BUMP / RE-SENT -- I think I got lost in a flurry of responses to another subject: Richard –

This usually happens because you're compiling with one standard library (e.g. libc++, the default on clang) while your boost libraries were compiled with another (e.g. libstdc++, the default on gcc).

That makes sense. Does my MacOS program need to be compiled for GCC (IE GNU C++17 / GNU C++20), and then linked with libstdc++ to be compatible with the default build of Boost, or the opposite: build Boost and link with libc++? I saw that there's a list of toolsets, but in the "getting started with unix variants", included with the 1_85_0 .zip package, there is no listing for toolset=clang or toolset=clang-darwin. Nor is there any mention of clang on: https://www.boost.org/doc/libs/1_85_0/more/getting_started/unix-variants.htm... Stephen