Need help translating int and const * char C++ to Rust via CXX

emcp · November 12, 2022, 4:16pm

I am almost there.. I have refactored the C++ project I was originally working in. Rust is now the goal and I am wrapping the C++ dependency via CXX.. (And ConanRS)

but I am getting an error for what seems like basic translation

main.rs

use std::ffi::c_char;

#[cxx::bridge(namespace = "com::enserio")]
mod ffi {

    unsafe extern "C++" {
        include!("twsapi_grpc_server/include/twsapi-client.h");
        include!("twsapi_grpc_server/include/AvailableAlgoParams.h");
        include!("twsapi_grpc_server/include/AccountSummaryTags.h");
        include!("twsapi_grpc_server/include/Utils.h");

        type TWSApiClient;

        fn new_twsapi_client() -> UniquePtr<TWSApiClient>;
        // C++ VERSION = int32_t connect(const char * host, int32_t port, int32_t client_id = 0);
        fn init_connect(&self, host: *const c_char , port: i32, client_id: i32) -> i32;
    }
}

fn main() {
    println!("Starting TWSApiClient connect()");
    let client = ffi::new_twsapi_client();
    client.connect("127.0.0.1", 4002, 10);
    println!("Finished TWSApiClient connect()");
}

client.cpp

...
    int32_t TWSApiClient::init_connect(const char *host, int32_t port, int32_t client_id)
    {
...

The Error

  error[cxxbridge]: pointer argument requires that the function be marked unsafe
     ┌─ src/main.rs:16:32
     │
  16 │         fn init_connect(&self, host: *const c_char , port: i32, client_id: i32) -> i32;
     │                                ^^^^^^^^^^^^^^^^^^^ pointer argument requires that the function be marked unsafe

If I take away the *const c_char and instead try &c_char` .. it gives

/src/main.rs.cc:55:150: error: cannot convert ‘int32_t (com::enserio::TWSApiClient::*)(const char*, int32_t, int32_t)’ {aka ‘int (com::enserio::TWSApiClient::*)(const char*, int, int)’} to ‘int32_t (com::enserio::TWSApiClient::*)(const char&, int32_t, int32_t) const’ {aka ‘int (com::enserio::TWSApiClient::*)(const char&, int, int) const’} in initialization
  cargo:warning=   55 |   ::std::int32_t (::com::enserio::TWSApiClient::*init_connect$)(const char &, ::std::int32_t, ::std::int32_t) const = &::com::enserio::TWSApiClient::init_connect;
  cargo:warning=      |                                                                                                                                                      ^~~~~~~~~~~~
  exit status: 1

are my integers looking okay? should I just change to a std::string ?

alice · November 12, 2022, 4:18pm

As the error says, your function should be unsafe:

unsafe fn init_connect(&self, host: *const c_char , port: i32, client_id: i32) -> i32;

H2CO3 · November 12, 2022, 4:18pm

If you control the C++ side, then probably yes, and stick to a safe function, unless you have a good reason not to. string is safer in C++ too than a raw char pointer, and (for this reason) it's probably easier to translate correctly in an automatic way.

Two caveats apply, though:

You can't get a CxxString by-value on the Rust side
This will require you to have an std::string instance, so if all you have a const char *, then this will incur an allocation. (Given that this seems to be networking code, that cost is probably negligible to however long you have to wait for the network anyway.)

emcp · November 12, 2022, 4:42pm

im actually quite lost now.. because I went ahead trying to just get it to work by removing ALL function parameters... and still not working... seems to indicate one of the two sides of my code has a const... but I see no const on either side

my latest take

main.rs

#[cxx::bridge(namespace = "com::enserio")]
mod ffi {

    unsafe extern "C++" {
        include!("twsapi_grpc_server/include/twsapi-client.h");
        include!("twsapi_grpc_server/include/AvailableAlgoParams.h");
        include!("twsapi_grpc_server/include/AccountSummaryTags.h");
        include!("twsapi_grpc_server/include/Utils.h");

        type TWSApiClient;

        fn new_twsapi_client() -> UniquePtr<TWSApiClient>;
        //int32_t connect(const char * host, int32_t port, int32_t client_id = 0);
        fn init_connect(&self) -> bool;
    }
}

client.cpp

    bool TWSApiClient::init_connect()
    {

and header

    public:
        bool init_connect();

The Error

 cargo:warning=/home/emcp/Dev/git/JRGEMCP_Bootstrapping/bootstrap-rust-cpp-conan/target/debug/build/twsapi_grpc_server-f1ae9ab28f1e650b/out/cxxbridge/sources/twsapi_grpc_server/src/main.rs.cc: In function ‘bool com::enserio::com$enserio$cxxbridge1$TWSApiClient$init_connect(const TWSApiClient&)’:
  
cargo:warning=/home/emcp/Dev/git/JRGEMCP_Bootstrapping/bootstrap-rust-cpp-conan/target/debug/build/twsapi_grpc_server-f1ae9ab28f1e650b/out/cxxbridge/sources/twsapi_grpc_server/src/main.rs.cc:54:96: error: cannot convert ‘bool (com::enserio::TWSApiClient::*)()’ to ‘bool (com::enserio::TWSApiClient::*)() const’ in initialization
 
cargo:warning=   54 |   bool (::com::enserio::TWSApiClient::*init_connect$)() const = &::com::enserio::TWSApiClient::init_connect;
  cargo:warning=      |                                                                                                ^~~~~~~~~~~~
  exit status: 1

H2CO3 · November 12, 2022, 4:47pm

Your init_connect() takes &self, which is a pointer-to-immutable Self. It's the equivalent of C++'s const member function. However, on the C++ side, you don't declare it as const. You need to either:

change the C++ declaration to bool init_connect() const if you don't need the function to mutate the object, or
change the Rust declaration to fn init_connect(&mut self) -> bool if you do need mutable access.

emcp · November 12, 2022, 4:51pm

okay wow that was it.. I am learning so much of BOTH languages now.. thank you to you and @alice !

H2CO3 · November 12, 2022, 4:51pm

In general, C++ is mutable-by-default, whereas Rust is immutable-by-default.

emcp · November 12, 2022, 5:05pm

I successfully changed the reference.. but now seems another error has popped up. I ended up having to do this

  --- stderr

  error[cxxbridge]: needs a cxx::ExternType impl in order to be used as a non-pinned mutable reference in signature of `init_connect`
     ┌─ src/main.rs:10:9
     │
  10 │         type TWSApiClient;
     │         ^^^^^^^^^^^^^^^^^ needs a cxx::ExternType impl in order to be used as a non-pinned mutable reference in signature of `init_connect`


  error[cxxbridge]: mutable reference to opaque C++ type requires a pin -- use `self: Pin<&mut TWSApiClient>`
     ┌─ src/main.rs:14:25
     │
  14 │         fn init_connect(&mut self) -> bool;
     │                         ^^^^^^^^^ mutable reference to opaque C++ type requires a pin -- use `self: Pin<&mut TWSApiClient>`

Final working reference

        fn init_connect(self: Pin<&mut TWSApiClient>) -> bool;

H2CO3 · November 12, 2022, 5:09pm

Yeah, that's because C++ has move constructors, so in general when you transfer ownership of a C++ object, it might need to run arbitrary user-defined code in order to maintain internal invariants (this is how C++ can represent self-referential types, for example).

Mutable references can give away ownership in both directions (e.g. simple assignment or mem::replace()), so the Rust side needs to make sure it doesn't actually move the object by-value. This is what Pin is for. It's a marker type that restricts regular mutable references, so they can't be (safely) used for moving the referent value.

emcp · November 12, 2022, 5:30pm

Great to know..

I successfully compile the CXX , but seeing the following

fn main() {

    println!("Starting TWSApiClient connect()");
    let client = ffi::new_twsapi_client();

    client.init_connect(4002, 333);
    println!("Finished TWSApiClient connect()");

}


error[E0599]: no method named `init_connect` found for struct `UniquePtr<TWSApiClient>` in the current scope
  --> src/main.rs:22:12
   |
22 |     client.init_connect(4002, 333);
   |            ^^^^^^^ method not found in `UniquePtr<TWSApiClient>`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `twsapi_grpc_server` due to previous error

I will recheck my work vs. the Demo https://github.com/dtolnay/cxx/blob/master/demo/src/main.rs

Since i got the basic no-parameters working.. I re-added the integers successfully.

system · February 10, 2023, 5:31pm

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Need help translating int and const * char C++ to Rust via CXX

main.rs

main.rs

client.cpp

The Error

Final working reference

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