[問題] c++ virtual method的故事

[a9228097 (逞強是我的專長)]

最近看到LLVM裡面的某一點coding standard讓我一直想不透

If a class is defined in a header file and has a vtable (either it has
virtual methods or it derives from classes with virtual methods), it must
always have at least one out-of-line virtual method in the class. Without
this, the compiler will copy the vtable and RTTI into every .o file that
#includes the header, bloating .o file sizes and increasing link times.

大意就是,在一個定義在header file裡的class有virtual method or 它繼承的base
class有virtual method的話,
就必須至少定義一個virtual method

沒有的話compiler會copy vtable 和RTTI到每個有include這個header的.o檔

我的問題是,為什麼有一個virtual method compiler就不copy了?
所以vtable會放哪?
想知道整個故事是到底怎麼回事


有去看過more effective c++ item24 但是還是不太懂
只知道compiler copy是要解決vtable放哪的問題

請問有人知道嗎
感謝

====

知道了
http://www.angelfire.com/weird2/darkmoon/MEC/MI24_FR.HTM
裡面提到
A more common design is to employ a heuristic to determine which object file
should contain the vtbl for a class. Usually this heuristic is as follows: a
class's vtbl is generated in the object file containing the definition (i.e.,
the body) of the "first non-inline non-pure" virtual function in that class.

If all virtual functions in a class are declared inline, the heuristic fails,
and most heuristic-based implementations then generate a copy of the class's
vtbl in every object file that uses it.


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◆ From: 61.230.112.113
→ johnny1972:你自己的翻譯漏了最重要的一個字 out-of-line
推 purpose:純猜測。假設有3個虛函式，當其中一個虛函式定義在 a.cpp
→ purpose:則 compiler 會覺得複製 vtable 可能不完整，就不複製了？
→ purpose:相對的，當3個虛函式都可以從標頭檔取得，則資訊必然充足
→ firose:應該是全都 inline 了所以要在這個編譯單元內找到 vtable
→ a9228097:out-of-line = non-inline, 我還自動忽略那個字..
※ 編輯: a9228097        來自: 61.230.112.113       (04/12 00:38)
推 purpose:所以他是指，從一開始 vtbl 就只想放在一處儲存而已
→ purpose:只是挑選策略失敗，就乾脆海派點，每個有用到的地方都塞
→ tinlans:我 virtual function 從來不 inline，這東西能 inline 嗎?
→ tinlans:所以當初我在 LLVM 看到那條完全無視，感覺是說廢話。
→ a9228097:我是看到source裡有"virtual method anchor"才看到這個
→ a9228097:規定, 才疑惑為啥要這樣規定