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 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.

前置宣告

What are Forward declarations in C++ - GeeksforGeeks
 
 
 

 

 

REF:
https://stackoverflow.com/questions/2832714/header-files-inclusion-forward-declaration

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