How it works

Short description:

at compile-time: use aggregate initialization to detect fields count in user-provided structure

BOOST_PFR_USE_CPP17 == 1:
1. at compile-time: structured bindings are used to decompose a type T to known amount of fields
BOOST_PFR_USE_CPP17 == 0 && BOOST_PFR_USE_LOOPHOLE == 1:
1. at compile-time: use aggregate initialization to detect fields count in user-provided structure
2. at compile-time: make a structure that is convertible to anything and remember types it has been converted to during aggregate initialization of user-provided structure
3. at compile-time: using knowledge from previous steps create a tuple with exactly the same layout as in user-provided structure
4. at compile-time: find offsets for each field in user-provided structure using the tuple from previous step
5. at run-time: get pointer to each field, knowing the structure address and each field offset
6. at run-time: a tuple of references to fields is returned => all the tuple methods are available for the structure

BOOST_PFR_USE_CPP17 == 0 && BOOST_PFR_USE_LOOPHOLE == 0:

at run-time: T is constructed and field I is aggregate initialized using a separate instance of structure that is convertible to anything

	Note
	Additional care is taken to make sure that all the information about `T` is available to the compiler and that operations on `T` have no side effects, so the compiler can optimize away the unnecessary temporary objects.

at compile-time: I += 1
at compile-time: if I does not equal fields count goto step c. from inside of the conversion operator of the structure that is convertible to anything
at compile-time: using knowledge from previous steps create a tuple with exactly the same layout as in user-provided structure
at compile-time: find offsets for each field in user-provided structure using the tuple from previous step
at run-time: get pointer to each field, knowing the structure address and each field offset

at run-time: a tuple of references to fields is returned => all the tuple methods are available for the structure

Long description of some basics: Antony Polukhin: Better C++14 reflections. Long description of some basics of C++14 with BOOST_PFR_USE_LOOPHOLE == 0: Antony Polukhin: C++14 Reflections Without Macros, Markup nor External Tooling. Description of the BOOST_PFR_USE_LOOPHOLE == 1 technique by its inventor Alexandr Poltavsky in his blog.

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