Hi All
I just read the C++FAQ . I have something confused. The question
is that :

If you provide constructive operators, they should allow promotion of
the left-hand operand (at least in the case where the class has a
single-parameter ctor that is not marked with the explicit keyword).
For example, if your class Fraction supports promotion from int to
Fraction (via the non-explicit ctor Fraction::Fraction(int)), and if
you allow x - y for two Fraction objects, you should also allow 42 -
y. In practice that simply means that your operator-() should not be a
member function of Fraction. Typically you will make it a friend, if
for no other reason than to force it into the public: part of the
class, but even if it is not a friend, it should not be a member.


Why the operator-() should not be a member function? Can someone give
a example?

On Dec 6, 8:00 pm, dolphin <jdxyw2...@gmail.com> wrote:
> Hi All
> I just read the C++FAQ . I have something confused. The question
> is that :
>
> If you provide constructive operators, they should allow promotion of
> the left-hand operand (at least in the case where the class has a
> single-parameter ctor that is not marked with the explicit keyword).
> For example, if your class Fraction supports promotion from int to
> Fraction (via the non-explicit ctor Fraction::Fraction(int)), and if
> you allow x - y for two Fraction objects, you should also allow 42 -
> y. In practice that simply means that your operator-() should not be a
> member function of Fraction. Typically you will make it a friend, if
> for no other reason than to force it into the public: part of the
> class, but even if it is not a friend, it should not be a member.
>
> Why the operator-() should not be a member function? Can someone give
> a example?

Aah, another thread with the same issue. See this one -
http://groups.google.com/group/comp....f7add269?tvc=2

Short answer: It can not be a member because the member would never
get invoked!

On 12ÔÂ6ÈÕ, ÏÂÎç11ʱ32·Ö, Abhishek Padmanabh <abhishek.padman...@gmail.com>
wrote:
> On Dec 6, 8:00 pm, dolphin <jdxyw2...@gmail.com> wrote:
>
>
>
>
>
> > Hi All
> > I just read the C++FAQ . I have something confused. The question
> > is that :
>
> > If you provide constructive operators, they should allow promotion of
> > the left-hand operand (at least in the case where the class has a
> > single-parameter ctor that is not marked with the explicit keyword).
> > For example, if your class Fraction supports promotion from int to
> > Fraction (via the non-explicit ctor Fraction::Fraction(int)), and if
> > you allow x - y for two Fraction objects, you should also allow 42 -
> > y. In practice that simply means that your operator-() should not be a
> > member function of Fraction. Typically you will make it a friend, if
> > for no other reason than to force it into the public: part of the
> > class, but even if it is not a friend, it should not be a member.
>
> > Why the operator-() should not be a member function? Can someone give
> > a example?
>
> Aah, another thread with the same issue. See this one -http://groups.google.com/group/comp.lang.c++/browse_thread/thread/138...
>
> Short answer: It can not be a member because the member would never
> get invoked!- Òþ²Ø±»ÒýÓÃÎÄ×Ö -
>
> - ÏÔʾÒýÓõÄÎÄ×Ö -

Do you mean that operator+(T , T) and operator*(T,T) also should not
be member function?

On Thu, 6 Dec 2007 07:00:20 -0800 (PST) in comp.lang.c++, dolphin
<jdxyw2004@gmail.com> wrote,
>Why the operator-() should not be a member function? Can someone give
>a example?

#include <complex>
int main()
{
std::complex<double> v(0.5);
v - 1.0; // works whether or not - is a member
1.0 - v; // Works only if - is a non-member.
}