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  "title": "C++ `std::map` 与 `std::unordered_map` 的区别",
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      "text": "核心差异",
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  "contentMarkdown": "# C++ `std::map` 与 `std::unordered_map` 的区别\n\n> [!note] 名称\n> C++ 标准库没有名为 `hashmap` 的标准容器；对应的哈希关联容器是 `std::unordered_map`。\n\n## 核心差异\n\n| 维度 | `std::map` | `std::unordered_map` |\n| --- | --- | --- |\n| 组织方式 | 有序关联容器，通常由平衡搜索树实现 | 无序关联容器，通常由哈希表实现 |\n| 键顺序 | 按比较器排序 | 没有稳定的键顺序保证 |\n| 查找/插入/删除 | `O(log n)` | 平均 `O(1)`，最坏 `O(n)` |\n| 范围查询 | 支持 `lower_bound`、有序遍历 | 不适合按键范围查询 |\n| 主要失效条件 | 删除元素使被删元素的迭代器失效 | rehash 可使所有迭代器失效 |\n| 额外要求 | 键需要严格弱序比较 | 键需要哈希函数与相等判断一致 |\n\n标准规定的是行为和复杂度要求，不强制某一种底层数据结构。常见实现中 `map` 使用红黑树，`unordered_map` 使用桶数组与链式/节点结构，但不应把实现细节当成跨平台保证。\n\n## 如何选择\n\n选择 `std::map`：\n\n- 需要按键排序遍历；\n- 需要范围查询或相邻键；\n- 希望复杂度有稳定的对数上界；\n- 无法提供可靠、廉价的哈希函数。\n\n选择 `std::unordered_map`：\n\n- 主要是精确键查询；\n- 不依赖遍历顺序；\n- 哈希分布可控，且平均常数时间带来实际收益。\n\n哈希表不一定更快：小数据量、缓存局部性、节点分配、哈希成本、负载因子和碰撞都会影响结果。应使用真实键和值在目标平台做基准测试。\n\n## 常用控制项\n\n```cpp\n#include <unordered_map>\n\nstd::unordered_map<int, int> values;\nvalues.reserve(1000);       // 预计元素数，减少 rehash\nvalues.max_load_factor(0.8f);\n```\n\n自定义键时，若 `a == b`，哈希函数必须保证两者哈希值相同；反过来哈希值相同不要求键相等。\n\n## 参考资料\n\n- [Microsoft Learn：`map` class](https://learn.microsoft.com/en-us/cpp/standard-library/map-class?view=msvc-170)\n- [Microsoft Learn：`unordered_map` class](https://learn.microsoft.com/en-us/cpp/standard-library/unordered-map-class?view=msvc-170)\n"
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