Golang 快速删除map所有元素
发布时间：2022-01-12 编辑：jiaochengji.com
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<svg xmlns="http://www.w3.org/2000/svg" style="display: none;"><path stroke-linecap="round" d="M5,0 0,2.5 5,5z" id="raphael-marker-block" style="-webkit-tap-highlight-color: rgba(0, 0, 0, 0);"/></svg><h2>1. 所有Go版本通用方法：</h2>
重新申请一个新的map，旧的map交给GC去回收。
<pre><code>a := make(map[string]int) a["a"] = 1 a["b"] = 2 // clear all a = make(map[string]int) </code></pre> <h2>2. Go 1.11版本以上用法：</h2>通过Go的内部函数mapclear方法删除。这个函数并没有显示的调用方法，当你使用for循环遍历删除所有元素时，Go的编译器会优化成Go内部函数mapclear。
<pre><code>package main func main() { m := make(map[byte]int) m[1] = 1 m[2] = 2 for k := range m { delete(m, k) } } </code></pre>把上述源代码直接编译成汇编（默认编译是会优化的）：
<pre><code>go tool compile -S map_clear.go </code></pre>可以看到编译器把源码9行的for循环直接优化成了mapclear去删除所有元素。如下：
再来看看关闭优化后的结果：
关闭优化选项后，Go编译器直接通过循环遍历来删除map里面的元素。
由上可知，遍历删除在经过编译器优化后会调用mapclear一次性删除map所有元素，那这个mapclear函数是如何实现的，效率如何？
这部分代码涉及到内存管理和GC，只能看懂个大概，后续再补充。
实现思路：
<ul><li>清空统计数据，如元素个数、溢出数等。</li><li>重新申请一个新的extra，原有的extra交给GC。</li><li>释放桶内存块。</li></ul><pre><code class="lang-go hljs"><span class="token keyword">func</span> <span class="token function">mapclear</span><span class="token punctuation">(</span>t <span class="token operator">*</span>maptype<span class="token punctuation">,</span> h <span class="token operator">*</span>hmap<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token operator">...</span> <span class="token comment">// 把oldbuckets置nil，如果有oldbuckets就让GC处理</span> h<span class="token punctuation">.</span>oldbuckets <span class="token operator">=</span> <span class="token boolean">nil</span> <span class="token comment">// 初始化溢出数、元素个数</span> h<span class="token punctuation">.</span>nevacuate <span class="token operator">=</span> <span class="token number">0</span> h<span class="token punctuation">.</span>noverflow <span class="token operator">=</span> <span class="token number">0</span> h<span class="token punctuation">.</span>count <span class="token operator">=</span> <span class="token number">0</span> <span class="token comment">// 重新申请一个新的extra，旧的交给GC回收</span> <span class="token keyword">if</span> h<span class="token punctuation">.</span>extra <span class="token operator">!=</span> <span class="token boolean">nil</span> <span class="token punctuation">{</span> <span class="token operator">*</span>h<span class="token punctuation">.</span>extra <span class="token operator">=</span> mapextra<span class="token punctuation">{</span><span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token comment">// 清空bucket</span> <span class="token boolean">_</span><span class="token punctuation">,</span> nextOverflow <span class="token operator">:=</span> <span class="token function">makeBucketArray</span><span class="token punctuation">(</span>t<span class="token punctuation">,</span> h<span class="token punctuation">.</span>B<span class="token punctuation">,</span> h<span class="token punctuation">.</span>buckets<span class="token punctuation">)</span> <span class="token operator">...</span> <span class="token punctuation">}</span> <span class="token keyword">func</span> <span class="token function">makeBucketArray</span><span class="token punctuation">(</span>t <span class="token operator">*</span>maptype<span class="token punctuation">,</span> b <span class="token builtin">uint8</span><span class="token punctuation">,</span> dirtyalloc unsafe<span class="token punctuation">.</span>Pointer<span class="token punctuation">)</span> <span class="token punctuation">(</span>buckets unsafe<span class="token punctuation">.</span>Pointer<span class="token punctuation">,</span> nextOverflow <span class="token operator">*</span>bmap<span class="token punctuation">)</span> <span class="token punctuation">{</span> base <span class="token operator">:=</span> <span class="token function">bucketShift</span><span class="token punctuation">(</span>b<span class="token punctuation">)</span> nbuckets <span class="token operator">:=</span> base <span class="token operator">...</span> <span class="token comment">// 没有分配过内存，则申请一个新的</span> <span class="token keyword">if</span> dirtyalloc <span class="token operator">==</span> <span class="token boolean">nil</span> <span class="token punctuation">{</span> buckets <span class="token operator">=</span> <span class="token function">newarray</span><span class="token punctuation">(</span>t<span class="token punctuation">.</span>bucket<span class="token punctuation">,</span> <span class="token function">int</span><span class="token punctuation">(</span>nbuckets<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token comment">// 直接释放整个buckets</span> buckets <span class="token operator">=</span> dirtyalloc size <span class="token operator">:=</span> t<span class="token punctuation">.</span>bucket<span class="token punctuation">.</span>size <span class="token operator">*</span> nbuckets <span class="token keyword">if</span> t<span class="token punctuation">.</span>bucket<span class="token punctuation">.</span>kind<span class="token operator">&</span>kindNoPointers <span class="token operator">==</span> <span class="token number">0</span> <span class="token punctuation">{</span> <span class="token function">memclrHasPointers</span><span class="token punctuation">(</span>buckets<span class="token punctuation">,</span> size<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span> <span class="token function">memclrNoHeapPointers</span><span class="token punctuation">(</span>buckets<span class="token punctuation">,</span> size<span class="token punctuation">)</span> <span class="token punctuation">}</span> <span class="token punctuation">}</span> <span class="token operator">...</span> <span class="token punctuation">}</span> </code></pre>总结：
使用mapclear方法清空map时，做的工作就是初始化和释放申请内存块，效率很高。
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