{{#title Built-in bindings — Rust ♡ C++}} # Built-in bindings reference In addition to all the primitive types (i32 <=> int32_t), the following common types may be used in the fields of shared structs and the arguments and returns of extern functions.

name in Rust	name in C++	restrictions
String	rust::String
&str	rust::Str
&[T]	rust::Slice<const T>	^{cannot hold opaque C++ type}
&mut [T]	rust::Slice<T>	^{cannot hold opaque C++ type}
CxxString	std::string	^{cannot be passed by value}
Box<T>	rust::Box<T>	^{cannot hold opaque C++ type}
UniquePtr<T>	std::unique_ptr<T>	^{cannot hold opaque Rust type}
SharedPtr<T>	std::shared_ptr<T>	^{cannot hold opaque Rust type}
[T; N]	std::array<T, N>	^{cannot hold opaque C++ type}
Vec<T>	rust::Vec<T>	^{cannot hold opaque C++ type}
CxxVector<T>	std::vector<T>	^{cannot be passed by value, cannot hold opaque Rust type}
mut T, const T	T, const T	^{fn with a raw pointer argument must be declared unsafe to call}
fn(T, U) -> V	rust::Fn<V(T, U)>	^{only passing from Rust to C++ is implemented so far}
Result<T>	throw/catch	^{allowed as return type only}

The C++ API of the `rust` namespace is defined by the *include/cxx.h* file in the CXX GitHub repo. You will need to include this header in your C++ code when working with those types. **When using Cargo and the cxx-build crate, the header is made available to you at `#include "rust/cxx.h"`.** The `rust` namespace additionally provides lowercase type aliases of all the types mentioned in the table, for use in codebases preferring that style. For example `rust::String`, `rust::Vec` may alternatively be written `rust::string`, `rust::vec` etc. ## Pending bindings The following types are intended to be supported "soon" but are just not implemented yet. I don't expect any of these to be hard to make work but it's a matter of designing a nice API for each in its non-native language.

name in Rust	name in C++
BTreeMap<K, V>	^tbd
HashMap<K, V>	^tbd
Arc<T>	^tbd
Option<T>	^tbd
^tbd	std::map<K, V>
^tbd	std::unordered_map<K, V>