An example Polyglot project that demonstrates how Rust can use C++ functions that use std::string

example.cpp

#include <iostream>
#include <string>

std::string cppStdString()
{
     return "This is a C++ std::string";
}

std::string lotsOfStdStrings(std::string a, std::string b)
{
    return a + b;
}

example.h

// While polybuild is not explicitly told to use this file, it is assumed that you will provide
// headers for any C++ code that you want exposed. After all, your other C++ code wouldn't be
// able to use it otherwise.

#include <string>

std::string cppStdString();
std::string lotsOfStdStrings(std::string a, std::string b);

example.proxy.cpp

// The content below was generated by Polyglot to wrap each std::string function to
// take and return C-style strings instead.

// *** WARNING: autogenerated file, do not modify. Changes will be overwritten. ***
// Generated by Polyglot version 0.0.1-devel at Tue Dec 26 17:18:00 2023.
// This file contains type proxies for C++.

#include <cstring>

#include "../../example.h"

extern "C" const char *cppStdString_polyglot_typeproxy()
{
	return strdup(cppStdString().c_str());
}
extern "C" const char *lotsOfStdStrings_polyglot_typeproxy(const char *a, const char *b)
{
	return strdup(lotsOfStdStrings(a, b).c_str());
}

example.rs

// The content below was generated by Polyglot. It uses Rust's FFI library to convert
// C-style strings into usable String objects.

// *** WARNING: autogenerated file, do not modify. Changes will be overwritten. ***
// Generated by Polyglot version 0.0.1-devel at Tue Dec 26 17:18:00 2023.
// This file contains symbols that have been exported from C++ into Rust.

// This import supports various Polyglot features; however, it may not be used in every
// Polyglot wrapper file.
use std::ffi::CString;

extern {
	#[link_name = "cppStdString_polyglot_typeproxy"] fn cppStdString_polyglot_typeproxy() -> *mut i8;
}

#[allow(non_snake_case)]
pub fn cppStdString() -> String {
	unsafe {
		CString::from_raw(cppStdString_polyglot_typeproxy())
			.into_string()
			.expect("Failed to convert C-style string to String in cppStdString")
	}
}

extern {
	#[link_name = "lotsOfStdStrings_polyglot_typeproxy"] fn lotsOfStdStrings_polyglot_typeproxy(a: *const i8, b: *const i8) -> *mut i8;
}

#[allow(non_snake_case)]
pub fn lotsOfStdStrings(a: String, b: String) -> String {
	unsafe {
		CString::from_raw(lotsOfStdStrings_polyglot_typeproxy(
			CString::new(a).expect("Failed to convert parameter a of lotsOfStdStrings into CString").into_raw(), 
			CString::new(b).expect("Failed to convert parameter b of lotsOfStdStrings into CString").into_raw())
		)
			.into_string()
			.expect("Failed to convert C-style string to String in lotsOfStdStrings")
	}
}

extern {
}

main.rs

mod example;

fn main() {
	println!("{}", example::cppStdString());
	println!("{}", example::lotsOfStdStrings("This is a ".to_string(), "two-part string".to_string()));
}

polybuild.yml

# This file tells polybuild which sources to use

name: example
sources:
    - example.cpp
    - main.rs