Reproduction of USD plugin loading failure with libc++.
The issue here is that the type ShipFactoryBase is considered equal in the main program
and the plugin by GNU libstdc++ which uses strcmp for type equality. LLVM's
libc++ on the other hand uses a more sophisticated type equality check and therefore
considers ShipFactoryBase to be different in the plugin and main program, resulting in a
dynamic_cast between types to return NULL. A static_cast, reinterpret_cast or a
C-style cast will work. The issue here seems to be that libloader.so is built with
hidden visibility and ShipFactoryBase is lacking an api export directive. If that is
added, things will work fine.
Running the example here (having Nix installed) should give this output:
❯ make run
nix build -f ./default.nix libstd
Running with GNU libstdc++ (built with Clang):
🐄 [libstdc++] Casting away 🏝 using C++ standard library "libstdc++ (GNU)"... 🚢
🐄 [libstdc++] 🃏 Type for ShipFactoryBase in host and plugin are equal? ✅ yes
🐄 [libstdc++] 🚢 Ship type: ⛵
nix build -f ./default.nix libcxx
Running with LLVM libc++ (built with Clang):
🐉 [libc++] Casting away 🏝 using C++ standard library "libc++ (LLVM)"... 🚢
🐉 [libc++] 🃏 Type for ShipFactoryBase in host and plugin are equal? ❌ no
🐉 [libc++] 😭 Failed to cast factory to a ship factory when using libc++ (LLVM)!
nix build -f ./default.nix libcxxStrcmp
Running with LLVM libc++ (built with Clang, using strcmp fallback through _LIBCXXABI_FORGIVING_DYNAMIC_CAST):
🐉 [libc++ (strcmp)] Casting away 🏝 using C++ standard library "libc++ w/ strcmp (LLVM)"... 🚢
🐉 [libc++ (strcmp)] 🃏 Type for ShipFactoryBase in host and plugin are equal? ❌ no
🐉 [libc++ (strcmp)] 🚢 Ship type: ⛵
To make it clear that the compiler has nothing to do with it, we use Clang together with libstdc++ as well as libc++.
According to this, the difference between the c++ stdlib implementations seems to be known and unlikely to change.