[cfe-dev] Friend functions default parameters behavior

[Nikola Smiljanic]

This is taken from clang source at the point where the error is emitted:

// C++11 [dcl.fct.default]p4: If a friend declaration specifies a
// default argument expression, that declaration shall be a definition
// and shall be the only declaration of the function or function
// template in the translation unit.

Not sure how wide spread this construct is, I'm seeing it for the first
time. The code is not hard to fix and error seems appropriate to me.


On Wed, Jul 23, 2014 at 9:48 PM, Александр Овчинников <sanek23994 at gmail.com>
wrote:

> Hi all,
> In the http://clang.debian.net/ project Sylvestre, Arthur and I are
> trying to rebuild Debian with clang. And we have the problems with default
> arguments in the friend methods. (
> http://clang.debian.net/status.php?version=3.4.2&key=WRONG_DEFAULT_DECLARATION).
> This issue affects 19 packages in the Debian rebuild.
>
> In the process of the rebuild Debian packages we had created simple
> example which contains different behavior with gcc and clang. Please see
> below:
>
> *Simple example *
> int func1(int i, int j) { return i + j; }
>
> class foo {
> friend int func1(int i, int j = 0);
> public:
>     int func();
> };
>
> int foo::func() { return func1(1); }
>
> int main() {
>     foo a;
>     a.func();
>     return 0;
> }
>
> Clang compilation error:
> test.cpp:8:12: error: friend declaration specifying a default argument
> must be the only declaration
> friend int func1(int i, int j = 0);
>            ^
> test.cpp:1:5: note: previous declaration is here
> int func1(int i, int j)
>     ^
> test.cpp:15:12: error: no matching function for call to 'func1'
>     return func1(1);
>            ^~~~~
> test.cpp:1:5: note: candidate function not viable: requires 2 arguments,
> but 1 was provided
>
> int func1(int i, int j)
>     ^
> 2 errors generated.
>
> The provided example successfully complies by gcc. So as it seems for the
> clang successfully compilation failed packages requires patches.
> As I understand the following code is correct way to using default
> arguments with friend functions:
>
> int func1(int i, int j = 0);
>
> class foo {
> friend int func1(int i, int j);
> public:
>     int func();
> };
>
> int foo::func() { return func1(1); }
>
> int main() {
>     foo a;
>     a.func();
>     return 0;
> }
>
> int func1(int i, int j) { return i + j; }
>
> Even if clang seems to follow the C++ specification, I wonder if clang is
> not too strict here.
> For example, this patch, to make sure that the code compiles with clang, I
> had to basically get ride of the default feature:
>
> https://bugs.debian.org/cgi-bin/bugreport.cgi?msg=5;filename=gfan.diff;att=1;bug=755308
> to be able to compile the code with clang without touching the API.
>
>
> --
> Alexander
>
> Best regards.
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>
>
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