Re: Why can not use reference in operator function?

[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.