C++ - Memory Layout of Virtual Inherit (虚继承的内存布局)

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#layout #c++ #compiler #table #function #inheritance

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本文介绍了虚拟表（Virtual Table）的概念及其实现原理。对于单一继承的情况，编译器会在编译时为每个拥有虚函数的类设置静态查找表，即虚拟表。该表中包含指向最派生函数的指针。而对于多重继承或多级继承的情况，则会通过增加额外的指针来支持上转型和下转型，如GCC编译器中增加了指向偏移值的指针，而Visual C++则引入了vbptr（虚拟基类表指针）。

Virtual Table是如何实现的呢？

1. single inherit的简单情况

A virtual table is a static look up array that the compiler sets up at compile time.

A virtual table contains one entry for each virtual function that can be called by objects of the class. Each entry in this table is simply a function pointer that points to the most-derived function accessible by that class.

Second, the compiler also adds a hidden pointer to the classes that have virtual functions or inherit from a base having virtual functions

Ref:

Virtual table (http://www.learncpp.com/cpp-tutorial/125-the-virtual-table/ )

2. multiple inherit/virtual inherit的复杂情况

In gcc some more vptr are added to implementing upcast/downcast, which point to offset value in Virtual table. Refer to:

Memory Layout for Multiple and Virtual Inheritance (gcc http://www.phpcompiler.org/articles/virtualinheritance.html )

In Visual C++, virtual inherit will add a vbptr (virtual base table pointer) to each derived classes. Refer to:

C++: Under the Hood by Jan Gray; it talks about class layout techniques and the virtual function call mechanism.

(http://www.openrce.org/articles/files/jangrayhood.pdf )

C++: Under the Hood是英文的；有一个中文的站点，详细讲解了包括virtual inherit在内的各种情况的object的layout，很不错！见http://www.cnblogs.com/Binhua-Liu/archive/2010/06/16/1759019.html