我不明白 [英] I don't get it
问题描述
A级{
公开：
char text_a [100];
A（）{* text_a = 0; }
~A（）{}
};
// --------------- -------------------------------------------------- ------------
B级{
公开：
char text_b [100];
B（）{* text_b = 0; }
~B（）{}
};
// --------------- -------------------------------------------------- ------------
class C：public A，public B {
public：
char text_c [100];
C（）{* text_c = 0; }
~C（）{}
};
// --------------- -------------------------------------------------- --------------
无效测试（）{
B * bp1，* bp2;
C c，* cp1，* cp2，* cp3;
void * p;
strcpy（c.text_a，" hello a" ;）;
strcpy（c.text_b，" hello b"）;
strcpy（c.text_c，" hello c"）;
cp1 =& c;
p = cp1;
bp1 = cp1; // ok
bp2 =（B *）p; //结果bp2错了！
cp2 =（C *）p; // ok
cp3 =（C *）bp2; //结果cp3错了！这是合乎逻辑的，因为bp2
已经错了。
}
// -------------- -------------------------------------------------- -------------
所以热点是bp2 =（B *）p;
这有什么问题???
我可以想象有人说'p没有指向B对象'。
但是如果你想一想，从概念上来说，确实如此，imho。
所以编译这个更加技术性了！？
也许我'愚蠢和/或缺少关于C ++必不可少的东西。
如果是这样，请给我一个指向我可以学习这个权利的链接。
干杯，
Jo
class A {
public:
char text_a[100];
A() { *text_a=0; }
~A() {}
};
//-----------------------------------------------------------------------------
class B {
public:
char text_b[100];
B() { *text_b=0; }
~B() {}
};
//-----------------------------------------------------------------------------
class C : public A, public B {
public:
char text_c[100];
C() { *text_c=0; }
~C() {}
};
//-------------------------------------------------------------------------------
void test() {
B *bp1,*bp2;
C c,*cp1,*cp2,*cp3;
void *p;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp1=cp1; // ok
bp2=(B*)p; // resulting bp2 is WRONG!
cp2=(C*)p; // ok
cp3=(C*)bp2; // resulting cp3 is WRONG! Which is logical because bp2
is already wrong.
}
//-----------------------------------------------------------------------------
So the hot spot is the bp2=(B*)p;
What''s wrong with that???
I can imagine someone saying "p is not pointing to a B object".
But if you think about it, conceptually, it does, imho.
So is it more a technical matter of compiling this!?
Maybe i''m stupid and/or missing something essential about C++.
If so, please give me a link to where i can study this right.
Cheers,
Jo
推荐答案
6月18日，11：17 am，Jo< jo.lan ... @ telenet.bewrote：
On Jun 18, 11:17 am, Jo <jo.lan...@telenet.bewrote:
A级{
public：
char text_a [100];
A（）{* text_a = 0; }
~A（）{}};
// ------------------- -------------------------------------------------- --------
B级{
公开：
char text_b [100] ;
B（）{* text_b = 0; }
~B（）{}};
// ------------------- -------------------------------------------------- --------
C级：公众A，公共B {
公开：
char text_c [100];
C（）{* text_c = 0; }
~C（）{}};
// ------------------- -------------------------------------------------- ----------
无效测试（）{
B * bp1，* bp2;
C c，* cp1，* cp2，* cp3;
void * p;
strcpy（c.text_a，" hello a"）;
strcpy（c.text_b，" hello b"）;
strcpy（c.text_c，" hello c"）;
cp1 =& c;
p = cp1;
bp1 = cp1; // ok
bp2 =（B *）p; //结果bp2错了！
cp2 =（C *）p; // ok
cp3 =（C *）bp2; //结果cp3错了！这是合乎逻辑的，因为bp2
已经错了。}
// ------------------ -------------------------------------------------- ---------
所以热点是bp2 =（B *）p;
什么''那个错了???
我可以想象有人说'p没有指向B对象'。
但是如果你想一想，从概念上讲，它确实如此。
所以编译这个更加技术性了！？
也许我是傻瓜和/或缺少关于C ++的必要条件。
如果是这样，请给我一个指向我可以正确学习的地方的链接。
干杯，
Jo
class A {
public:
char text_a[100];
A() { *text_a=0; }
~A() {}};
//-----------------------------------------------------------------------------
class B {
public:
char text_b[100];
B() { *text_b=0; }
~B() {}};
//-----------------------------------------------------------------------------
class C : public A, public B {
public:
char text_c[100];
C() { *text_c=0; }
~C() {}};
//-------------------------------------------------------------------------------
void test() {
B *bp1,*bp2;
C c,*cp1,*cp2,*cp3;
void *p;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp1=cp1; // ok
bp2=(B*)p; // resulting bp2 is WRONG!
cp2=(C*)p; // ok
cp3=(C*)bp2; // resulting cp3 is WRONG! Which is logical because bp2
is already wrong.}
//-----------------------------------------------------------------------------
So the hot spot is the bp2=(B*)p;
What''s wrong with that???
I can imagine someone saying "p is not pointing to a B object".
But if you think about it, conceptually, it does, imho.
So is it more a technical matter of compiling this!?
Maybe i''m stupid and/or missing something essential about C++.
If so, please give me a link to where i can study this right.
Cheers,
Jo
我重新编写了你的测试函数 -
int main （）{b / b b * bp1，* bp2;
C c，* cp1;
void * p;
int a;
strcpy（c.text_a，" hello a"）;
strcpy（c.text_b，"你好b"）;
strcpy（c.text_c，" hello c"）;
cp1 =& c;
p = cp1;
bp2 = （B *）p;
std :: cout<< p<<" \ n";
std :: cout<< bp2;
std :: cin>> a;
}
我在p和bp2中打印了值。两者都是一样的。
在我的m / c上，O / P是 -
0x22fe28
0x22fe28
为什么你认为在你的程序中导致bp2错误？
I re-wrote your test function like this -
int main() {
B *bp1,*bp2;
C c,*cp1;
void *p;
int a;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp2=(B*)p;
std::cout<<p<<"\n";
std::cout<<bp2;
std::cin>>a;
}
I printed the value in p and also in bp2. Both are one and the same.
On my m/c the O/P was -
0x22fe28
0x22fe28
Why do you think, in your program, resulting bp2 is WRONG?
Bharath写道：
Bharath wrote:
> 6月18日上午11点17分，Jo< jo.lan ... @ telenet.bewrote：
>On Jun 18, 11:17 am, Jo <jo.lan...@telenet.bewrote:
>> A级{
公开：
char text_a [100];
A（）{* text_a = 0 ; }
~A（）{}};
// --------------------------- --------------------------------------------------
B班{
公共：
[100];
B（）{* text_b = 0; }
~B（）{}};
// --------------------------- --------------------------------------------------
C班：公众A，公共B {
公共：
char text_c [100];
C（）{* text_c = 0; }
~C（）{}};
// --------------------------- -------------------------------------------------- -
void test（）{
B * bp1，* bp2;
C c，* cp1，* cp2，* cp3;
void * p;
strcpy（c.text_a，" hello a"）;
strcpy（c.text_b，" hello b"）;
strcpy（c.text_c，"你好c"）;
cp1 =& c;
p = cp1;
bp1 = cp1; // ok
bp2 =（B *）p; //结果bp2错了！
cp2 =（C *）p; // ok
cp3 =（C *）bp2; //结果cp3错了！这是合乎逻辑的，因为bp2
已经错了。}
// -------------------------- -------------------------------------------------- -
所以热点是bp2 =（B *）p;
那有什么问题???
我可以想象有人说p没有指向B对象。
但是如果你想一想，从概念上来说，确实如此。我不知道。
所以它更像是一个技术问题编译这个！？
也许我很愚蠢和/或缺少一些关于C ++的重要信息。
如果是这样，请给我一个链接，我可以在哪里学习这个权利。
干杯，
>>class A {
public:
char text_a[100];
A() { *text_a=0; }
~A() {}};
//-----------------------------------------------------------------------------
class B {
public:
char text_b[100];
B() { *text_b=0; }
~B() {}};
//-----------------------------------------------------------------------------
class C : public A, public B {
public:
char text_c[100];
C() { *text_c=0; }
~C() {}};
//-------------------------------------------------------------------------------
void test() {
B *bp1,*bp2;
C c,*cp1,*cp2,*cp3;
void *p;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp1=cp1; // ok
bp2=(B*)p; // resulting bp2 is WRONG!
cp2=(C*)p; // ok
cp3=(C*)bp2; // resulting cp3 is WRONG! Which is logical because bp2
is already wrong.}
//-----------------------------------------------------------------------------
So the hot spot is the bp2=(B*)p;
What''s wrong with that???
I can imagine someone saying "p is not pointing to a B object".
But if you think about it, conceptually, it does, imho.
So is it more a technical matter of compiling this!?
Maybe i''m stupid and/or missing something essential about C++.
If so, please give me a link to where i can study this right.
Cheers,
Jo
我重写了你的测试功能 -
int main（）{
B * bp1，* bp2;
C c，* cp1;
void * p;
int a;
strcpy（c.text_a，" hello a"）;
strcpy（c.text_b，" hello b"）;
strcpy （c.text_c，hello c）;
cp1 =& c;
p = cp1;
bp2 =（B *）p;
std :: cout<< p<<" \ n" ;;
std :: cout<< bp2;
std :: cin>> a;
}
我在p和bp2中打印了值。两者都是同一个。
在我的m / c上，O / P是 -
0x22fe28
0x22fe28
为什么你认为，在你的程序中，结果bp2错了？
I re-wrote your test function like this -
int main() {
B *bp1,*bp2;
C c,*cp1;
void *p;
int a;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp2=(B*)p;
std::cout<<p<<"\n";
std::cout<<bp2;
std::cin>>a;
}
I printed the value in p and also in bp2. Both are one and the same.
On my m/c the O/P was -
0x22fe28
0x22fe28
Why do you think, in your program, resulting bp2 is WRONG?
因为我遇到了运行时问题。
所以我将所有内容缩小到上面的代码。 />
在我的电脑上运行它会产生错误的bp2指针。我可以在
" Watch"中找到它。小组。
我也做了类似的事情：在测试结束时/主要
功能我做了一个
OutputDebugString（cp3-> text_a）;
OutputDebugString（cp3-> text_b）;
OutputDebugString（cp3-> text_c）;
这为文本提供了垃圾!!!
使用Visual C ++编译7.1.3088
Because i got runtime problems.
So i narrowed down everything to the above code.
Running this on my PC gives a wrong bp2 pointer. I can see it in the
"Watch" panel.
And i also did a similar thing than you: at the end of the test/main
function i did a
OutputDebugString(cp3->text_a);
OutputDebugString(cp3->text_b);
OutputDebugString(cp3->text_c);
This gives rubbish for text a!!!
Compilation is done with Visual C++ 7.1.3088
Jo写道：
Jo wrote:
A级{
public：
char text_a [100];
A（）{* text_a = 0; }
~A（）{}
};
// --------------- -------------------------------------------------- ------------
B级{
公开：
char text_b [100];
B（）{* text_b = 0; }
~B（）{}
};
// --------------- -------------------------------------------------- ------------
class C：public A，public B {
public：
char text_c [100];
C（）{* text_c = 0; }
~C（）{}
};
// --------------- -------------------------------------------------- --------------
void test（）{
B * bp1，* bp2;
C c，* cp1，* cp2，* cp3;
void * p;
strcpy（c.text_a， hello a）;
strcpy（c.text_b，" hello b"）;
strcpy（c.text_c，" hello c"）;
cp1 =& c;
p = cp1;
bp1 = cp1; // ok
bp2 =（B *）p; //结果bp2错了！
cp2 =（C *）p; // ok
cp3 =（C *）bp2; //结果cp3错了！这是合乎逻辑的，因为bp2已经错了。
}
// -------------- -------------------------------------------------- -------------
所以热点是bp2 =（B *）p;
这有什么问题???
我可以想象有人说'p没有指向B对象'。
class A {
public:
char text_a[100];
A() { *text_a=0; }
~A() {}
};
//-----------------------------------------------------------------------------
class B {
public:
char text_b[100];
B() { *text_b=0; }
~B() {}
};
//-----------------------------------------------------------------------------
class C : public A, public B {
public:
char text_c[100];
C() { *text_c=0; }
~C() {}
};
//-------------------------------------------------------------------------------
void test() {
B *bp1,*bp2;
C c,*cp1,*cp2,*cp3;
void *p;
strcpy(c.text_a,"hello a");
strcpy(c.text_b,"hello b");
strcpy(c.text_c,"hello c");
cp1=&c;
p=cp1;
bp1=cp1; // ok
bp2=(B*)p; // resulting bp2 is WRONG!
cp2=(C*)p; // ok
cp3=(C*)bp2; // resulting cp3 is WRONG! Which is logical because bp2 is
already wrong.
}
//-----------------------------------------------------------------------------
So the hot spot is the bp2=(B*)p;
What''s wrong with that???
I can imagine someone saying "p is not pointing to a B object".
正确。
Right.
但是如果你想一想，从概念上讲，它确实如此。
But if you think about it, conceptually, it does, imho.
不，它没有。
No, it doesn''t.
>
那么编译这个就更具技术性了！？
>
So is it more a technical matter of compiling this!?
没有代码错误。
No the code is wrong.
>
也许我愚蠢和/或缺少关于C ++的重要信息。
>
Maybe i''m stupid and/or missing something essential about C++.
我怀疑你是傻瓜但是你肯定错过了必不可少的东西。
I doubt you''re stupid but you''re certainly missing something essential.
> ;
如果是这样，请给我一个链接，我可以在哪里学习这个权利。
>
If so, please give me a link to where i can study this right.
我不知道为什么你认为代码应该是正确的，这是我的问题。
也许你在期待一个普通的演员像dynamic_cast一样工作？
写的时候
bp2 =（B *）p; //结果bp2错了！
你期望程序''发现'那个B对象隐藏了
对象里面是p指向？
如果是google for dynamic_cast，或者更好的还是读一本好书。
john
I have no idea why you think that code should be right, that is my problem.
Perhaps you are expecting an ordinary cast to work like a dynamic_cast?
When you write
bp2=(B*)p; // resulting bp2 is WRONG!
you are expecting the program to ''discover'' that there a B object hidden
inside the object that p is pointing to?
If so google for dynamic_cast, or better still read a good book.
john
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