Example: String interning library

C++ Header: interner.hpp

#ifndef INTERNER_HPP
#define INTERNER_HPP

#include <string>
#include <unordered_set>

class StringInterner {
    std::unordered_set<std::string> strings;

public:
    // Returns a pointer to the interned string (valid for lifetime of interner)
    const char* intern(const char* s) {
        auto [it, _] = strings.emplace(s);
        return it->c_str();
    }

    size_t count() const {
        return strings.size();
    }
};

#endif

C header file: interner.h

// interner.h (C API for FFI)
#ifndef INTERNER_H
#define INTERNER_H

#include <stddef.h>

#ifdef __cplusplus
extern "C" {
#endif

typedef struct StringInterner StringInterner;

StringInterner* interner_new(void);
void interner_free(StringInterner* interner);
const char* interner_intern(StringInterner* interner, const char* s);
size_t interner_count(const StringInterner* interner);

#ifdef __cplusplus
}
#endif

C++ implementation (interner.cpp)

#include "interner.hpp"
#include "interner.h"

extern "C" {

StringInterner* interner_new(void) {
    return new StringInterner();
}

void interner_free(StringInterner* interner) {
    delete interner;
}

const char* interner_intern(StringInterner* interner, const char* s) {
    return interner->intern(s);
}

size_t interner_count(const StringInterner* interner) {
    return interner->count();
}

}

#![allow(unused)]
fn main() {
use std::ffi::{CStr, CString};
use std::marker::PhantomData;
use std::os::raw::c_char;

#[repr(C)]
pub struct StringInternerRaw {
    _opaque: [u8; 0],
    _pin: PhantomData<(*mut u8, std::marker::PhantomPinned)>,
}

unsafe extern "C" {
    fn interner_new() -> *mut StringInternerRaw;

    fn interner_free(interner: *mut StringInternerRaw);

    fn interner_intern(
        interner: *mut StringInternerRaw,
        s: *const c_char,
    ) -> *const c_char;

    fn interner_count(interner: *const StringInternerRaw) -> usize;
}
}