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Make vec!() macro create a vector with precise capacity #16204

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Make vec!() macro create a vector with precise capacity #16204

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1653c86
Add utility macro to count arguments in a comma-separated list
bkoropoff Aug 2, 2014
224cd1e
Make vec!() macro create a vector with a precise capacity
bkoropoff Aug 2, 2014
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11 changes: 10 additions & 1 deletion src/libstd/macros.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -348,12 +348,21 @@ macro_rules! try(
($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
)

/// Expands to an expression which evaluates to the number of
/// comma-separated expression parameters passed to it.
#[macro_export]
macro_rules! count_args(
($e:expr $(,$e_rest:expr)*) => (1u + count_args!($($e_rest),*));
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This pattern looks weird, shouldn't it be ($e:expr, $(e_rest:expr),*)?

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That doesn't work with one argument i.e. count_args!(foo).

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Ah. You can always add a one-argument case though. But I suppose if this pattern works, then it doesn't matter.

() => (0u);
($($e:expr),*,) => (count_args!($($e),*))
)

/// Create a `std::vec::Vec` containing the arguments.
#[macro_export]
macro_rules! vec(
($($e:expr),*) => ({
// leading _ to allow empty construction without a warning.
let mut _temp = ::std::vec::Vec::new();
let mut _temp = ::std::vec::Vec::with_capacity(count_args!($($e),*));
$(_temp.push($e);)*
_temp
});
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