Direct call cpp from Rust!

directcpp is yet another method to interop c++ with rust.

It's designed to be lightweight, efficient, MSVC-friendly, no cpp side code generation, work with outer c++ build tools like visual studio.

If you're looking for a way to work with c++ and rust with Visual Studio, directcpp might be the result.

why not use crate cxx

Crate cxx is a powerful crate for c++/rust interop. However, in my environment visual studio is important, I need it to debug my c++ code. For MSVC, crate cxx is not friendly enough. It has pre-compiled c++ code and cannot be compiled in the debug configuration. Specifically, it's because code compiled with the iterator debugging doesn't work together with code compiled without it.

At the same time, I do not want the code generator to make all the code required for interaction. That seems to create a natural barrier between C++ and Rust. I hope that Rust can use C++ code like a normal lib. Of course, it doesn't come without a cost, and I will try to use these interfaces as little as possible. Therefore, I hope that the interfaces that need to be used can be arbitrary and do not need to strictly follow a certain paradigm.

Of course, security is still very important, and I don't want to make too many compromises on the C++ side and make a lot of strange functions for Rust. The same goes for the rust side. Don't convert some structures into a form that can be recognized by c++ in order to call c++ -- though I did that at the beginning -- because it looked quite cumbersome. Some shitty code takes up large chunks of my text, making it difficult for me to read my own code.

Therefore, I chose to generate a small number of very free interfaces so that rust can freely call C++ code with 99% security. I dare not say it is 100%, although crate cxx does this. After all, when you call C++ from Rust, you can cache any pointer you get from C++, which is your freedom, but it is also the price you pay for your freedom.

Getting started

If you want to get started with directcpp, you can start by reading the test code of it. They are in the directory test_proj in github where I also demonstrate how to make visual studio and rust coexist peacefully, and debug version compilation is also supported as I used a hacker's method to allow rust to link to the msvcrtd debugging library.

You can try the simplest one first:

use directcpp;

#[directcpp::bridge]
extern "C++" {
	pub fn on_start();
}

And in your cpp file, write:

void on_start(void)
{
	std::cout << "on cpp side start!" << std::endl;
}

Note extern "C" is not needed. Rust would directly call the mangled version of the c++ function. This is why it's called directcpp.

You can also pass a reference of a struct into c++ and get another struct back:

#[directcpp::bridge]
extern "C++" {
	pub fn on_magic(magic: &MagicIn) -> MagicOut;
}

To do this , the MagicIn and MagicOut struct should have exactly the same memory layout between c++ and rust. I provided the rust_spt.h in github which you can freely include that. This is the price for no code generation on c++ side, and gives the freedom to write and compile c++ code arbitrarily.

You can also get a class object pointer from c++ side and call its member functions while keeping the safety by SharedPtr/std::shared_ptr in rust/c++, respectively. Check code in test_proj.

You may need to generate some code in your way, thought, as I used rust2h.py to generate some c++ structure from rust, keeping the same memory layout of the them, like MagicIn and MagicOut. The rust2h.py should not be considered be a part of directcpp, but they with other stuff all work together to make the final product. So directcpp is a loosely organized library which offers the most freedom, but it may take you a little time to get used to it.

I have put rust2h.py into tools folder. However, convert such structures is not hard, you can even do that by some text manipulation tools like sed.

Special Mappings

Normally directcpp maps a name in argument as the same named type with C++. However, there are some special types:

• &CStr maps to: const char* in C++
• &str maps to const char*, size_t in C++.
• &[u8] maps to const uint8_t*, size_t in C++.
• String maps to RustString

Tested On

• windows with MSVC, x86_64
• Linux, x86_64
• MacOS, x86_64
• MacOS, aarch64