Struct VaList 

#[repr(transparent)]
pub struct VaList<'a> {
    inner: VaListInner,
    _marker: PhantomCovariantLifetime<'a>,
}

🔬This is a nightly-only experimental API. (c_variadic #44930)

A variable argument list, ABI-compatible with va_list in C.

This type is created in c-variadic functions when ... is desugared. A VaList is automatically initialized (equivalent to calling va_start in C).

#![feature(c_variadic)]

use std::ffi::VaList;

/// # Safety
/// Must be passed at least `count` arguments of type `i32`.
unsafe extern "C" fn my_func(count: u32, ap: ...) -> i32 {
    unsafe { vmy_func(count, ap) }
}

/// # Safety
/// Must be passed at least `count` arguments of type `i32`.
unsafe fn vmy_func(count: u32, mut ap: VaList<'_>) -> i32 {
    let mut sum = 0;
    for _ in 0..count {
        sum += unsafe { ap.next_arg::<i32>() };
    }
    sum
}

assert_eq!(unsafe { my_func(1, 42i32) }, 42);
assert_eq!(unsafe { my_func(3, 42i32, -7i32, 20i32) }, 55);

The VaList::next_arg method reads the next argument from the variable argument list, and is equivalent to C va_arg.

Cloning a VaList performs the equivalent of C va_copy, producing an independent cursor that arguments can be read from without affecting the original. Dropping a VaList performs the equivalent of C va_end.

A VaList can be used across an FFI boundary, and fully matches the platform’s va_list in terms of layout and ABI.

Fields

inner: VaListInner

🔬This is a nightly-only experimental API. (c_variadic #44930)

_marker: PhantomCovariantLifetime<'a>

🔬This is a nightly-only experimental API. (c_variadic #44930)

Implementations

impl VaList<'_>

pub(crate) const fn duplicate(&self) -> Self

🔬This is a nightly-only experimental API. (c_variadic #44930)

impl<'f> VaList<'f>

pub const unsafe fn next_arg<T: VaArgSafe>(&mut self) -> T

🔬This is a nightly-only experimental API. (c_variadic #44930)

Read the next argument from the variable argument list.

Only types that implement VaArgSafe can be read from a variable argument list.

Safety

This function is safe to call only if all of the following conditions are satisfied:

• There is another c-variadic argument to read.
• The actual type of the argument U is compatible with T (as defined below).
• If U and T are both integer types, then the value passed by the caller must be representable in both types.

Types T and U are compatible when:

• T and U are the same type.
• T and U are integer types of the same size.
• T and U are both pointers, and their target types are compatible.
• T is a pointer to c_void and U is a pointer to i8 or u8, or vice versa.

Trait Implementations

impl<'f> Clone for VaList<'f>

fn clone(&self) -> Self

Clone the VaList, producing a second independent cursor into the variable argument list.

Corresponds to va_copy in C.

fn clone_from(&mut self, source: &Self)

where Self:,

Performs copy-assignment from source.

impl Debug for VaList<'_>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'f> Drop for VaList<'f>

fn drop(&mut self)

Drop the VaList.

Corresponds to va_end in C.

fn pin_drop(self: Pin<&mut Self>)

🔬This is a nightly-only experimental API. (pin_ergonomics #130494)

Execute the destructor for this type, but different to Drop::drop, it requires self to be pinned.