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Extend explanation of lazy_static

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Philipp Oppermann
2018-06-17 20:08:06 +02:00
parent 01ac5b5ea0
commit c39835e61e
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blog/content/second-edition/posts/03-vga-text-buffer/index.md
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@@ -443,7 +443,9 @@ The issue about `ColorCode::new` would be solvable by using [`const` functions],
[`const` functions]: https://doc.rust-lang.org/unstable-book/language-features/const-fn.html [`const` functions]: https://doc.rust-lang.org/unstable-book/language-features/const-fn.html
### Lazy Statics ### Lazy Statics
Fortunately the `lazy_static` macro exists. Instead of evaluating a `static` at compile time, the macro performs the initialization when the `static` is referenced the first time. Thus, we can do almost everything in the initialization block and are even able to read runtime values. The one-time initialization of statics with non-const functions is a common problem in Rust. Fortunately, there already exists a good solution in a crate named [lazy_static]. This crate provides a `lazy_static!` macro that defines a lazily initialized `static`. Instead of computing its value at compile time, the `static` laziliy initializes itself when it's accessed the first time. Thus, the initialization happens at runtime so that arbitrarily complex initialization code is possible.
[lazy_static]: https://docs.rs/lazy_static/1.0.1/lazy_static/
Let's add the `lazy_static` crate to our project: Let's add the `lazy_static` crate to our project:
@@ -461,7 +463,8 @@ extern crate lazy_static;
version = "1.0" version = "1.0"
features = ["spin_no_std"] features = ["spin_no_std"]
``` ```
We need the `spin_no_std` feature, since we don't link the standard library.
We need the `spin_no_std` feature, since we don't link the standard library. We also need the `#[macro_use]` attribute on the `extern crate` line to import the `lazy_static!` macro.
With `lazy_static`, we can define our static `WRITER` without problems: With `lazy_static`, we can define our static `WRITER` without problems: