
../llvm

• http://clang.llvm.org/get_started.html
• https://llvm.org/docs/CMake.html

libcxxabi is required or linking libcxx will fail with:

ld: library not found for -lcxxabi_shared clang: error: linker command failed with exit code 1 (use -v to see invocation)

DLLVM_CCACHE_BUILD requires https://ccache.dev (use brew if you wish). First build will be very slow. Rebuilds will be faster.

---

After the above is done and ninja compiles around 3000 files, run

 ninja install all 
 ninja install build-xcode-toolchain

Find the created toolchain in location you chose above/Toolchains. Copy it to ~/Library/Developer/Toolchains/

---

If Xcode is open, close it and reopen. In Xcode app menu > Toolchains > choose the new one, llvm12git.

Create a new c++ project normally and go to its project's build settings.

Search for COMPILER_INDEX_STORE_ENABLE or Enable index-while-building functionality and set it to "No". Otherwise, build fails with "unrecognised option" about indexing.

---

Change "C++ language dialect" to "c++20" or "c++2a"

Build the project normally. However, warnings may not go away while the code successfully builds due to indexing disabled. :( Adding header search path helps with warnings.

• Adding system header search path to Xcode

---

Make sure to check feature status:

• http://clang.llvm.org/cxx_status.html
• https://en.cppreference.com/w/cpp/20

Code I tested:

#include <compare>
#include <concepts>

struct Aggr {
  int i;
  char c;

  auto operator<=>(Aggr const &) const = default;
};
struct A {
  int x;
  int y;
  int z;
};
int main()
{

  // A a{.y = 2,.x = 1};  // error; designator order does not match declaration
  // order
  A b{.x = 1, .z = 2};  // ok, b.y initialized to 0
  return 0;

}