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# Backends for cppyy

The cppyy module needs a backend to provide the C++ reflection information on which the Python bindings are build. The backend is called through a C-API, which can be found in the PyPy sources in: pypy/module/cppyy/include/capi.h. There are two kinds of API calls: querying about reflection information, which are used during the creation of Python-side constructs, and making the actual calls into C++. The objects passed around are all opaque: cppyy does not make any assumptions about them, other than that the opaque handles can be copied. Their definition, however, appears in two places: in the C code (in capi.h), and on the RPython side (in capi_types.py), so if they are changed, they need to be changed on both sides.

There are two places where selections in the RPython code affect the choice (and use) of the backend. The first is in pypy/module/cppyy/capi/__init__.py:

    # choose C-API access method:
    from pypy.module.cppyy.capi.loadable_capi import *
    #from pypy.module.cppyy.capi.builtin_capi import *

The default is the loadable C-API. Comment it and uncomment the builtin C-API line, to use the builtin version.

Next, if the builtin C-API is chosen, the specific backend needs to be set as well (default is Reflex). This second choice is in pypy/module/cppyy/capi/builtin_capi.py:

    import reflex_capi as backend
    #import cint_capi as backend

After those choices have been made, built pypy-c as usual.

When building pypy-c from source, keep the following in mind. If the loadable_capi is chosen, no further prerequisites are needed. However, for the build of the builtin_capi to succeed, the `ROOTSYS` environment variable must point to the location of your ROOT (or standalone Reflex in the case of the Reflex backend) installation, or the `root-config` utility must be accessible through `$PATH` (e.g. by adding `$ROOTSYS/bin` to `PATH`). In case of the former, include files are expected under `$ROOTSYS/include` and libraries under `$ROOTSYS/lib`.

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