编译器会自动的为每个对象创建一个隐藏的“虚指针”__vptr,它指向类 A 的虚表,如下图所示: C++语言这样实现虚函数机制的空间开销是微乎其微的,事实上,每一个对象只需要一个额外的“虚指针”__vptr就能够调用类的虚函数。同样的,时间开销也很小:相比于常规函数的调用,虚函数的...
则有两个vptr。此外A是一个空类,因此编译器会自动地添加一个字节。再加上边界对齐的3个字节。因此8+8+3*4+1+3=32。 windows下的解析如下: 关键是Y,Z和A Y类似于linux的Y。但是VS并没有给空类生成一个字节的。因此3*4+4=16。 Z,VS编译器中,是将virtual base class table和virtual function table区...
This pointer is called virtual table pointer, sometimes 'vptr'. In VC++ compiler, the objects will have a pointer named '__vfptr' in them and in some other compiler it's '__vptr_X', where X is the class name. Now __vfptr is not directly accessible from your code. For example, if...
Virtual Table is a table of function pointers, which is created for each class that has virtual functions, it stores addresses of virtual functions of the class. Whereas each object of that class with virtual functions contains a hidden vptr, where this vptr points to the vtable of its class...
{ std::cout << "in Base::fun" << std::endl; } }; Base b; b.vptr = addr...
Virtual table pointer In theory, it is said that vptr pointer, virtual table poiner, or vpointer, is stored in every class that has at least one virtual method. Let us puzzle out what a thing is this. For this, let us write a simple demo program on C++. 123456789101112131415...
为一个已有虚函数的类增加了一个或多个虚函数并没有为该类的每一个对象增加一个vptr。 每一个多态类增加了至少多增加了一个vtbl到程序数据区。 多态类的每一个构造函数必须初始化vptr 每一个虚函数调用必须定位通过查询vtbl以定位函数地址(通常需要2-4条额外的机器指令) ...
Although the compiler does this automatically, we’ll put it in the next example just to show where it’s added:class Base { public: VirtualTable* __vptr; virtual void function1() {}; virtual void function2() {}; }; class D1: public Base { public: void function1() override {};...
shape_virtual_table circle_vtbl = { &circle::area, &circle::name, &circle::put }; (我说“类似这样”,因为这样的代码实际通不过编译,这样的代码只是演示下vtbl的通常的布局)使用这样的转换模式, a = ps->area(); 转换成 a = (*ps->vtbl->area)(ps); ...
Every object has a VPtr and is stored in the 1st memory location of the object memory. VPtr will have the address of the Virtual Table which has the function addresses of all the virtual functions available. Using the Vptr we can easily access the VirtualTable of the class and make ...