What is the best way to convert from a C++ std::vector to a Rust Vec?

Question

I wrote a test program to check that the data will be read correctly on the Rust.

C++ code:

std::vector<float> vec;
vec.push_back(10);
vec.push_back(20);
std::cout << "len is " << get_len(&vec);

Rust code:

#[no_mangle]
pub extern "C" fn get_len(vec: *const Vec<f32>) -> i32 {
    let vec= unsafe { &*vec};
    vec.len() as i32
}

My output looks like this and is always different:

len is 603268424

C++'s `std::vector` has no guarantees on layout (it's very likely to be different across standard library implementations). You need to reduce it to something that both languages can understand, possibly something like passing `vec.data()` and `vec.size()` separately. — milleniumbug, Jun 06 '19 at 16:58

score 11 · Accepted Answer · edited Jun 06 '19 at 19:29

One thing to know is that C++ doesn't allow giving away the ownership of the data vector. This is a limitation of the C++ design. There are some tricks but I don't advise using them. To stay "safe", you can only borrow a C++ vector.

This can be done easily, using vector::data() and vector::size():

#include <cstddef>
#include <vector>

extern "C" void show_vector(float const *data, std::size_t size);

int main() {
    std::vector<float> vec{10, 20};
    show_vector(vec.data(), vec.size());
}

The Rust side can use slice::from_raw_parts(), creating a view of our vector without taking ownership.

use libc::{c_float, size_t};

extern "C" fn show_vector(data: *const c_float, size: size_t) {
    let vec = unsafe { std::slice::from_raw_parts(data, size) };
    for x in vec {
        println!("{}", x);
    }
}

What is the best way to convert from a C++ std::vector to a Rust Vec?

1 Answers1