[問題] c++ virtual method的故事
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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