Boost.OpenMethod on Compiler Explorer

Compiler Explorer can include a header from a URL, but only one file at a time: it does not resolve the includes inside the file it fetches. dev/flatten.py thus rewrites each public header into a self-sufficient one, and CI publishes them at the root of https://boostorg.github.io/openmethod, next to the documentation.

The URL is the point: CE's Libraries dropdown carries released Boost, and these headers are develop. Class registration by reflection, for one, lands in Boost 1.93, so an example that shows it has to fetch the headers rather than pick a version from the dropdown.

An example on Compiler Explorer therefore includes exactly what it would include locally, one line per header, in the same order:

#include <https://boostorg.github.io/openmethod/boost/openmethod.hpp>
#include <https://boostorg.github.io/openmethod/boost/openmethod/initialize.hpp>

Every path under include/boost/ is available under that URL - the interops, the policies, inplace_vptr.hpp. Two things to know:

• boost/openmethod.hpp comes first. It is the only self-contained file; the others check that it has been included and stop with an #error otherwise.
• Select a Boost version in the Libraries dropdown all the same. The flattened headers still include Boost.Mp11, Boost.DynamicBitset and the rest from Boost itself; only boost/openmethod comes from the URLs.

-std=c++17 -O3 -DNDEBUG is a good set of options to look at the generated code.

Reflection

reflection.cpp registers the classes by C++26 reflection (P2996). It includes the same two headers as the other examples, but needs a compiler that implements P2996 - x86-64 gcc (trunk) on Compiler Explorer - and -std=c++26 -freflection.

Bulldog is the point of the example. No overrider mentions it, and no BOOST_OPENMETHOD_CLASSES lists it; the scan started by BOOST_OPENMETHOD_REGISTER_CLASSES() finds it deriving from Dog, registers it, and poke dispatches it to the overrider for Dog.

On Compiler Explorer: https://godbolt.org/z/8T8z9zWfK

The sources

The sources in this directory are the examples published on Compiler Explorer. They are built and run as part of the test suite, and again against the flattened headers by dev/check-flat.sh, so a broken flattening is caught before it reaches the site.

To generate the headers locally:

python3 dev/flatten.py               # writes flat/boost/...
BOOST_SRC_DIR=/path/to/boost dev/check-flat.sh