我总是对 c++ 虚拟表的内存布局感到困惑。这是我用来研究它的示例代码:
#include <cstdio>
#include <iostream>
using namespace std;
class Point
{
public:
Point()
{
cout<<"Point constructor"<<endl;
}
virtual void func_hs()
{
cout<<"Point::func_hs"<<endl;
printf("the address of this --func_hs:%p\n",&Point::func_hs);
}
virtual void func_zzy()
{
cout<<"Point::func_zzy"<<endl;
printf("the address of this --func_zzy:%p\n",&Point::func_zzy);
}
void printVt()
{
printf("the address of object,this:%p\nthe address of vt:%p\n",
this,(void*)*(int*)this);
}
void callVtFuncs(int num=2)
{
typedef void (*Funp)(void);
for(int i=0;i<num;i++)
{
Funp funp=(Funp)*((int*)*(int*)this+i);
printf("%p\n",((int*)*(int*)this+i));
printf("Point::callVtFuncs=>address of this fun:%p\n",funp);
if(i==2||i==3)
{
continue;
}
funp();
}
}
void printVirtualFunAddress()
{
cout<<endl<<endl;
printf("func_hs:%p\nfunc_zzy:%p\n",&Point::func_hs,&Point::func_zzy);
}
virtual ~Point()
{
cout<<"Point destructor"<<endl;
}
virtual void func_zzzy()
{
cout<<"Point::func_zzzy"<<endl;
printf("the address of this --func_zzzy:%p\n",&Point::func_zzzy);
}
protected:
float x,y,z;
};
int main(int argc, char *argv[])
{
Point point;
point.printVt();
point.callVtFuncs(5);
point.printVirtualFunAddress();
return 0;
}
我在课堂上放了4个虚函数,并打印出地址信息。这是输出:
Point constructor
the address of object,this:0xbffff620
the address of vt:0x8048db8
0x8048db8
Point::callVtFuncs=>address of this fun:0x8048914
Point::func_hs
the address of this --func_hs:0x1
0x8048dbc
Point::callVtFuncs=>address of this fun:0x8048966
Point::func_zzy
the address of this --func_zzy:0x5
0x8048dc0
Point::callVtFuncs=>address of this fun:0x8048b0a
0x8048dc4
Point::callVtFuncs=>address of this fun:0x8048b56
0x8048dc8
Point::callVtFuncs=>address of this fun:0x8048b74
Point::func_zzzy
the address of this --func_zzzy:0x11
func_hs:0x1
func_zzy:(nil)
func_zzzy:0x5
Point destructor
我完全不明白为什么最后一个输出是 'funz_zzy:(nil)' 和 'funz_zzy:0x5' 但上面是 0x5 和 0x11。
这是一些调试信息:(linux 32bit)
(gdb) x/16a 0x8048da8
0x8048da8: 0xa7025 0x0 0x0 0x8048dd4 <_ZTI5Point>
0x8048db8 <_ZTV5Point+8>: 0x8048914 <Point::func_hs()> 0x8048966 <Point::func_zzy()> 0x8048b0a <Point::~Point()> 0x8048b56 <Point::~Point()>
0x8048dc8 <_ZTV5Point+24>: 0x8048b74 <Point::func_zzzy()> 0x696f5035 0x746e 0x804a248 <_ZTVN10__cxxabiv117__class_type_infoE@@CXXABI_1.3+8>
0x8048dd8 <_ZTI5Point+4>: 0x8048dcc <_ZTS5Point> 0x3b031b01 0x80 0xf
我不明白为什么有两个 Point::~Point()?0x804a248 处的信息是否代表类的类型信息?
其他一些信息:
(gdb) x/1a 0x8048dd4
0x8048dd4 <_ZTI5Point>: 0x804a248 <_ZTVN10__cxxabiv117__class_type_infoE@@CXXABI_1.3+8>
0x8048dd4 是做什么用的?