C++核心準則C.89:保證哈希不會拋出異常

白云母晶簇

C.89: Make a hash noexcept

C.89:保證哈希不會拋出異常

Reason（原因）

Users of hashed containers use hash indirectly and don't expect simple access to throw. It's a standard-library?

requirement.

哈希容器的用戶間接地使用哈希功能，不希望簡單的操作發(fā)生異常。這是標準庫的要求。

Example, bad（反面示例）

template<>
struct hash { ?// thoroughly bad hash specialization
 ? ?using result_type = size_t;
 ? ?using argument_type = My_type;

 ? ?size_t operator() (const My_type & x) const
 ? ?{
 ? ? ? ?size_t xs = x.s.size();
 ? ? ? ?if (xs < 4) throw Bad_My_type{}; ? ?// "Nobody expects the Spanish inquisition!"
 ? ? ? ?return hash()(x.s.size()) ^ trim(x.s);
 ? ?}
};

int main()
{
 ? ?unordered_map m;
 ? ?My_type mt{ "asdfg" };
 ? ?m[mt] = 7;
 ? ?cout << m[My_type{ "asdfg" }] << '\n';
}

If you have to define a hash specialization, try simply to let it combine standard-library hash specializations with ^ (xor). That tends to work better than "cleverness" for non-specialists.

如果你已經(jīng)定義了哈希特化，爭取簡單地實現(xiàn)為通過異或和標準庫哈希特化的組合。

Enforcement（實現(xiàn)建議）

• Flag throwing hashes.

• 提示拋出異常的哈希。

原文鏈接

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c89-make-a-hash-noexcept

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