C++核心准则C.89:保证哈希不会抛出异常

C.89: Make a hash noexcept

C.89:保证哈希不会抛出异常

Reason（原因）

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

requirement.

哈希容器的用户间接地使用哈希功能，不希望简单的操作发生异常。这是标准库的要求。

Example, bad（反面示例）

template<>
struct hash<My_type> {  // 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<size_t>()(x.s.size()) ^ trim(x.s);
    }
};

int main()
{
    unordered_map<My_type, int> 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.

如果你已经定义了哈希特化，争取简单地实现为通过异或和标准库哈希特化的组合。

Enforcement（实现建议）

• Flag throwing hashes.
• 提示抛出异常的哈希。

原文链接

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

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