warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

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最近复习到C++的虚函数，想输出一下虚函数表的地址，但是在编译的时候遇到下面这个错误：

my_virtual.cpp: In function ‘int main()’:
my_virtual.cpp:29:39: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     int *vtbl_addr=((int *)*(int *)(&b));  //②
                                       ^
my_virtual.cpp:33:63: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     printf("第一个虚函数地址:%p\n",(void *)*(vtbl_addr));
                                                               ^
my_virtual.cpp:34:67: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     printf("第二个虚函数地址:%p\n",(void *)*(vtbl_addr + 1));
                                                                   ^
my_virtual.cpp:37:28: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     pfun = (Fun)(*vtbl_addr);  //vitural f();
                            ^
my_virtual.cpp:41:34: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
     pfun = (Fun)(*(vtbl_addr + 1));  //vitural g();
                                  ^

源码如下：

#include<iostream>
#include<stdio.h>
#include<stdlib.h>
using namespace std;

class Base {
public:
    virtual void f() { cout << "Base::f" << endl; }
    virtual void g() { cout << "Base::g" << endl; }
};

typedef void(*Fun)(void);  //函数指针

int main()
{
    Base b;

    printf("sizeof(int)=%d,sizeof(int *)=%d,sizeof(void *)=%d\n",(int)sizeof(int),(int)sizeof(int *),(int)sizeof(void *));
    
    //int vtbl_addr=((int *)*(int *)(&b));   //①
    //int *vtbl_addr=((int *)*(int *)(&b));  //②
    unsigned long int *vtbl_addr=((unsigned long int *)*(unsigned long int *)(&b)); //③

    printf("虚表地址:%p\n", (void *)vtbl_addr);
    printf("第一个虚函数地址:%p\n",(void *)*(vtbl_addr));
    printf("第二个虚函数地址:%p\n",(void *)*(vtbl_addr + 1));
    
    Fun pfun = NULL;
    pfun = (Fun)(*vtbl_addr);  //vitural f();
    printf("f() addr:%p\n", (void *)pfun);
    pfun();

    pfun = (Fun)(*(vtbl_addr + 1));  //vitural g();
    pfun();
    printf("g() addr:%p\n", (void *)pfun);

    return 0;
}

调试过程：

• 最开始是用的一个int类型去保存虚函数表的地址，在windows下编译、运行是通过的，但是放在win10的子系统下编译时碰到了上述警告，提示大小不一样，才想到64位系统指针的大小是8byte
• 然后写下了第二句话，用指针保存地址这总没错吧，但是编译的时候还是碰到了这个警告
• 又试了dynamic_cast来强制转换，emmm，还是不行，最后看cpp参考手册的时候看到dereference这个单词，才想到(int *)(&b)是把b的地址转化为(int *)，那么解引用的时候得到就是一个int类型，就是说还是4byte，转化为8byte的指针当然会出错了
• 最后第三种写法是编译运行通过了