WIP add book entry for instantiating in Rust

[BenFordTytherington]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (64d4ee6) to head (e4ab2f6).

Additional details and impacted files

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  Lines        12612     12612           
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  Hits         12612     12612           

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[LeonMatthesKDAB]

@BenFordTytherington Please also document that you can add constructor arguments if required :)

[BenFordTytherington]

@BenFordTytherington Please also document that you can add constructor arguments if required :)

I wasn't aware that was an option with these templates, what would that syntax look like?

[LeonMatthesKDAB]

@BenFordTytherington , it follows the same pattern as our construct function, e.g. here:

cxx-qt/crates/cxx-qt-lib/src/core/qvector/qvector_i64.rs

Line 28 in 64d4ee6

fn construct(_: &QVector_i64) -> QVector_i64;

You just declare the appropriate parameters and if the declared parameters match an existing constructor it will just work :)
And you can even create multiple constructors that way by renaming them.

Most QObjects for example can be created with an optional QObject* parent, so that could be a good example to show, e.g.:

#[namespace = "rust::cxxqtlib1"]
unsafe extern "C++" {
    include!("cxx-qt-lib/common.h");

    #[rust_name = "my_object_new"]
    fn make_unique() -> UniquePtr<MyObject>;
    
    #[rust_name = "my_object_new_with_parent"]
    fn make_unique(parent: *mut QObject) -> UniquePtr<MyObject>;
}

I think the docs should also mention that these functions are provided by CXX-Qt-lib as helper functions (which explains why they are in the rust::cxxqtlib1 namespace.
It's important to note that this is not std::make_unique.
These wrappers are specifically designed for better CXX compatibility.