多级/多级继承期间对象的大小 [英] Size of objects during Multilevel/Multiple inheritance

问题描述

Below is a pseudo declaration for a multilevel inheritance.

Base class ( protected int data)

derived1 : virtual public base ( protected int data1 )

derived2 : virtual public base ( protected int data2)

derived3 : derived1,derived2 ( private int data3 )

Main(){ base b;    derived1 d1;   derived2 d2;   derived3 d3; }

sizeof(b) // 4 which is correct as only int (4bytes)
sizeof(d1) // 12 why not 8 -> 4(base) + 4(derived)
sizeof(d2) // ??? whatever applies above should apply here
sizeof(d3) // 24 why not 12 -> 4(base) + 4(derived1/derived2) + 4(d3).

大小是否也包括虚拟表.同样，这里没有虚拟表，因为没有定义虚拟函数.请帮助澄清我的疑问.

Does size also include virtual tables also. Again here there cannot be virtual table as no virtual function defined. Please help in clarifying my doubt.

PS:到目前为止我所了解的:

PS: What I have understood till now:

Unless the function is declared virtual in base class,

base *bptr;
 derived d;
 bptr = &d;
 bptr->fun();  // will call the base class function.

But if the fun() is declared virtual then the above code will call derived class fun().

推荐答案

首先，在上述实现中，您需要返回count的类型而不是void.

First of all, in your implementation above, you need to return the type of count instead of void.

例如，假设您已声明int count.

然后，您需要在'postfix'版本中返回int，在'prefix'版本中返回int&或const int&.

Then you need to return int in the 'postfix' version, and int& or const int& in the 'prefix' version.

尝试b = a++和b = ++a，您将看到(当然，您需要每个函数都返回一个值).

Try b = a++ and b = ++a, and you will see (of course, you'll need each function to return a value).

这两个版本之间的区别仅在于返回值. "prefix ++"在操作之前返回count 的值，而"postfix ++"在操作之后返回count 的值.

The difference between these two versions is only in the return value. The 'prefix++' returns the value of count before the operation, and the 'postfix++' returns the value of count after the operation.

此外，由于其性质，'postfix ++'可以仅返回要递增的变量的副本(例如，int)，而'prefix ++'也可以 返回该变量的引用(例如int&).

In addition, due to its nature, the 'postfix++' can only return a copy of the variable being incremented (e.g., int), whereas the 'prefix++' can also return a reference of that variable (e.g., int&).

由于您没有在实现中返回任何内容，因此您无法利用这两个版本之间的差异.

Since you are not returning anything in your implementation, you cannot make any use of the difference between these two versions.

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