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@xxxxxxxxx>:
> Please reply to the mailing list, not me.
>
> On Nov 25, 2013 1:23 PM, "Parmenides" <mobile.parmenides@xxxxxxxxx> 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@xxxxxxxxx>:
>> > On 23 November 2013 16:54, Parmenides <mobile.parmenides@xxxxxxxxx>
>> > 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.
