From b99a2542f38a2d08b863d2e2ab73bfd64647e520 Mon Sep 17 00:00:00 2001 From: Tim Chevalier Date: Wed, 19 Dec 2012 21:32:19 -0700 Subject: [PATCH] docs: mention static methods /cc #4217 --- doc/rust.md | 19 +++++++++++++++++-- doc/tutorial.md | 26 ++++++++++++++++++++++++++ 2 files changed, 43 insertions(+), 2 deletions(-) diff --git a/doc/rust.md b/doc/rust.md index 8bff1aa37af..f1064c395aa 100644 --- a/doc/rust.md +++ b/doc/rust.md @@ -1195,8 +1195,23 @@ Values with a trait type can have [methods called](#method-call-expressions) on for any method in the trait, and can be used to instantiate type parameters that are bounded by the trait. -Trait methods may be static. -Currently, implementations of static methods behave like functions declared in the implementation's module. +Trait methods may be static, +which means that they lack a `self` argument. +This means that they can only be called with function call syntax (`f(x)`) +and not method call syntax (`obj.f()`). +The way to refer to the name of a static method is to qualify it with the trait name, +treating the trait name like a module. +For example: + +~~~~ +trait Num { + static pure fn from_int(n: int) -> self; +} +impl float: Num { + static pure fn from_int(n: int) -> float { n as float } +} +let x: float = Num::from_int(42); +~~~~ Traits can have _constraints_ for example, in diff --git a/doc/tutorial.md b/doc/tutorial.md index c7b75993f11..8c57f2883ad 100644 --- a/doc/tutorial.md +++ b/doc/tutorial.md @@ -2076,6 +2076,32 @@ the preferred way to use traits polymorphically. This usage of traits is similar to Haskell type classes. +## Static methods + +Traits can define _static_ methods, which don't have an implicit `self` argument. +The `static` keyword distinguishes static methods from methods that have a `self`: + +~~~~ +trait Shape { + fn area() -> float; + static fn new_shape(area: float) -> Shape; +} +~~~~ + +Constructors are one application for static methods, as in `new_shape` above. +To call a static method, you have to prefix it with the trait name and a double colon: + +~~~~ +# trait Shape { static fn new_shape(area: float) -> self; } +# use float::consts::pi; +# use float::sqrt; +struct Circle { radius: float } +impl Circle: Shape { + static fn new_shape(area: float) -> Circle { Circle { radius: sqrt(area / pi) } } +} +let s: Circle = Shape::new_shape(42.5); +~~~~ + ## Trait constraints We can write a trait declaration that is _constrained_ to only be implementable on types that