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{{#title *mut T, *const T — Rust ♡ C++}}
# *mut T, *const T
Generally you should use references (`&mut T`, `&T`) or [std::unique_ptr\<T\>]
where possible over raw pointers, but raw pointers are available too as an
unsafe fallback option.
[std::unique_ptr\<T\>]: uniqueptr.md
### Restrictions:
Extern functions and function pointers taking a raw pointer as an argument must
be declared `unsafe fn` i.e. unsafe to call. The same does not apply to
functions which only *return* a raw pointer, though presumably doing anything
useful with the returned pointer is going to involve unsafe code elsewhere
anyway.
## Example
This example illustrates making a Rust call to a canonical C-style `main`
signature involving `char *argv[]`.
```cpp
// include/args.h
#pragma once
void parseArgs(int argc, char *argv[]);
```
```cpp
// src/args.cc
#include "example/include/args.h"
#include <iostream>
void parseArgs(int argc, char *argv[]) {
std::cout << argc << std::endl;
for (int i = 0; i < argc; i++) {
std::cout << '"' << argv[i] << '"' << std::endl;
}
}
```
```rust,noplayground
// src/main.rs
use std::env;
use std::ffi::CString;
use std::os::raw::c_char;
use std::os::unix::ffi::OsStrExt;
use std::ptr;
#[cxx::bridge]
mod ffi {
extern "C++" {
include!("example/include/args.h");
unsafe fn parseArgs(argc: i32, argv: *mut *mut c_char);
}
}
fn main() {
// Convert from OsString to nul-terminated CString, truncating each argument
// at the first inner nul byte if present.
let args: Vec<CString> = env::args_os()
.map(|os_str| {
let bytes = os_str.as_bytes();
CString::new(bytes).unwrap_or_else(|nul_error| {
let nul_position = nul_error.nul_position();
let mut bytes = nul_error.into_vec();
bytes.truncate(nul_position);
CString::new(bytes).unwrap()
})
})
.collect();
// Convert from Vec<CString> of owned strings to Vec<*mut c_char> of
// borrowed string pointers.
//
// Once extern type stabilizes (https://github.com/rust-lang/rust/issues/43467)
// and https://internals.rust-lang.org/t/pre-rfc-make-cstr-a-thin-pointer/6258
// is implemented, and CStr pointers become thin, we can sidestep this step
// by accumulating the args as Vec<Box<CStr>> up front, then simply casting
// from *mut [Box<CStr>] to *mut [*mut CStr] to *mut *mut c_char.
let argc = args.len();
let mut argv: Vec<*mut c_char> = Vec::with_capacity(argc + 1);
for arg in &args {
argv.push(arg.as_ptr() as *mut c_char);
}
argv.push(ptr::null_mut()); // Nul terminator.
unsafe {
ffi::parseArgs(argc as i32, argv.as_mut_ptr());
}
// The CStrings go out of scope here. C function must not have held on to
// the pointers beyond this point.
}
```
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