MSVC中的虚拟继承错误

Question

似乎我的问题是MSVC中的一个错误.我正在使用带有Service Pack 1的Visual Studio 2008,我的代码适用于GCC(在codepad.org上测试过).

有关此错误的任何官方信息？任何想法如何解决它？VS2010中是否修复了该错误？所有的见解将不胜感激.

代码:

struct Base {
    Base(int i = 0) : i(i) {}
    virtual ~Base() {}
    virtual Base *clone() const = 0;

protected:
    int i;
};

struct A : virtual public Base {
    A() {}
    virtual A *clone() const = 0;
};

struct B : public A {
    B() {}
    B *clone() const { return new B(*this); }

    /// MSVC debugger shows that 'b' is for some reason missing the Base
    /// portion of it's object ("Error: expression cannot be evaluated")
    /// and trying to access 'b.i' causes an unhandled exception.
    ///
    /// Note: This only seems to occur with MSVC
    B(const B &b) : Base(b.i), A() {}
};

void foo(const A &elem) {
    A *a = elem.clone();
    if (a) delete a;
}

int main() {
    A *a = new B;
    foo(*a);
    delete a;
}

Answer 1

看起来好像编译器this在调用时没有正确调整指针A::clone.如果你删除声明A::clone然后一切正常.

深入挖掘,当你拥有时A::clone,vtable看起来像这样:

    [0x0]   0x002f1136 [thunk]:B::`vector deleting destructor'`vtordisp{4294967292,0}' (unsigned int)   void *
    [0x1]   0x002f11e0 [thunk]:B::clone`vtordisp{4294967292,0}' (void)  void *
    [0x2]   0x002f12ad [thunk]:B::clone`vtordisp{4294967292,4}' (void)  void *
    [0x3]   0x002f12a3 B::clone(void)   void *

和foo调用elem.__vfptr[2],this错误地偏移了-4个字节.没有A::clone,vtable看起来像这样:

    [0x0]   0x00ee1136 [thunk]:B::`vector deleting destructor'`vtordisp{4294967292,0}' (unsigned int)   void *
    [0x1]   0x00ee11e0 [thunk]:B::clone`vtordisp{4294967292,0}' (void)  void *
    [0x2]   0x00ee12a3 B::clone(void)   void *

和foo电话elem.__vfptr[1].这根本不会调整this(代码假设this将等于Base而不是B).

因此,看起来编译器假定这A::clone是一个新的虚方法,并且Base::clone在确定是否A需要新的虚拟表时不会覆盖,但后来其他一些代码确定A不需要虚拟表.您可以通过比较sizeof(B)或不使用新的虚函数来验证这一点:

struct A : virtual public Base {
    A() {}
    virtual A *clone() const = 0;
}; //sizeof(B)==16

struct A : virtual public Base {
    A() {}
    virtual A *clone() const = 0;
virtual const A *clone2() const { return this; }
}; //sizeof(B)==20

所以这是一个编译器错误.