# Embedding Julia: reading std::vector from C++?

**URL:** https://discourse.julialang.org/t/embedding-julia-reading-std-vector-from-c/100857
**Category:** General Usage
**Tags:** embedding
**Created:** [June 26, 2023, 7:21pm UTC](https://discourse.julialang.org/t/embedding-julia-reading-std-vector-from-c/100857 "2023-06-26T19:21:12Z")
**Posts on this page:** 2
**Page:** 1

<div class="post-metadata">

### Author: ![frylock](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frylock/32/50213_2.png) [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [June 26, 2023, 7:21pm UTC](https://discourse.julialang.org/t/embedding-julia-reading-std-vector-from-c/100857/1 "2023-06-26T19:21:13Z")

</div>

I’m aware of [existing](https://github.com/JuliaInterop/Cxx.jl) … [packages](https://github.com/JuliaInterop/CxxWrap.jl) for wrapping C++, this is really more of a learning exercise for me (and I’m trying to learn enough to read what is in the existing pacages). I knew that it was possible to get C++ to give me a pointer to the contiguous memory of the `std::vector` contents, so I figured that I would hand it over to Julia like this:

```plaintext
#include <vector>
#include <julia.h>
#include <iostream>

JULIA_DEFINE_FAST_TLS

int main(int argc, char *argv[]) {
    std::vector<int> stuff = {1, 2, 3, 4}; // ordinarily, I'd use `auto` but wanted to be explicit

    auto contents = stuff.data();
    std::cout << "stuff.data(): " << stuff.data() << "\n";
    std::cout << "stuff.data()[0]: " << contents[0] << "\n"; // prints 1
    std::cout << "stuff.data()[1]: " << contents[1] << "\n"; // prints 2
    std::cout << "stuff.data()[2]: " << contents[2] << "\n"; // prints 3
    std::cout << "stuff.data()[3]: " << contents[3] << "\n"; // prints 4
    std::cout << "stuff.size(): " << stuff.size() << std::endl;

    jl_init();

    jl_value_t *array_type = jl_apply_array_type((jl_value_t *)jl_int64_type, 1);
    jl_array_t *julia_stuff = jl_ptr_to_array_1d(array_type, contents, stuff.size(), 0);

    // Have Julia print your vector.
    jl_function_t *prnt = jl_get_function(jl_base_module, "println");
    jl_call1(prnt, (jl_value_t *)julia_stuff);

    jl_atexit_hook(0);
    return EXIT_SUCCESS;
}

```

Built it with:

```bash
function build_cpp_thing {
    source_file=$1
    basename="${source_file%%.*}"
    g++ -I$JULIA_DIR/usr/include/julia \
        -L$JULIA_DIR/usr/lib \
        -Wl,-rpath,$JULIA_DIR/usr/lib \
        -g \
        $source_file -ljulia -o $basename
}

```

however, I seem to get garbage out of the print line:

```julia
stuff.data(): 0x563fc0cd4b20
stuff.data()[0]: 1
stuff.data()[1]: 2
stuff.data()[2]: 3
stuff.data()[3]: 4
stuff.size(): 4
[8589934593, 17179869187, 0, 1041]

```

??

---

<div class="post-metadata">

### Author: ![frylock](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/frylock/32/50213_2.png) [@frylock](https://discourse.julialang.org/u/frylock)
#### Post date: [June 26, 2023, 7:31pm UTC](https://discourse.julialang.org/t/embedding-julia-reading-std-vector-from-c/100857/2 "2023-06-26T19:31:43Z")

</div>

Oh no - I see my problem, I specified `jl_int64_type` instead of `jl_int32_type`.  
🤪