variant-boost-python

Demonstrates passing and returning std::variant using Boost.Python

Motivation

In Python I enjoy the freedom of dynamic typing, but in C++ I like strong, compiler-checked types. Boost.Python provides a way to cleanly cross that boundary. But it doesn't have built-in support for every type or design pattern.

In particular it's often the case that Python values supplied to, or returned from, C++ classes and methods can only be one of a small number of types. One way to express this is to use overloads for each possible type. Another is to pass and return sum types like std::variant.

Goals

1. For a parameter of C++ type std::variant<A, B, C> I want Python users to be able to pass any of the Python equivalents of A, B, or C
2. An exception should be thrown if the Python value is not one of the variant's types
3. The Python value None should only be accepted if std::monostate is one of the member types
4. Member types wrapped with std::reference_wrapper should be accepted without copying the wrapped type
5. A variant return value should be converted to whichever of the variant types was present

In this way the use of variant should be invisible to Python users (outside of type checking) and appear like the dynamic typing they are used to.