做 個 有 趣 的 人

 This will reduce compile times etc. If that doesn't compile go for the #include. You have to go for the #include if you need to do any of the following:

1. Access a member or function of the class.
2. Use pointer arithmetic.
3. Use sizeof.
4. Any RTTI information.
5. new/delete, copy etc.
6. Use it by value.
7. Inherit from it.
8. Have it as a member.
9. Instance in a function.

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