返回信息流class A {int a;};
class B : virtual A {int b;};
class C : virtual A {int c;};
class D : B, C {};
这时取sizeof
A 4
B 12
C 12
D 20
如果D是
class D : virtual B, C {};
取sizeof
D 20
如果D是
class D : virtual B, virtual C {};
取sizeof
D 24
为啥是24啊，求大神解答[ema0]
这是一条镜像帖。来源：北邮人论坛 / cpp / #83180同步于 2014/10/8
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2014/10/8镜像同步6 回复
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摘自《Inside the C++ Object Model》:
In the case of virtual inheritance, only a single occurrence of the base class is maintained (called a subobject)
regardless of how many times the class is derived from within the inheritance chain. iostream, for example,
contains only a single instance of the virtual ios base class.
How might a derived class internally model its base class instance? In a simple base class object model, each
base class might be assigned a slot within the derived class object. Each slot holds the address of the base
class subobject. The primary drawback to this scheme is the space and access-time overhead of the
indirection. A benefit is that the size of the class object is unaffected by changes in the size of its associated
base classes.
Alternatively, one can imagine a base table model. Here, a base class table is generated for which each slot
contains the address of an associated base class, much as the virtual table holds the address of each virtual
function. Each class object contains a bptr initialized to address its base class table. The primary drawback to
this strategy, of course, is both the space and access-time overhead of the indirection. One benefit is a
uniform representation of inheritance within each class object. Each class object would contain a base table
pointer at some fixed location regardless of the size or number of its base classes. A second benefit would be
the ability to grow, shrink, or otherwise modify the base class table without changing the size of the class
objects themselves.
VC编译器就是按照第一种实现的，所以有两个bptr, 听说g++是按照第二种方法实现的
【 在 ashjn2011 的大作中提到: 】
: 摘自《Inside the C++ Object Model》:
: In the case of virtual inheritance, only a single occurrence of the base class is maintained (called a subobject)
: regardless of how many times the class is derived from within the inheritance chain. iostream, for example,
: ...................
谢谢解答。我使用的是vs，一开始我也以为有2个vbptr，所以做了以下实验：
class A {
int a;
};
class B {
int b;
};
class C :virtual A, virtual B {
};
结果 sizeof(C) 的结果是 12， 也就是只有1个vbptr，这个很奇怪啊。。