Calling C++ from Rust

Until this point, all Zngur features discussed was about using Rust code inside C++, but no C++/Rust interop tool is complete without supporting the C++ to Rust direction. In fact, this direction is arguably more important since in a C++/Rust project most of the existing code is in C++ and Rust code needs a way to use them in order to be useful. So Zngur also supports this direction.

Zngur general idea is that Rust semantics is a subset of C++ semantics, so we should use Rust things in C++ and avoid bringing C++ things in Rust (See Design decisions). So, even in the C++ to Rust direction, Zngur operates only on Rust types. For example, Zngur allows you to call a C++ function that takes Rust types in inputs in Rust, but you can't call a function that takes a C++ object. Or you can write an impl block for a Rust type in C++ and call those methods in Rust, but you can't call C++ methods of a C++ object in Rust.

So you can't call your C++ code directly in Rust, and you need to write a Rusty wrapper for your C++ code. This is often unavoidable even if the interop tool supports calling C++ directly if you have a meaningful amount of Rust code, since C++ value cannot be owned in Rust, and Rust is more expressive in API and function signatures (e.g. it has lifetimes) so such a wrapper is required anyway. Zngur tries to enable writing a Rusty wrapper for your C++ code as idiomatic as a normal Rust code, and all features in this section try to help in achieving this goal.