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Rust元编程

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chlict edited this page Apr 26, 2020 · 1 revision

html的例子

# #![allow(unused_must_use)]
macro_rules! write_html {
    ($w:expr, ) => (());

    ($w:expr, $e:tt) => (write!($w, "{}", $e));

    ($w:expr, $tag:ident [ $($inner:tt)* ] $($rest:tt)*) => {{
        write!($w, "<{}>", stringify!($tag));
        write_html!($w, $($inner)*);
        write!($w, "</{}>", stringify!($tag));
        write_html!($w, $($rest)*);
    }};
}

fn main() {
#   // FIXME(#21826)
    use std::fmt::Write;
    let mut out = String::new();

    write_html!(&mut out,
        html[
            head[title["Macros guide"]]
            body[h1["Macros are the best!"]]
        ]);

    assert_eq!(out,
        "<html><head><title>Macros guide</title></head>\
         <body><h1>Macros are the best!</h1></body></html>");
}

过程宏

宏也有库：syn和quote

也就是说，在编译器处理到 hw! 宏的时候会将 hw! 所有参数打包为 TokenStream 交给上面定义的函数执行，然后用返回的 TokenStream 替换掉原先宏所占据的源码位置。

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