c*******9
发帖数: 6411 | 1 【 以下文字转载自 Quant 讨论区 】
发信人: jsyzghan (民兵营长), 信区: Quant
标 题: 码工试题
发信站: BBS 未名空间站 (Thu Mar 11 10:54:48 2010, 美东)
N年之前的,某著名银行的一老俄问的.现在也没想出来.
Given these classes:
struct A
{
A() { throw 0; }
};
struct B : A
{
B();
};
With absolutely no change to the above, implement B::B() so that it does
not throw. |
X****r
发帖数: 3557 | 2 这个可能吗？要是B::B()既没有完成又不throw，到时候岂不是还要调用~B:B()？
这不是自己挖个坑跳下去嘛。
【在 c*******9 的大作中提到】
: 【 以下文字转载自 Quant 讨论区 】
: 发信人: jsyzghan (民兵营长), 信区: Quant
: 标 题: 码工试题
: 发信站: BBS 未名空间站 (Thu Mar 11 10:54:48 2010, 美东)
: N年之前的,某著名银行的一老俄问的.现在也没想出来.
: Given these classes:
: struct A
: {
: A() { throw 0; }
: };
|
X****r
发帖数: 3557 | 3 当然如果是脑筋急转弯的话……
struct BB {
BB() { /* I don't throw! */}
};
#define B BB
【在 X****r 的大作中提到】
: 这个可能吗？要是B::B()既没有完成又不throw，到时候岂不是还要调用~B:B()？
: 这不是自己挖个坑跳下去嘛。
|
h*****0
发帖数: 4889 | 4 B::B() : A( *( (A*) (new int) ) ) {}
【在 c*******9 的大作中提到】
: 【 以下文字转载自 Quant 讨论区 】
: 发信人: jsyzghan (民兵营长), 信区: Quant
: 标 题: 码工试题
: 发信站: BBS 未名空间站 (Thu Mar 11 10:54:48 2010, 美东)
: N年之前的,某著名银行的一老俄问的.现在也没想出来.
: Given these classes:
: struct A
: {
: A() { throw 0; }
: };
|
X****r
发帖数: 3557 | 5 这个倒是个解法……虽然一样没有实际的用处，不过比我上面那个强得多。
【在 h*****0 的大作中提到】
: B::B() : A( *( (A*) (new int) ) ) {}
|
D****A
发帖数: 360 | 6 clever!
It could be even simpler. I tried this:
B::B()
:A(*(A *)0)
{}
【在 h*****0 的大作中提到】
: B::B() : A( *( (A*) (new int) ) ) {}
|
X****r
发帖数: 3557 | 7 dereferencing a null pointer is technically undefined behavior...
【在 D****A 的大作中提到】
: clever!
: It could be even simpler. I tried this:
: B::B()
: :A(*(A *)0)
: {}
|
D****A
发帖数: 360 | 8 yea... dereferencing a null ptr is undefined, but in this code, the *0 is
passed as argument to a copy constructor which does nothing. ;)
Runtime error can be arbitrary. ``new int'' is not safe either.
So, come on, its just a trick.
【在 X****r 的大作中提到】
: dereferencing a null pointer is technically undefined behavior...
|
d****p
发帖数: 685 | 9 In this case (the A is an empty class) the copy constructor of A is
optimized away when initializing B and the
null pointer does no harm.
If you however add an int member variable in A, you will have a segmentation
fault.
So perhaps the following is OK:
B::B() : A(*(static_cast(malloc(sizeof(A)))) {}
【在 D****A 的大作中提到】
: yea... dereferencing a null ptr is undefined, but in this code, the *0 is
: passed as argument to a copy constructor which does nothing. ;)
: Runtime error can be arbitrary. ``new int'' is not safe either.
: So, come on, its just a trick.
|
D****A
发帖数: 360 | 10 你说的对，其实你这个也不完美
不如 B::B() :A(*(A *)this) {}
segmentation
【在 d****p 的大作中提到】
: In this case (the A is an empty class) the copy constructor of A is
: optimized away when initializing B and the
: null pointer does no harm.
: If you however add an int member variable in A, you will have a segmentation
: fault.
: So perhaps the following is OK:
: B::B() : A(*(static_cast(malloc(sizeof(A)))) {}
|
|
|
d****p
发帖数: 685 | 11 If in B's object layout A is placed after B, we don't know what will happen
code will put B's virtual table pointer into A :-(
Basically when we try to initialize A during B's initialization list, point
this is totally not ready and it is a bad
idea to even think about using it. |
h*****0
发帖数: 4889 | 12 我感觉this指针在那个地方，会通不过编译。
happen
point
【在 d****p 的大作中提到】
: If in B's object layout A is placed after B, we don't know what will happen
: code will put B's virtual table pointer into A :-(
: Basically when we try to initialize A during B's initialization list, point
: this is totally not ready and it is a bad
: idea to even think about using it.
|
D****A
发帖数: 360 | 13 完全没问题的，要不然C++的子类static cast成父类的实现就有问题了
【在 h*****0 的大作中提到】
: 我感觉this指针在那个地方，会通不过编译。
:
: happen
: point
|
d****p
发帖数: 685 | 14 hmm.. At that time, this is firmly valid (though not property initialized)
so I guess compiler will allow dereferencing this in the derived class's
ctor's initialization list; it is however programmer's responsibility to
make sure it is used properly.
Consider the following legitimate example:
class Foo
{
....int a;
public:
....Foo(int a) : a(a) {}
....int IncA() { return a; }
};
class Bar : public Foo
{
... int b;
public:
....Bar(int x) : Foo(x), b(this->IncA()) {}
};
In the list after Foo is i
【在 h*****0 的大作中提到】
: 我感觉this指针在那个地方，会通不过编译。
:
: happen
: point
|
D****A
发帖数: 360 | 15 the use of ``this'' is totally benign here since the compiler knows exactly
where the A part in B is, no matter what implementation it is. Therefore
the copy constructor copies A to itself. Typically &B equls &static_cast(
B)
but that's not even necessary here.
happen
point
【在 d****p 的大作中提到】
: If in B's object layout A is placed after B, we don't know what will happen
: code will put B's virtual table pointer into A :-(
: Basically when we try to initialize A during B's initialization list, point
: this is totally not ready and it is a bad
: idea to even think about using it.
|
D****A
发帖数: 360 | 16 class Foo
{
....int a;
public:
....Foo(int a) : a(a) {}
....int IncA() { return a; }
};
class Bar : public Foo
{
... int b;
public:
....Bar(int x) : Foo(x), b(this->IncA()) {}
};
In the list after Foo is initialized, it is ok to invoke IncA via this->IncA
() which will be translated to Foo::IncA since IncA is not virtual.
In case IncA is virtual, I agree with you that a good compiler should forbit
using "this" to call any virtual function since at that time vtable is
perhaps not ready.
In the ca
【在 h*****0 的大作中提到】
: 我感觉this指针在那个地方，会通不过编译。
:
: happen
: point
|
d****p
发帖数: 685 | 17 You are right :-)
exactly
>(
【在 D****A 的大作中提到】
: the use of ``this'' is totally benign here since the compiler knows exactly
: where the A part in B is, no matter what implementation it is. Therefore
: the copy constructor copies A to itself. Typically &B equls &static_cast(
: B)
: but that's not even necessary here.
:
: happen
: point
|
d****p
发帖数: 685 | 18
. If it is pure
virtual you'll have compiler error anyways.
Suppose IncA() is virtual.
Compiler normally constructs Bar's vtable in Bar's constructor body; so in
the initialization list this->IncA()
will look for B's vtable which is unfortunately not ready.
Anyway I guess at that time Foo's vtable may be ready and the form (static_
castthis)->IncA() may work.
【在 D****A 的大作中提到】
: class Foo
: {
: ....int a;
: public:
: ....Foo(int a) : a(a) {}
: ....int IncA() { return a; }
: };
: class Bar : public Foo
: {
: ... int b;
|
d****p
发帖数: 685 | 19
Correction :-(
this->AnyFunc() seems always a static resolution and has nothing to do with
vtable.
【在 d****p 的大作中提到】
:
: . If it is pure
: virtual you'll have compiler error anyways.
: Suppose IncA() is virtual.
: Compiler normally constructs Bar's vtable in Bar's constructor body; so in
: the initialization list this->IncA()
: will look for B's vtable which is unfortunately not ready.
: Anyway I guess at that time Foo's vtable may be ready and the form (static_
: castthis)->IncA() may work.
|
