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From: Jeffrey Schwab on 16 Jul 2010 01:19
On 7/15/10 7:46 PM, Seungbeom Kim wrote:

> struct A
> {
> A(int);
> friend A operator+(const A&, int);
> A operator+(const A&);
> };
>
> int main()
> {
> A(1)+2;
> }
>
> Comeau C++ gives an error for this:
>
> error: more than one operator "+" matches these operands:
> function "operator+(const A&, int)"
> function "A::operator+(const A&)"
> operand types are: A + int
> A(1)+3;
> ^


GCC agrees with Comeau about the error, but accepts the code if A is
passed by value in the friend operator:

friend A operator+(A, int);

Apparently, binding A(1) to the reference-type parameter counts as an
implicit conversion for purposes of overload resolution, at least as
implemented by GCC.

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From: Francis Glassborow on 16 Jul 2010 01:32
Seungbeom Kim wrote:
> On 2010-07-13 19:48, Pete Becker wrote:
>> Off hand, it looks like ob1 + ob2 is returning a Test object, and since
>> operator+ takes a non-const Test object, it can't be called with a
>> temporary. Both free functions should take their Test object by const
>> reference or by value, and the member operator+ should be marked const.
>
> I found something interesting when the member operator+ is not const.
> A minimal example:
>
> struct A
> {
> A(int);
> friend A operator+(const A&, int);
> A operator+(const A&);
> };
>
> int main()
> {
> A(1)+2;
> }
>
> Comeau C++ gives an error for this:
>
> error: more than one operator "+" matches these operands:
> function "operator+(const A &, int)"
> function "A::operator+(const A &)"
> operand types are: A + int
> A(1)+3;
> ^
>


No you are missing the point. Neither of your operator+ functions is an
exact match. the friend version matches on the second parameter but
needs the first one to be promoted from 'A' to 'A const' (the
restriction on using temporaries as arguments does not change the basic
type of the temporary). The member function requires a conversion from
int to A. The best match rules do not help because they do not apply
when the mismatches are on different parameters.

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