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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