C++ Q48: illegal operation (C33) - 未名空间MITBBS历史存档

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C++ Q48: illegal operation (C33)

C++ Q48: illegal operation (C33)# JobHunting - 待字闺中

c*e2010-08-13 07:08

1 楼

If a class has + and = operator overloaded, and three objects a, b, c
constructed, which of the following is illegal?
a) (c=a+a)=b+c;
b) a=a=b+c;
More detail:
____________________
class X {
public:
X& operator=(const X& rhs);
const X& operator+(const X& rhs) const;
private;
int n;
};
int main() {
X a, b, c;
// Statement goes here
return 0;
}

P*b2010-08-13 07:08

2 楼

这个是不是a？

【在 c**********e 的大作中提到】

: If a class has + and = operator overloaded, and three objects a, b, c
: constructed, which of the following is illegal?
: a) (c=a+a)=b+c;
: b) a=a=b+c;
: More detail:
: ____________________
: class X {
: public:
: X& operator=(const X& rhs);
: const X& operator+(const X& rhs) const;

s*t2010-08-13 07:08

3 楼

只能是a了

【在 P*******b 的大作中提到】

: 这个是不是a？

x*y2010-08-13 07:08

4 楼

It really depends on how you overload these operators...If you follow the
convention of built in types and types in standard libraries ( return a non-
const reference to *this) a) actually works; but for b), the self-assignment
can often lead to mistake for badlly overloaded = operator...

【在 c**********e 的大作中提到】

c*e2010-08-13 07:08

5 楼

More detail is given -- I was lazy in typing the code -- now I have done it.

non-
assignment

【在 x***y 的大作中提到】

: It really depends on how you overload these operators...If you follow the
: convention of built in types and types in standard libraries ( return a non-
: const reference to *this) a) actually works; but for b), the self-assignment
: can often lead to mistake for badlly overloaded = operator...

P*b2010-08-13 07:08

6 楼

题不全吧，都是对的。

【在 c**********e 的大作中提到】

s*t2010-08-13 07:08

7 楼

似乎没有答案啊。

【在 c**********e 的大作中提到】

x*y2010-08-13 07:08

8 楼

Then, I think the answer is a.
The problem is this declaration:
const X& operator+(const X& rhs) const;
then in a)
a+a is a.operator+(a) and notice a reference is returned。
we know that we can not return a reference to a local stack, a local static,
or a heap-allocated object, so the only possibility is that a itself is
returned. But as the + operation, we return the sum, in which the a's value
must be changed, which voilate the signature of pass by reference to const
and const function...

it.

【在 c**********e 的大作中提到】

: More detail is given -- I was lazy in typing the code -- now I have done it.
:
: non-
: assignment

s*t2010-08-13 07:08

9 楼

It seems to me that it depends on how operator+ is defined.
We can return a reference to a heap-allocated object and a local static.
Actually, we sometimes need to return a reference to a local static,
due to initialization order problem.
We can create a heap-allocated object, so it is not clear to me that a's
value has to be changed.

static,
value

【在 x***y 的大作中提到】

: Then, I think the answer is a.
: The problem is this declaration:
: const X& operator+(const X& rhs) const;
: then in a)
: a+a is a.operator+(a) and notice a reference is returned。
: we know that we can not return a reference to a local stack, a local static,
: or a heap-allocated object, so the only possibility is that a itself is
: returned. But as the + operation, we return the sum, in which the a's value
: must be changed, which voilate the signature of pass by reference to const
: and const function...

x*y2010-08-13 07:08

10 楼

returning a reference to a heap-allocated in this one is a bad idea, as
there is no way for you to delete it; about reference to a local static
object, your statement is right, local static works, but it's weired to use
it here, especially it will create problems for multithreaded application
when + is such a freqently used operator.
Without definition of operators, the answer is that it depends. However, in
my opinion, from pratical perspective, either time overhead is very high for
synchroniz