* Why can not use reference in operator function?
@ 2013-11-23 17:03 Parmenides
2013-11-24 18:19 ` Jonathan Wakely
0 siblings, 1 reply; 4+ messages in thread
From: Parmenides @ 2013-11-23 17:03 UTC (permalink / raw)
To: gcc-help
Hi,
I have the following code:
#include <iostream>
#include <cstring>
using namespace std;
class Int{
int x;
public:
Int(int i = 0)
{
x = i;
}
~Int()
{
}
void show()
{
cout << x << endl;
}
friend Int operator+(Int &a, Int &b) // Because 'b' is a reference to
object rather than an object
{
return Int(a.x+b.x);
}
};
int main()
{
Int i(3), j;
j = i + Int(6); // This can not call constructor.
j.show();
return 0;
}
Gcc issues error message, but VC++ 2010 compile it successfully. I
tried to modify
friend Int operator+(Int &a, Int &b)
to
friend Int operator+(Int a, Int b)
both compiler can get it pass. Why Gcc does not want to convert a
'int' to a 'Int' object in the '+' operator function when its second
parameter is a reference to object rather than an objetc?
I further modified
j = i + Int(6);
to
j = i + 6;
both compiler can get it pass again. Therefore, I think it seem that
Gcc encourage programmers to use implict conversion like 'i + 6'
rather that explict conversion like 'i + Int(6)'. Is this right? If
so, does this practice comfore to the C++ standard?
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Why can not use reference in operator function?
2013-11-23 17:03 Why can not use reference in operator function? Parmenides
@ 2013-11-24 18:19 ` Jonathan Wakely
[not found] ` <CAOXENUgy1FadKfQd6uv88qoYPUMR3z9sx083up3cWkbCNTFwcg@mail.gmail.com>
0 siblings, 1 reply; 4+ messages in thread
From: Jonathan Wakely @ 2013-11-24 18:19 UTC (permalink / raw)
To: Parmenides; +Cc: gcc-help
On 23 November 2013 16:54, Parmenides <mobile.parmenides@gmail.com> wrote:
> Hi,
>
> I have the following code:
>
> #include <iostream>
> #include <cstring>
> using namespace std;
>
> class Int{
> int x;
>
> public:
> Int(int i = 0)
> {
> x = i;
> }
>
> ~Int()
> {
> }
>
> void show()
> {
> cout << x << endl;
> }
>
> friend Int operator+(Int &a, Int &b) // Because 'b' is a reference to
> object rather than an object
> {
> return Int(a.x+b.x);
> }
> };
>
> int main()
> {
> Int i(3), j;
> j = i + Int(6); // This can not call constructor.
This creates a temporary object and temporaries cannot bind to
non-const references.
> j.show();
>
> return 0;
> }
>
> Gcc issues error message, but VC++ 2010 compile it successfully. I
This is a well-known VC++ bug, it allows temporaries to bind to
non-const references. That does not conform to the C++ standard.
> tried to modify
>
> friend Int operator+(Int &a, Int &b)
>
> to
>
> friend Int operator+(Int a, Int b)
>
> both compiler can get it pass. Why Gcc does not want to convert a
> 'int' to a 'Int' object in the '+' operator function when its second
> parameter is a reference to object rather than an objetc?
Because a temporary cannot bind to a non-const reference.
> I further modified
>
> j = i + Int(6);
>
> to
>
> j = i + 6;
>
> both compiler can get it pass again. Therefore, I think it seem that
> Gcc encourage programmers to use implict conversion like 'i + 6'
> rather that explict conversion like 'i + Int(6)'. Is this right?
No, GCC encourages you to use const references if you want to bind to
temporaries, as required by the C++ standard.
Your operator should have been declared
friend Int operator+(const Int& a, const Int& b)
> If
> so, does this practice comfore to the C++ standard?
G++ conforms to the standard in this regard.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Why can not use reference in operator function?
[not found] ` <CAH6eHdTHpCNtMb0EOrs034mifZ9j-r=CwyYgkTHM2J1uN6dmVg@mail.gmail.com>
@ 2013-11-26 13:23 ` Parmenides
2013-11-26 13:41 ` Jonathan Wakely
0 siblings, 1 reply; 4+ messages in thread
From: Parmenides @ 2013-11-26 13:23 UTC (permalink / raw)
To: gcc-help
Hi Wakely,
Thanks for your reply. The code has gotten compiled successfully with
both compliers. I wonder what is the difference between 'Int &' and
'const Int &'. Why the former fails, while the latter causes the
conversion from an 'int' to an 'Int' object?
2013/11/26 Jonathan Wakely <jwakely.gcc@gmail.com>:
> Please reply to the mailing list, not me.
>
> On Nov 25, 2013 1:23 PM, "Parmenides" <mobile.parmenides@gmail.com> wrote:
>>
>> Hi Wakely,
>>
>> Thanks for your reply. The code has gotten compiled successfully with
>> both compliers. I wonder what is the difference between 'Int &' and
>> 'const Int &'. Why the former fails, while the latter causes the
>> conversion from an 'int' to an 'Int' object?
>>
>>
>> 2013/11/24 Jonathan Wakely <jwakely.gcc@gmail.com>:
>> > On 23 November 2013 16:54, Parmenides <mobile.parmenides@gmail.com>
>> > wrote:
>> >> Hi,
>> >>
>> >> I have the following code:
>> >>
>> >> #include <iostream>
>> >> #include <cstring>
>> >> using namespace std;
>> >>
>> >> class Int{
>> >> int x;
>> >>
>> >> public:
>> >> Int(int i = 0)
>> >> {
>> >> x = i;
>> >> }
>> >>
>> >> ~Int()
>> >> {
>> >> }
>> >>
>> >> void show()
>> >> {
>> >> cout << x << endl;
>> >> }
>> >>
>> >> friend Int operator+(Int &a, Int &b) // Because 'b' is a reference to
>> >> object rather than an object
>> >> {
>> >> return Int(a.x+b.x);
>> >> }
>> >> };
>> >>
>> >> int main()
>> >> {
>> >> Int i(3), j;
>> >> j = i + Int(6); // This can not call constructor.
>> >
>> > This creates a temporary object and temporaries cannot bind to
>> > non-const references.
>> >
>> >> j.show();
>> >>
>> >> return 0;
>> >> }
>> >>
>> >> Gcc issues error message, but VC++ 2010 compile it successfully. I
>> >
>> > This is a well-known VC++ bug, it allows temporaries to bind to
>> > non-const references. That does not conform to the C++ standard.
>> >
>> >> tried to modify
>> >>
>> >> friend Int operator+(Int &a, Int &b)
>> >>
>> >> to
>> >>
>> >> friend Int operator+(Int a, Int b)
>> >>
>> >> both compiler can get it pass. Why Gcc does not want to convert a
>> >> 'int' to a 'Int' object in the '+' operator function when its second
>> >> parameter is a reference to object rather than an objetc?
>> >
>> > Because a temporary cannot bind to a non-const reference.
>> >
>> >> I further modified
>> >>
>> >> j = i + Int(6);
>> >>
>> >> to
>> >>
>> >> j = i + 6;
>> >>
>> >> both compiler can get it pass again. Therefore, I think it seem that
>> >> Gcc encourage programmers to use implict conversion like 'i + 6'
>> >> rather that explict conversion like 'i + Int(6)'. Is this right?
>> >
>> > No, GCC encourages you to use const references if you want to bind to
>> > temporaries, as required by the C++ standard.
>> >
>> > Your operator should have been declared
>> >
>> > friend Int operator+(const Int& a, const Int& b)
>> >
>> >
>> >> If
>> >> so, does this practice comfore to the C++ standard?
>> >
>> > G++ conforms to the standard in this regard.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Why can not use reference in operator function?
2013-11-26 13:23 ` Parmenides
@ 2013-11-26 13:41 ` Jonathan Wakely
0 siblings, 0 replies; 4+ messages in thread
From: Jonathan Wakely @ 2013-11-26 13:41 UTC (permalink / raw)
To: Parmenides; +Cc: gcc-help
On 26 November 2013 13:17, Parmenides wrote:
> Hi Wakely,
>
> Thanks for your reply. The code has gotten compiled successfully with
> both compliers. I wonder what is the difference between 'Int &' and
> 'const Int &'.
That is a basic C++ question, so is not suitable for this mailing
list. See http://www.parashift.com/c++-faq/ref-to-const.html
> Why the former fails, while the latter causes the
> conversion from an 'int' to an 'Int' object?
Again, this mailing list is not the right place to try and learn C++.
The conversion from int to Int creates a temporary object, and
temporary Int objects cannot bind to Int& references, they can only
bind to const Int& reference. That's how C++ works.
^ permalink raw reply [flat|nested] 4+ messages in thread
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2013-11-23 17:03 Why can not use reference in operator function? Parmenides
2013-11-24 18:19 ` Jonathan Wakely
[not found] ` <CAOXENUgy1FadKfQd6uv88qoYPUMR3z9sx083up3cWkbCNTFwcg@mail.gmail.com>
[not found] ` <CAH6eHdTHpCNtMb0EOrs034mifZ9j-r=CwyYgkTHM2J1uN6dmVg@mail.gmail.com>
2013-11-26 13:23 ` Parmenides
2013-11-26 13:41 ` Jonathan Wakely
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