Compilation errors when using newer C++ standards

[radioactiveman]

Out of curiosity I have tried to compile Audacious with the gnu++20 standard.
Both audacious and audacious-plugins then fail to compile null checks like this:

String foo;
if (foo != nullptr)
...

Error message:

../src/libaudgui/infowin.cc:519:32: error: use of overloaded operator '!=' is ambiguous (with operand types 'String' and 'std::nullptr_t')
    g_return_if_fail (filename != nullptr);
                      ~~~~~~~~ ^  ~~~~~~~
/usr/include/glib-2.0/glib/gmessages.h:640:19: note: expanded from macro 'g_return_if_fail'
    if (G_LIKELY (expr)) \
                  ^~~~
/usr/include/glib-2.0/glib/gmacros.h:1243:59: note: expanded from macro 'G_LIKELY'
#define G_LIKELY(expr) (__builtin_expect (_G_BOOLEAN_EXPR(expr), 1))
                                                          ^~~~
/usr/include/glib-2.0/glib/gmacros.h:1242:66: note: expanded from macro '_G_BOOLEAN_EXPR'
#define _G_BOOLEAN_EXPR(expr) _G_BOOLEAN_EXPR_IMPL (__COUNTER__, expr)
                                                                 ^~~~
/usr/include/glib-2.0/glib/gmacros.h:1236:8: note: expanded from macro '_G_BOOLEAN_EXPR_IMPL'
   if (expr)                                    \
       ^~~~
../src/libaudcore/objects.h:155:10: note: candidate function
    bool operator==(const String & b) const { return raw_equal(raw, b.raw); }
         ^
../src/libaudgui/infowin.cc:519:32: note: built-in candidate operator!=(const char *, const char *)
    g_return_if_fail (filename != nullptr);
                               ^
../src/libaudgui/infowin.cc:519:32: note: built-in candidate operator!=(const volatile char *, const volatile char *)

How can this be solved? Ideally in our String class, so we do not have to change all explicit null checks to this which compiles fine.

String foo;
if (foo)
...

Another question I have is why do we use the gnu c++ standard.
Audacious and -plugins compile successfully with c++17 for example, at least on Linux.

[jlindgren90]

I don't think we should be comparing a String with nullptr. I'd rather replace those occurrences with the boolean-style check.

Regarding the gnu++ vs c++ thing, I think there were some older versions of gcc that didn't fully support the c++ variants. I tried gcc 5.4.0 just now and it worked fine though, so maybe it's not an issue any more.

[radioactiveman]

All right, thanks John. Fixed with c67afc5 and audacious-media-player/audacious-plugins@6009062.

Using c++20 instead of gnu++20 caused the Windows build to fail with the error M_PI was not declared in this scope.
So let's better keep using the gnu std.

audacious/src/libaudcore/equalizer.cc

Line 64 in b7f0019

float th = 2 * (float)M_PI * fc;