Question

所以回到基础，我试图把我的头围绕着vtable和诸如此类的东西。在下面的示例中，如果我将B*传递给某个函数，那么该函数如何知道调用C对象的vtable中的方法而不是vtable中的方法A的？是否有两个单独的VTable传递给该对象？接口指针真的只是vtable（因为接口，IIRC，不能包含属性声明）？

我想说的是，直到我真正尝试这个代码，我假设你不能一次继承多个接口/类（并且所有接口都必须是线性的，可以这么说，以便vtable建立在自己身上。

如果我对vtable如何工作的想法是正确的（我现在知道它不是），那么传递B*并调用B::OutB()就会调用{{1相反（显然不是这种情况）。

有人可以解释一下吗？

A:OutA()

输出（如预期的那样）：

// Includes
#include <windows.h>
#include <iostream>

interface A
{
public:
    virtual void OutA() = 0;
};

interface B
{
public:
    virtual void OutB() = 0;
};

class C : public A, public B
{
public:
    void OutA();
    void OutB();
};

void C::OutA()
{
    printf("Out A\n");
}

void C::OutB()
{
    printf("Out B\n");
}

int main()
{
    C obj;
    obj.OutA();
    obj.OutB();

    A* ap = (A*)&obj;
    B* bp = (B*)&obj;

    ap->OutA();
    bp->OutB();

    system("pause");

    // Return
    return 0;
}

Answer 1

我不知道interface是什么，但是如果您正在编写C ++并且A和B是类，那么C将包含两个子对象：{{ 1}}和A，每个子对象都有自己的vtable指针。

将C ++编译为C时，我们可以：

请参阅http://ideone.com/TioyX

输出：

#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>

const int debug = 0;

void __pure_virtual_called() {
    fputs ("pure virtual function called\n", stderr);
    abort();
}

/* Translation of:

class A
{
public:
    virtual void OutA() = 0;
};
*/

struct A;

typedef struct  {
    void (*ptr__OutA) (struct A *__this);
} vtable__A;

typedef struct A {
    vtable__A *__vptr;
} A;

/* translation A::OutA() 
 * pure virtual function */
void A__OutA (A *__this) {
     __pure_virtual_called();
}

vtable__A vtable__A__A = { .ptr__OutA = A__OutA };

void A__constructor (A *__this) {
    if (debug)
        printf ("A__constructor %p\n", (void*)__this);

    /* dynamic type is initialised to A */
    __this->__vptr = &vtable__A__A;
}

/* Translation of:

class B
{
public:
    virtual void OutB() = 0;
};

*/

struct B;

typedef struct {
    void (*ptr__OutB)(struct B *__this);
} vtable__B;

typedef struct B {
    vtable__B *__vptr;
} B;

/* translation B::OutB() 
 * pure virtual function */
void B__OutB (B *__this) {
     __pure_virtual_called();
}

vtable__B vtable__B__B = { .ptr__OutB = B__OutB };

void B__constructor (B *__this) {
    if (debug)
        printf ("B__constructor %p\n", (void*)__this);

    /* dynamic type is initialised to B */
    __this->__vptr = &vtable__B__B;
}
/* Translation of:

class C : public A, public B
{
public:
    void OutA(); // overrides A::OutA()
    void OutB(); // overrides B::OutB()
    // note :
    // no new virtual function
};

*/

/* no new virtual function 
 * so no specific vtable type! */

typedef struct {
/* no additional vptr, we already have 2! */
    A base__A;
    B base__B;
} C;

/******* upcasts 
 * translation of 
 * static_cast<C*> (p) 
 */

/* translation of 
 * A *p;
 * static_cast<C*> (p);
 */
C *static_cast__A__C (A *__ptr) {
    /* 
     * base__A is first member of C
     * so offsetof(C, base__A) == 0
     * can skip the pointer adjustment
     */ 
    return (C*)__ptr;
}

/* translation of 
 * B *p;
 * static_cast<C*> (p);
 */
C *static_cast__B__C (B *__ptr) {
    /* locate enclosing C object: 
     * __base__B is not first member
     * need to adjust pointer
     */
    return (C*)((char*)__ptr - offsetof(C, base__B));
}

/* translation of virtual functions of C 
 * overriding function declarations from A
 */

/* translation of C::OutA() */

/* C::OutA() called from C */
void C__OutA (C *__this) {
    printf("Out A this=%p\n", (void*)__this);
}

/* C::OutA() called from A */
void C__A__OutA (A *__this) {
    if (debug)
            printf ("C__A__OutA %p\n", (void*)__this);
    C__OutA (static_cast__A__C (__this));
}

vtable__A vtable__A__C = { .ptr__OutA = C__A__OutA };

/* translation of virtual functions of C 
 * overriding function declarations from B
 */

/* translation of C::OutB() */

/* C::OutB() called from C */
void C__OutB (C *__this) {
    printf("Out B this=%p\n", (void*)__this);
}

/* C::OutB() called from B */
void C__B__OutB (B *__this) {
    if (debug)
            printf ("C__B__OutB %p\n", (void*)__this);
    C__OutB (static_cast__B__C (__this));
}

vtable__B vtable__B__C = { .ptr__OutB = C__B__OutB };

void C__constructor (C *__this) {
    if (debug)
        printf ("C__constructor %p\n", (void*)__this);
    /* construct subobjects */
    A__constructor (&__this->base__A);
    B__constructor (&__this->base__B);

    /* adjust dynamic type of this to C */
    __this->base__A.__vptr = &vtable__A__C;
    __this->base__B.__vptr = &vtable__B__C;
}

/* calls to C virtual functions with a C* 
 */

/* translation of 
 * C *p;
 * p->OutA();
 *
 * is
 * ((A*)p)->OutA();
 *
 * because C::OutA() is overrides A::OutA()
 */
void dyn__C__OutA (C *__this) {
    A *base_ptr__A = &__this->base__A;
    base_ptr__A->__vptr->ptr__OutA (base_ptr__A);
}

/* translation of 

int main()
{
    C obj;
    obj.OutA();
    obj.OutB();

    A *ap = &obj;
    B *bp = &obj;
    C *cp = &obj;

    ap->OutA();
    bp->OutB();
    cp->OutA();

    // Return
    return 0;
}

 *
 */

int main () {
    /* translation of:
    C obj; 
    */
    C obj;
    C__constructor (&obj);

    /* translation of:
    obj.OutA();
    obj.OutB();
     * obj is a locally declared object
     * so dynamic type of obj is known as C
     * can make direct call to C::OutA(), C::OutB()
     */
    C__OutA (&obj);
    C__OutB (&obj);

    /* dumb (zero optimisation) translation of:
    A *ap = &obj;
    B *bp = &obj;
    C *cp = &obj;
    */
    A *ap = &obj.base__A;
    B *bp = &obj.base__B;
    C *cp = &obj;

    /* translation of:
    ap->OutA();
    bp->OutB();
    cp->OutA();

    * dumb compiler = no optimisation
    * so dynamic type of *ap, *bp, *cp is unknown
    * so make "virtual" calls using vtable
    */
    ap->__vptr->ptr__OutA(ap);
    bp->__vptr->ptr__OutB(bp);
    dyn__C__OutA (cp);

    /* note: obj lifetime ends now
     * C has a trivial destructor 
     * so no destructor call needed
     */

    return 0;
}

Answer 2

使用多重继承，对象内置部分，每个部分对应一个基类。这包括vtable指针。这是必要的，因为与指针或引用交互的代码将不知道它是否与基类或派生类一起使用，因此它们必须以相同的方式布局。

一个令人惊讶的结果是当你将指针强制转换为其中一个基类时，它的地址可能会改变！编译器在幕后生成一些代码，以调整指向对象的正确部分的指针。

C obj;
A* ap = (A*)&obj;
B* bp = (B*)&obj;
bool same = ((void*)ap) == ((void*)bp);  // false!

C ++多重继承和vtables

2 个答案: