Diamond多重继承与动态转换 [英] Diamond multiple inheritance vs. dynamic cast
问题描述
请注意这个简单的多重继承模型：
void main（）{
A级{
public：
virtual void print（）{cout<< " A" << endl;
};
class支持1：虚拟公共A {
public：
virtual void print（）{cout<< " supp1" << endl;
};
class Support2：虚拟公共A {
public：
virtual void print（）{cout<< " supp2" <<结束;
};
B类：虚拟公众A，公众支持1，公众支持2 {
public：
virtual void print（）{cout<< " supp1" <<结束;
};
A * ptr =新B;
B * lpb = dynamic_cast< B *>（ ptr）;
删除ptr;
}
....
我得到以下警告（并且程序崩溃）：
警告c4541：''dynamic_cast''用于多态类型''main :: A'与
/ GR-;可能导致不可预测的行为
.....
嗯，为什么这不起作用。如果B类派生自A（由于虚拟而持有
一个A的内部实例），那么动态演员
应该有效，对吧？
请帮帮我，谢谢。
Please observe this simple model of multiple inheritance:
void main() {
class A {
public:
virtual void print() { cout << "A" << endl;
};
class Support1 : virtual public A {
public:
virtual void print() { cout << "supp1" << endl;
};
class Support2 : virtual public A {
public:
virtual void print() { cout << "supp2" << endl;
};
class B : virtual public A, public Support1, public Support2 {
public:
virtual void print() { cout << "supp1" << endl;
};
A *ptr = new B;
B *lpb = dynamic_cast<B *>(ptr);
delete ptr;
}
....
I get the following warning (and the program crashes):
warning c4541: ''dynamic_cast'' used on polymorphic type ''main::A'' with
/GR-; unpredictable behaviour may result
.....
Well, why doesn''t this work. If class B is derived from A (and holds
one internal instance of A because of virtual), then a dynamic cast
should work, right?
Please help me with this and thank you.
推荐答案
ax **** @ gmail.com 写道：
请注意这个简单的多重继承模型：
void main（）{
Please observe this simple model of multiple inheritance:
void main() {
1.非法。 main（）*必须*返回int。
1. Illegal. main() *MUST* return int.
>
}
...
我收到以下警告（程序崩溃）：
警告c4541 ：''dynamic_cast''用于多态类型''main :: A''与
/ GR-;可能导致不可预测的行为
....
嗯，为什么这不起作用。如果B类派生自A（由于虚拟而持有
一个A的内部实例），那么动态演员
应该有效，对吧？
请帮帮我，谢谢。
>
[redacted]
}
...
I get the following warning (and the program crashes):
warning c4541: ''dynamic_cast'' used on polymorphic type ''main::A'' with
/GR-; unpredictable behaviour may result
....
Well, why doesn''t this work. If class B is derived from A (and holds
one internal instance of A because of virtual), then a dynamic cast
should work, right?
Please help me with this and thank you.
因为微软认为RTTI和dynamic_cast太贵了而且b $ b昂贵。除非明确启用。
Because Microsoft decided that RTTI and dynamic_cast were "too
expensive" unless explicitly enabled.
ax****@gmail.com 写道：
请注意这个简单的多重继承模型：
Please observe this simple model of multiple inheritance:
#include< ; iostream>
使用std :: cout;
使用std :: endl;
#include <iostream>
using std::cout;
using std::endl;
void main（）{
void main() {
int main（）{
int main() {
class A {
public：
virtual void print（）{cout<< " A" <<结束;
};
class A {
public:
virtual void print() { cout << "A" << endl;
};
在这个和所有后续的print（）函数中，你忘了一个右大括号。
In this and all following print() functions, you forgot a closing brace.
class Support1 ：虚拟公共A {
public：
virtual void print（）{cout<< " supp1" << endl;
};
class Support2：虚拟公共A {
public：
virtual void print（）{cout<< " supp2" <<结束;
};
B类：虚拟公众A，公众支持1，公众支持2 {
public：
virtual void print（）{cout<< " supp1" <<结束;
};
class Support1 : virtual public A {
public:
virtual void print() { cout << "supp1" << endl;
};
class Support2 : virtual public A {
public:
virtual void print() { cout << "supp2" << endl;
};
class B : virtual public A, public Support1, public Support2 {
public:
virtual void print() { cout << "supp1" << endl;
};
我假设你的意思是B不是supp1这里。
I assume you meant "B" not "supp1" here.
A * ptr = new B;
B * lpb = dynamic_cast< B *>（ptr）;
删除ptr;
A *ptr = new B;
B *lpb = dynamic_cast<B *>(ptr);
delete ptr;
这里你是通过指向A的指针删除指向B的指针。在这种情况下你必须使用虚拟析构函数
，否则你将会有未定义的
行为。这很可能是导致你崩溃的原因。
Here you are deleting a pointer to B through a pointer to A. You must
use virtual destructors in this case, or you will have undefined
behavior. This is most likely what is causing your crash.
}
...
我得到以下警告（并且程序崩溃）：
警告c4541：''dynamic_cast''用于多态类型''main :: A''with
/ GR-;不可预知的行为可能导致
}
...
I get the following warning (and the program crashes):
warning c4541: ''dynamic_cast'' used on polymorphic type ''main::A'' with
/GR-; unpredictable behaviour may result
g ++ 4.1.1没有给出这样的警告。
g++ 4.1.1 gives no such warning.
嗯，为什么没有'这个工作。如果B类派生自A（由于虚拟而持有
一个A的内部实例），那么动态演员
应该有效，对吧？
请帮帮我，谢谢。
Well, why doesn''t this work. If class B is derived from A (and holds
one internal instance of A because of virtual), then a dynamic cast
should work, right?
Please help me with this and thank you.
我认为你的问题不是多重继承或者是dynamic_cast<> ;.
Nate
请注意这个简单的多重继承模型：
Please observe this simple model of multiple inheritance:
>
class A {
public：
virtual void print（）{cout<< " A" << endl;
};
class支持1：虚拟公共A {
public：
virtual void print（）{cout<< " supp1" << endl;
};
class Support2：虚拟公共A {
public：
virtual void print（）{cout<< " supp2" <<结束;
};
B类：虚拟公众A，公众支持1，公众支持2 {
public：
virtual void print（）{cout<< " supp1" <<结束;
};
A * ptr =新B;
B * lpb = dynamic_cast< B *>（ ptr）;
删除ptr;
}
...
我得到以下警告（并且程序崩溃）：
警告c4541：''dynamic_cast''用于多态类型''main :: A ''
/ GR-;不可预测的行为可能导致
>
class A {
public:
virtual void print() { cout << "A" << endl;
};
class Support1 : virtual public A {
public:
virtual void print() { cout << "supp1" << endl;
};
class Support2 : virtual public A {
public:
virtual void print() { cout << "supp2" << endl;
};
class B : virtual public A, public Support1, public Support2 {
public:
virtual void print() { cout << "supp1" << endl;
};
A *ptr = new B;
B *lpb = dynamic_cast<B *>(ptr);
delete ptr;
}
...
I get the following warning (and the program crashes):
warning c4541: ''dynamic_cast'' used on polymorphic type ''main::A'' with
/GR-; unpredictable behaviour may result
三件事：
1）请发布至少编译的代码。请参阅
http ：//www.parashift.com/c++-faq-lit...t.html#faq-5.8 了解更多
发布信息。
2）你看到的警告是特定于Visual C ++的，所以
偏离主题。
< OT>
如果你在Visual Studio的帮助中查找C4541，你会发现这样的东西：
错误信息
''标识符''用于多态类型''类型''用/ GR-;
不可预测的行为可能导致
您试图使用需要运行的功能时间类型信息
没有启用运行时类型信息。用/ GR重新编译。
换句话说，更改编译设置以启用RTTI。这将是
修复警告。
< / OT>
3）即使警告已经修复，这仍然赢了不行。 A上需要一个
虚拟析构函数。参见
http://www.parashift.com/c++-faq-lit....html#faq-20.7 。
像
虚拟~A（）{}
Michael
Three things:
1) Please post code that at least compiles. See
http://www.parashift.com/c++-faq-lit...t.html#faq-5.8 for more
info on posting.
2) The warning you are seeing is Visual C++ specific, and so is
off-topic.
<OT>
If you were to look up C4541 in the help for Visual Studio, you would
find something like this:
Error Message
''identifier'' used on polymorphic type ''type'' with /GR-;
unpredictable behavior may result
You tried to use a feature that requires run-time type information
without enabling run-time type information. Recompile with /GR.
In other words, change your compile settings to enable RTTI. That will
fix the warning.
</OT>
3) Even with the warning fixed, this still won''t work. You need a
virtual destructor on A. See
http://www.parashift.com/c++-faq-lit....html#faq-20.7.
Something like
virtual ~A() {}
Michael
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