diff options
Diffstat (limited to '2023/05')
| -rw-r--r-- | 2023/05/01/Rust-虚表布局规则介绍/index.html | 21 | ||||
| -rw-r--r-- | 2023/05/02/Rust-Partial-语义/index.html | 11 | ||||
| -rw-r--r-- | 2023/05/03/Rust-NewType-模式/index.html | 11 | ||||
| -rw-r--r-- | 2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html | 5 |
4 files changed, 24 insertions, 24 deletions
diff --git a/2023/05/01/Rust-虚表布局规则介绍/index.html b/2023/05/01/Rust-虚表布局规则介绍/index.html index f4502594..84a18b0c 100644 --- a/2023/05/01/Rust-虚表布局规则介绍/index.html +++ b/2023/05/01/Rust-虚表布局规则介绍/index.html @@ -196,39 +196,38 @@ <blockquote> <p>注意：Rust 虚表及其结构属于 Rust 语言的内部实现细节，不保证稳定性。本文所介绍的虚表布局仅反映本文创作时最新的 Rust 虚表结构[1]，在将来 Rust 虚表结构可能会发生变化。一个 Rust 程序的正确性不应该以任何方式依赖于 Rust 虚表的结构。</p> </blockquote> -<h2 id="基本结构"><a href="#基本结构" class="headerlink" title="基本结构"></a>基本结构</h2><p>Rust 程序中的所有虚表均以一个固定结构的 header 开头。Header 中按顺序包含三个usize 大小的字段：drop_in_place ，size 和 align 。在 header 之后是一系列的 usize 大小字段，其数量以及含义在每个虚表中可能都不同。</p> +<h2 id="基本结构"><a class="header-anchor" href="#基本结构">¶</a>基本结构</h2> +<p>Rust 程序中的所有虚表均以一个固定结构的 header 开头。Header 中按顺序包含三个usize 大小的字段：drop_in_place ，size 和 align 。在 header 之后是一系列的 usize 大小字段，其数量以及含义在每个虚表中可能都不同。</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">+---------------+</span><br><span class="line">| drop_in_place |</span><br><span class="line">+---------------+</span><br><span class="line">| size |</span><br><span class="line">+---------------+</span><br><span class="line">| align |</span><br><span class="line">+---------------+</span><br><span class="line">| entry1 |</span><br><span class="line">+---------------+</span><br><span class="line">| entry2 |</span><br><span class="line">+---------------+</span><br><span class="line">| entry3 |</span><br><span class="line">+---------------+</span><br></pre></td></tr></table></figure> <p>虚表 header 中的drop_in_place 是一个函数指针，其指向的函数能够原地 drop 当前胖指针所引用的对象。size 和 align 两个域分别给出对象的大小和内存对齐，这两个域共同构成一个 std::alloc::Layout 结构，可用于释放当前胖指针所引用的对象所占据的内存。虚表 header 的存在使得 trait object 总是能被销毁和释放。例如当销毁一个 <code>Box<dyn Trait></code> 时，<code>Box::<dyn Trait>::drop</code> 会首先调用虚表中的 drop_in_place 函数原地销毁 Box 所引用的对象，然后再调用 dealloc 函数并传递虚表中的 size 和 align 释放堆空间。</p> <p>在虚表 header 之后是一系列的字段。在最普遍的情况下，每个字段代表一个指向 trait 所定义的函数的指针。例如，对于下列 object safe 的 trait:</p> <figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">pub</span> <span class="keyword">trait</span> <span class="title class_">Trait</span> {</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun1</span>(&<span class="keyword">self</span>);</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun2</span>(&<span class="keyword">self</span>);</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">fun3</span>(&<span class="keyword">self</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> - <p>如果类型T 实现了 Trait，那么为 T 生成的 Trait 虚表的结构为：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">+--------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+--------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun1 |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun2 |</span><br><span class="line">+--------------------------+</span><br><span class="line">| fn <T as Trait>::fun3 |</span><br><span class="line">+--------------------------+</span><br></pre></td></tr></table></figure> - <p>Trait 中的函数按照声明顺序依次排列在虚表 header 之后。当通过一个指向 T 对象的 &dyn Trait 调用 fun2 函数时，程序会先从虚表的第 5 个域中得到为 T 实现的 Trait::fun2 函数的地址，然后再调用之。</p> -<h2 id="Super-Trait"><a href="#Super-Trait" class="headerlink" title="Super Trait"></a>Super Trait</h2><p>Object safe 的 trait 可以有 super trait。例如：</p> +<h2 id="Super-Trait"><a class="header-anchor" href="#Super-Trait">¶</a>Super Trait</h2> +<p>Object safe 的 trait 可以有 super trait。例如：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">pub trait Grand {</span><br><span class="line"> fn grand_fun1(&self);</span><br><span class="line"> fn grand_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Parent : Grand {</span><br><span class="line"> fn parent_fun1(&self);</span><br><span class="line"> fn parent_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Trait : Parent {</span><br><span class="line"> fn fun(&self);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> - <p>如果类型T 实现了 Trait，那么此时为 T 生成的 Trait 虚表的结构为：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">+-------------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Grand>::grand_fun1 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Grand>::grand_fun2 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Parent>::parent_fun1 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Parent>::parent_fun2 |</span><br><span class="line">+-------------------------------+</span><br><span class="line">| fn <T as Trait>::fun |</span><br><span class="line">+-------------------------------+</span><br></pre></td></tr></table></figure> - <p>可以看到，此时Trait 以及 Trait 的所有直接或间接父 trait 所定义的所有函数均包含在虚表 header 之后，且顺序为后序（即先排布 Trait 的父 trait 所定义的所有函数，最后再排布 Trait 所定义的所有函数）。这样的排布方式使得在得到 T 类型的 Trait 虚表的同时也同时得到了 T 类型的 Parent 虚表和 Grand 虚表。T 类型的 Grand 虚表恰好由 Trait 虚表的前五个域构成，T 类型的 Parent 虚表恰好由 Trait 虚表的前七项构成。这使得向上转换变得非常简单。</p> <p>所谓向上转换，即 Rust 允许将&dyn Trait 转换为 &dyn Parent 或 &dyn Grand 。在向上转换的过程中，胖指针的对象地址域保持不变，但 metadata 域可能需要进行调整，因为不同的 trait 可能具有不同的虚表地址。但在当前示例中，向上转换不需要调整 metadata 域，因为一个指向 Trait 虚表的指针同时也指向 Parent 虚表和 Grand 虚表。在后文中我们会进一步介绍需要调整 metadata 域的向上转换的情况。</p> <blockquote> <p>注意：目前 stable Rust 暂不支持向上转换。要使用向上转换特性，必须使用 nightly 工具链，并向源文件中添加 #![feature(trait_upcasting)] 特性开关。</p> </blockquote> -<h2 id="多重继承"><a href="#多重继承" class="headerlink" title="多重继承"></a>多重继承</h2><p>Trait 可以有多个 super trait。例如：</p> +<h2 id="多重继承"><a class="header-anchor" href="#多重继承">¶</a>多重继承</h2> +<p>Trait 可以有多个 super trait。例如：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line">pub trait Base {</span><br><span class="line"> fn base_fun1(&self);</span><br><span class="line"> fn base_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Left : Base {</span><br><span class="line"> fn left_fun1(&self);</span><br><span class="line"> fn left_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Right : Base {</span><br><span class="line"> fn right_fun1(&self);</span><br><span class="line"> fn right_fun2(&self);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">pub trait Trait : Left + Right {</span><br><span class="line"> fn fun(&self);</span><br><span class="line">}</span><br></pre></td></tr></table></figure> - <p>如果类型T 实现了 Trait，那么此时为 T 生成的 Trait 虚表的结构为：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">+-----------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Base>::base_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Base>::base_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Left>::left_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Left>::left_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Right>::right_fun1 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Right>::right_fun2 |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| ptr to <T as Right>::vtable |</span><br><span class="line">+-----------------------------+</span><br><span class="line">| fn <T as Trait>::fun |</span><br><span class="line">+-----------------------------+</span><br></pre></td></tr></table></figure> - <p>可以看到，此时Trait 及其所有直接或间接父 trait 所定义的所有函数仍然包含在虚表内，因此通过 &dyn Trait 调用的函数仍然可以直接从虚表内得到其实际目标函数的地址。另外，Trait 虚表内仍然包含有效的 Base 虚表和 Left 虚表。因此，将 &dyn Trait 向上转换为 &dyn Left 或 &dyn Base 仍然是极其简单的，不需要调整胖指针的 metadata 域。但是，将 &dyn Trait 向上转换为 &dyn Right 就需要调整 metadata 域了，因为 Trait 虚表内并不包含一个有效的 Right 虚表。这也是 Trait 虚表中 ptr to <code><T as Right>::vtable</code> 域的作用：在执行向上转换时，程序会读取 Trait 虚表的这个域作为得到的 &dyn Right 胖指针的 metadata 。这也是 <em>Rust 向上转换与 C++ 向上转换的一个很大不同：在 C++ 中的向上转换通常并不需要访问虚表（除非需要执行跨虚继承边界的转换），但在 Rust 中向上转换可能需要访问虚表。</em></p> <p>更加一般地，对于一个 object safe 的 traitTr，将其第一个父 trait、第一个父 trait 的第一个父 trait、…… 这一系列直接或间接父 trait 记为这个 trait 的 PrefixTrait 集合。在将 &dyn Tr 向上转换时，如果转换到的目标 trait 包含在 PrefixTrait 集合内，那么这个向上转换是平凡的：不需要调整胖指针的 metadata 域。否则，这个向上转换需要在 Tr 的虚表内读取目标 trait 的虚表指针作为转换结果的 metadata 。在 Tr 的虚表结构中，位于 PrefixTrait 集合中的父 trait 只需要排布他们所定义的函数即可；对于其他父 trait 还需要额外在虚表内排布一个指向其虚表的指针用于向上转换。</p> -<h2 id="向下转换"><a href="#向下转换" class="headerlink" title="向下转换"></a>向下转换</h2><p>Rust 提供了一个特殊的 trait：std::any::Any 。该 trait 支持向下转换，即可以将 &dyn Any 转换为 T 。转换过程中会对胖指针所指向的对象的实际类型进行检查，确认其确实是一个 T 类型的对象。Any trait 的虚表结构有一些特殊；在虚表 header 之后，Any 虚表仅包含一个域，这个域直接给出胖指针指向的对象的类型标识（由一个 std::any::TypeId 类型的值表示）。例如，对于任意的 T: ‘static，编译器为其生成的 Any 虚表为：</p> +<h2 id="向下转换"><a class="header-anchor" href="#向下转换">¶</a>向下转换</h2> +<p>Rust 提供了一个特殊的 trait：std::any::Any 。该 trait 支持向下转换，即可以将 &dyn Any 转换为 T 。转换过程中会对胖指针所指向的对象的实际类型进行检查，确认其确实是一个 T 类型的对象。Any trait 的虚表结构有一些特殊；在虚表 header 之后，Any 虚表仅包含一个域，这个域直接给出胖指针指向的对象的类型标识（由一个 std::any::TypeId 类型的值表示）。例如，对于任意的 T: 'static，编译器为其生成的 Any 虚表为：</p> <figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">+--------------------------+</span><br><span class="line">| fn drop_in_place(*mut T) |</span><br><span class="line">+--------------------------+</span><br><span class="line">| size of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| align of T |</span><br><span class="line">+--------------------------+</span><br><span class="line">| TypeId of T |</span><br><span class="line">+--------------------------+</span><br></pre></td></tr></table></figure> <p>在执行向下转换时，程序首先检查转换到的类型是否与虚表中给出的TypeId 所标识的类型一致。若类型检查通过，向下转换操作可以直接返回胖指针中的指针域作为转换结果。</p> -<h3 id="REFERENCE"><a href="#REFERENCE" class="headerlink" title="REFERENCE"></a>REFERENCE</h3><ol> +<h3 id="REFERENCE"><a class="header-anchor" href="#REFERENCE">¶</a>REFERENCE</h3> +<ol> <li>Vtable format to support dyn upcasting coercion <a target="_blank" rel="noopener" href="https://rust-lang.github.io/dyn-upcasting-coercion-initiative/design-discussions/vtable-layout.*html">https://rust-lang.github.io/dyn-upcasting-coercion-initiative/design-discussions/vtable-layout.*html</a>*</li> </ol> diff --git a/2023/05/02/Rust-Partial-语义/index.html b/2023/05/02/Rust-Partial-语义/index.html index b2745990..d56cbb23 100644 --- a/2023/05/02/Rust-Partial-语义/index.html +++ b/2023/05/02/Rust-Partial-语义/index.html @@ -195,15 +195,16 @@ <blockquote> <p>在Rust中，PartialEq和PartialOrd trait处理了不是所有值都可以相互比较的情况。</p> </blockquote> -<h2 id="PartialEq-Trait"><a href="#PartialEq-Trait" class="headerlink" title="PartialEq Trait"></a>PartialEq Trait</h2><p>PartialEq trait用于定义值相等性的比较。它的设计允许类型的值之间进行相等（<code>==</code>）和不等（<code>!=</code>）的比较。与其对应的 Eq trait 确保一个类型的所有值都是可以可靠比较的，即满足等价关系的特性，如自反性、对称性和传递性。</p> +<h2 id="PartialEq-Trait"><a class="header-anchor" href="#PartialEq-Trait">¶</a>PartialEq Trait</h2> +<p>PartialEq trait用于定义值相等性的比较。它的设计允许类型的值之间进行相等（<code>==</code>）和不等（<code>!=</code>）的比较。与其对应的 Eq trait 确保一个类型的所有值都是可以可靠比较的，即满足等价关系的特性，如自反性、对称性和传递性。</p> <figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">fn</span> <span class="title function_">eq</span>(&<span class="keyword">self</span>, other: &<span class="keyword">Self</span>) <span class="punctuation">-></span> <span class="type">bool</span>;</span><br><span class="line"><span class="keyword">fn</span> <span class="title function_">ne</span>(&<span class="keyword">self</span>, other: &<span class="keyword">Self</span>) <span class="punctuation">-></span> <span class="type">bool</span>;</span><br></pre></td></tr></table></figure> - <p>在大多数情况下，类型的值都能够完全比较相等性，这时可以实现Eq。然而，对于一些特殊类型的值，如浮点数，由于存在无穷大的正负值和NaN值，导致它们的比较更加复杂。例如，根据IEEE浮点数的标准，NaN与任何值（包括它自己）比较都不相等。</p> -<h2 id="PartialOrd-Trait"><a href="#PartialOrd-Trait" class="headerlink" title="PartialOrd Trait"></a>PartialOrd Trait</h2><p>PartialOrd trait用于定义值之间的大小比较。类似于PartialEq，它允许部分比较大小，返回一个Option，表示比较结果可能存在，也可能不存在（即比较无法进行时返回None）:</p> +<h2 id="PartialOrd-Trait"><a class="header-anchor" href="#PartialOrd-Trait">¶</a>PartialOrd Trait</h2> +<p>PartialOrd trait用于定义值之间的大小比较。类似于PartialEq，它允许部分比较大小，返回一个Option，表示比较结果可能存在，也可能不存在（即比较无法进行时返回None）:</p> <figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">fn</span> <span class="title function_">partial_cmp</span>(&<span class="keyword">self</span>, other: &<span class="keyword">Self</span>) <span class="punctuation">-></span> <span class="type">Option</span><Ordering>;</span><br></pre></td></tr></table></figure> - <p>在全部比较可能的场景，我们会使用Ord trait，它要求实现cmp方法，总是返回一个Ordering，表示两个值之间的确切比较关系。Ord是在所有值都能够比较时使用的，例如整数和字符串。</p> -<h2 id="设计用意和解决的问题"><a href="#设计用意和解决的问题" class="headerlink" title="设计用意和解决的问题"></a>设计用意和解决的问题</h2><p>Rust 设计 PartialEq 和 PartialOrd trait 主要出于以下几个理由：</p> +<h2 id="设计用意和解决的问题"><a class="header-anchor" href="#设计用意和解决的问题">¶</a>设计用意和解决的问题</h2> +<p>Rust 设计 PartialEq 和 PartialOrd trait 主要出于以下几个理由：</p> <ul> <li>非总序理念：并不是所有类型都有一个全局的排序方法。例如，复数之间就没有一个自然的大小顺序。为了避免为这些类型人为地赋予一个排序方法，Rust 提供了一个只需部分实现序列操作的选择。</li> <li>IEEE 浮点数标准：由于浮点数标准定义了特殊值（NaN, 正负无穷），以及NaN不等于自身的规则，浮点数在一些情况下不能进行相等性或大小比较。</li> diff --git a/2023/05/03/Rust-NewType-模式/index.html b/2023/05/03/Rust-NewType-模式/index.html index b14dadc3..24f2a78a 100644 --- a/2023/05/03/Rust-NewType-模式/index.html +++ b/2023/05/03/Rust-NewType-模式/index.html @@ -193,14 +193,15 @@ </div> <div class="post-content"> <p>New Type模式是一种软件设计模式，用于在已有类型的基础上创建一个新的类型。在Rust中，这通常是通过定义一个结构体，其中只包含一个单一成员。这个结构体（New Type）对外提供了一个新的、独立的类型，用于对原始类型增加额外的语义或限制。</p> -<h2 id="加强类型安全"><a href="#加强类型安全" class="headerlink" title="加强类型安全"></a>加强类型安全</h2><figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Meters</span>(<span class="type">f64</span>);</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Feet</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_meters</span> = <span class="title function_ invoke__">Meters</span>(<span class="number">100.0</span>);</span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_feet</span> = <span class="title function_ invoke__">Feet</span>(<span class="number">328.084</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 编译器会防止以下代码执行，因为类型不匹配</span></span><br><span class="line"><span class="comment">// let wrong_length = Meters(length_in_feet); // 编译错误</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 正确的构造</span></span><br><span class="line"><span class="keyword">fn</span> <span class="title function_">add_lengths</span>(length1: Meters, length2: Meters) <span class="punctuation">-></span> Meters {</span><br><span class="line"> <span class="title function_ invoke__">Meters</span>(length1.<span class="number">0</span> + length2.<span class="number">0</span>)</span><br><span class="line">}</span><br></pre></td></tr></table></figure> - +<h2 id="加强类型安全"><a class="header-anchor" href="#加强类型安全">¶</a>加强类型安全</h2> +<figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Meters</span>(<span class="type">f64</span>);</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">Feet</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_meters</span> = <span class="title function_ invoke__">Meters</span>(<span class="number">100.0</span>);</span><br><span class="line"><span class="keyword">let</span> <span class="variable">length_in_feet</span> = <span class="title function_ invoke__">Feet</span>(<span class="number">328.084</span>);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 编译器会防止以下代码执行，因为类型不匹配</span></span><br><span class="line"><span class="comment">// let wrong_length = Meters(length_in_feet); // 编译错误</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 正确的构造</span></span><br><span class="line"><span class="keyword">fn</span> <span class="title function_">add_lengths</span>(length1: Meters, length2: Meters) <span class="punctuation">-></span> Meters {</span><br><span class="line"> <span class="title function_ invoke__">Meters</span>(length1.<span class="number">0</span> + length2.<span class="number">0</span>)</span><br><span class="line">}</span><br></pre></td></tr></table></figure> <p>这个例子使用 newtype 模式避免将原始类型f64用于不同的量度，从而增强了类型的安全性。</p> -<h2 id="实现特定-trait"><a href="#实现特定-trait" class="headerlink" title="实现特定 trait"></a>实现特定 trait</h2><figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Kilometers</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">impl</span> <span class="title class_">Kilometers</span> {</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">to_miles</span>(&<span class="keyword">self</span>) <span class="punctuation">-></span> <span class="type">f64</span> {</span><br><span class="line"> <span class="keyword">self</span>.<span class="number">0</span> * <span class="number">0.621371</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">distance</span> = <span class="title function_ invoke__">Kilometers</span>(<span class="number">10.0</span>);</span><br><span class="line"><span class="built_in">println!</span>(<span class="string">"The distance in miles is {}"</span>, distance.<span class="title function_ invoke__">to_miles</span>());</span><br></pre></td></tr></table></figure> - +<h2 id="实现特定-trait"><a class="header-anchor" href="#实现特定-trait">¶</a>实现特定 trait</h2> +<figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">struct</span> <span class="title class_">Kilometers</span>(<span class="type">f64</span>);</span><br><span class="line"></span><br><span class="line"><span class="keyword">impl</span> <span class="title class_">Kilometers</span> {</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">to_miles</span>(&<span class="keyword">self</span>) <span class="punctuation">-></span> <span class="type">f64</span> {</span><br><span class="line"> <span class="keyword">self</span>.<span class="number">0</span> * <span class="number">0.621371</span></span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> <span class="variable">distance</span> = <span class="title function_ invoke__">Kilometers</span>(<span class="number">10.0</span>);</span><br><span class="line"><span class="built_in">println!</span>(<span class="string">"The distance in miles is {}"</span>, distance.<span class="title function_ invoke__">to_miles</span>());</span><br></pre></td></tr></table></figure> <p>这里，Kilometers有一个方法to_miles，该方法是不会影响其他f64数据的。如果我们有另一个表示温度的f64类型，就不会意外调用到与距离相关的方法。</p> <p>New Type模式同样适用于对<code>Box<dyn SomeTrait></code>类型的包装，这可以在需要动态分派（动态调用实现了某个接口的不同类型的对象的方法）的时候提供便利。通过创建一个New Type来包装这样的<code>Box<dyn SomeTrait></code>类型，可以提供自定义的方法或实现更多的trait，同时也可以让API更加清晰和易于使用。</p> -<h2 id="零成本抽象"><a href="#零成本抽象" class="headerlink" title="零成本抽象"></a>零成本抽象</h2><p>在Rust中，New Type模式不仅是类型安全的，还是一种零成本抽象。这是因为Rust编译器在编译时期会进行足够的优化，以确保New Type的使用没有运行时开销。 Rust的零成本抽象原则确保了抽象不会引入额外的运行时成本。例如，当你使用Meters这样的New Type时，Rust确保：</p> +<h2 id="零成本抽象"><a class="header-anchor" href="#零成本抽象">¶</a>零成本抽象</h2> +<p>在Rust中，New Type模式不仅是类型安全的，还是一种零成本抽象。这是因为Rust编译器在编译时期会进行足够的优化，以确保New Type的使用没有运行时开销。 Rust的零成本抽象原则确保了抽象不会引入额外的运行时成本。例如，当你使用Meters这样的New Type时，Rust确保：</p> <ol> <li>无额外内存开销：Meters只包含一个f64，在内存中的表现和单独的f64是一样的。</li> <li>无额外运行时开销：使用Meters时，性能和直接使用f64完全相同。编译器会移除任何关于New Type的包装和解包的代码。</li> diff --git a/2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html b/2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html index e208733c..a44f767e 100644 --- a/2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html +++ b/2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html @@ -194,13 +194,12 @@ <div class="post-content"> <p>代码：</p> <figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">...</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">handlers</span>(<span class="keyword">self</span>) <span class="punctuation">-></span> crate::server::request::Handlers {</span><br><span class="line"> <span class="built_in">vec!</span>[(</span><br><span class="line"> <span class="string">"/tree"</span>,</span><br><span class="line"> routing::<span class="title function_ invoke__">get</span>(<span class="keyword">move</span> || <span class="keyword">async</span> {</span><br><span class="line"> (</span><br><span class="line"> StatusCode::OK,</span><br><span class="line"> <span class="title function_ invoke__">Json</span>(json!(<span class="keyword">self</span>.<span class="title function_ invoke__">clone</span>().<span class="title function_ invoke__">tree</span>(<span class="keyword">self</span>.<span class="title function_ invoke__">clone</span>().root))),</span><br><span class="line"> )</span><br><span class="line"> }),</span><br><span class="line"> )]</span><br><span class="line"> }</span><br><span class="line">...</span><br></pre></td></tr></table></figure> - <p>编译错误：</p> <figure class="highlight text"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line">error: lifetime may not live long enough</span><br><span class="line"> --> src/storage/filesystem/mod.rs:45:34</span><br><span class="line"> |</span><br><span class="line">45 | routing::get(move || async {</span><br><span class="line"> | __________________________-------_^</span><br><span class="line"> | | | |</span><br><span class="line"> | | | return type of closure `{async block@src/storage/filesystem/mod.rs:45:34: 50:14}` contains a lifetime `'2`</span><br><span class="line"> | | lifetime `'1` represents this closure's body</span><br><span class="line">46 | | (</span><br><span class="line">47 | | StatusCode::OK,</span><br><span class="line">48 | | Json(json!(self.clone().tree(self.clone().root))),</span><br><span class="line">49 | | )</span><br><span class="line">50 | | }),</span><br><span class="line"> | |_____________^ returning this value requires that `'1` must outlive `'2`</span><br><span class="line"> |</span><br><span class="line"> = note: closure implements `Fn`, so references to captured variables can't escape the closure</span><br></pre></td></tr></table></figure> - <p>这是因为 <code>handlers</code> 里面的闭包捕获了一个引用，并且尝试返回一个包含该引用的值导致的。</p> <p>细说就是：闭包内部使用了 <code>self.clone()</code> 来获取一个新的实例，然后在异步块中返回一个 JSON 对象，这个 JSON 对象依赖于 <code>self.tree()</code> 的结果。因为闭包捕获了 <code>self</code> 的引用，所以它必须保证 <code>self</code> 在闭包执行完毕后仍然有效。</p> -<p>解决这个问题的思路是：确保闭包中的所有引用都在闭包执行完毕之前就不再被使用。<br>就是说，要将闭包的作用域限制在一个更短的生命周期内，或者使用其他方式来避免闭包捕获长期存在的引用:</p> +<p>解决这个问题的思路是：确保闭包中的所有引用都在闭包执行完毕之前就不再被使用。<br> +就是说，要将闭包的作用域限制在一个更短的生命周期内，或者使用其他方式来避免闭包捕获长期存在的引用:</p> <figure class="highlight rust"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">...</span><br><span class="line"> <span class="keyword">fn</span> <span class="title function_">handlers</span>(<span class="keyword">self</span>) <span class="punctuation">-></span> crate::server::request::Handlers {</span><br><span class="line"> <span class="keyword">let</span> <span class="variable">tree</span> = json!(<span class="keyword">self</span>.<span class="title function_ invoke__">clone</span>().<span class="title function_ invoke__">tree</span>(<span class="keyword">self</span>.<span class="title function_ invoke__">clone</span>().root));</span><br><span class="line"> <span class="built_in">vec!</span>[(</span><br><span class="line"> <span class="string">"/tree"</span>,</span><br><span class="line"> routing::<span class="title function_ invoke__">get</span>(<span class="keyword">move</span> || <span class="keyword">async</span> { (StatusCode::OK, <span class="title function_ invoke__">Json</span>(tree)) }),</span><br><span class="line"> )]</span><br><span class="line"> }</span><br><span class="line">...</span><br></pre></td></tr></table></figure> </div> |