Use `nullptr` for all null pointer constants

[myQwil]

Also added -Wzero-as-null-pointer-constant to cxx flags.

Ensuring that only nullptr is used with pointer types can prevent accidental assignments or comparisons with non-pointer types, reduce the risk of subtle bugs, and improve code reliability. It also helps to avoid ambiguity and improve clarity and intent.

[Wohlstand]

Be careful! Don't add this argument directly, add at least a macro that checks it and adds when compiler do really has it.

Here is an example of macro: https://github.com/WohlSoft/AudioCodecs/blob/master/audio_codec_common.cmake#L17-L22 And the example on how to use it:

ac_add_cxx_warning_flag(zero-as-null-pointer-constant ZERO_AS_NULLPTR_CONSTANT)

Anyway, please rename ac_add_cxx_warning_flag into gme_add_cxx_warning_flag.

[Wohlstand]

Looks much better now! Soon I'll take more deeper look and merge.

[sezero]

There seems to be misses? I get the following from my compiler:

/tmp/libgme/gme/gme.cpp: In function 'const char* gme_track_info(const Music_Emu*, gme_info_t**, int)':
/tmp/libgme/gme/gme.cpp:304:7: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  *out = NULL;
       ^
/tmp/libgme/gme/Nes_Apu.cpp: In constructor 'Nes_Apu::Nes_Apu()':
/tmp/libgme/gme/Nes_Apu.cpp:26:17: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  dmc.prg_reader = NULL;
                 ^
/tmp/libgme/gme/Nes_Apu.cpp:27:16: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  irq_notifier_ = NULL;
                ^
/tmp/libgme/gme/Nes_Apu.cpp:35:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  output( NULL );
               ^
In file included from /tmp/libgme/gme/nes_cpu_io.h:6:0,
                 from /tmp/libgme/gme/Nes_Cpu.cpp:28:
/tmp/libgme/gme/Nes_Fds_Apu.h: In constructor 'Nes_Fds_Apu::Nes_Fds_Apu()':
/tmp/libgme/gme/Nes_Fds_Apu.h:127:22: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  osc_output( 0, NULL );
                      ^
In file included from /tmp/libgme/gme/Nes_Fme7_Apu.cpp:3:0:
/tmp/libgme/gme/Nes_Fme7_Apu.h: In constructor 'Nes_Fme7_Apu::Nes_Fme7_Apu()':
/tmp/libgme/gme/Nes_Fme7_Apu.h:89:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  output( NULL );
               ^
/tmp/libgme/gme/Nes_Namco_Apu.cpp: In constructor 'Nes_Namco_Apu::Nes_Namco_Apu()':
/tmp/libgme/gme/Nes_Namco_Apu.cpp:20:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  output( NULL );
               ^
/tmp/libgme/gme/Nes_Vrc6_Apu.cpp: In constructor 'Nes_Vrc6_Apu::Nes_Vrc6_Apu()':
/tmp/libgme/gme/Nes_Vrc6_Apu.cpp:20:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  output( NULL );
               ^
In file included from /tmp/libgme/gme/Nes_Fds_Apu.cpp:3:0:
/tmp/libgme/gme/Nes_Fds_Apu.h: In constructor 'Nes_Fds_Apu::Nes_Fds_Apu()':
/tmp/libgme/gme/Nes_Fds_Apu.h:127:22: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  osc_output( 0, NULL );
                      ^
In file included from /tmp/libgme/gme/Nsf_Emu.cpp:13:0:
/tmp/libgme/gme/Nes_Fme7_Apu.h: In constructor 'Nes_Fme7_Apu::Nes_Fme7_Apu()':
/tmp/libgme/gme/Nes_Fme7_Apu.h:89:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  output( NULL );
               ^
In file included from /tmp/libgme/gme/Nsf_Emu.cpp:14:0:
/tmp/libgme/gme/Nes_Fds_Apu.h: In constructor 'Nes_Fds_Apu::Nes_Fds_Apu()':
/tmp/libgme/gme/Nes_Fds_Apu.h:127:22: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  osc_output( 0, NULL );
                      ^
In file included from /tmp/libgme/gme/Vgm_Emu.h:7:0,
                 from /tmp/libgme/gme/Vgm_Emu.cpp:3:
/tmp/libgme/gme/Vgm_Emu_Impl.h: In instantiation of 'Ym_Emu<Emu>::Ym_Emu() [with Emu = Ym2612_Nuked_Emu]':
/tmp/libgme/gme/Vgm_Emu_Impl.h:27:7:   required from here
/tmp/libgme/gme/Vgm_Emu_Impl.h:20:74: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  Ym_Emu()                        : last_time( disabled_time ), out( NULL ) { }
                                                                          ^
/tmp/libgme/gme/Vgm_Emu_Impl.h: In instantiation of 'Ym_Emu<Emu>::Ym_Emu() [with Emu = Ym2413_Emu]':
/tmp/libgme/gme/Vgm_Emu_Impl.h:27:7:   required from here
/tmp/libgme/gme/Vgm_Emu_Impl.h:20:74: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
/tmp/libgme/player/Music_Player.cpp: In constructor 'Music_Player::Music_Player()':
/tmp/libgme/player/Music_Player.cpp:49:14: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  track_info_ = NULL;
              ^
/tmp/libgme/player/Music_Player.cpp: In member function 'void Music_Player::stop()':
/tmp/libgme/player/Music_Player.cpp:68:7: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
  emu_ = NULL;
       ^
/tmp/libgme/player/Music_Player.cpp: In member function 'const char* Music_Player::start_track(int)':
/tmp/libgme/player/Music_Player.cpp:140:15: warning: zero as null pointer constant [-Wzero-as-null-pointer-constant]
   track_info_ = NULL;
               ^

P.S.: @Wohlstand: Can you apply the following to demo/CMakeLists.txt? demo/basics_multi.c seems to need it.

diff --git a/demo/CMakeLists.txt b/demo/CMakeLists.txt
index b4453f7..82991ce 100644
--- a/demo/CMakeLists.txt
+++ b/demo/CMakeLists.txt
@@ -6,4 +6,6 @@ include_directories(${CMAKE_SOURCE_DIR}/gme ${CMAKE_SOURCE_DIR})
 link_directories(${CMAKE_BINARY_DIR}/gme)

+set (CMAKE_C_STANDARD 99)
+
 add_executable(demo Wave_Writer.cpp basics.c)

[myQwil]

@sezero Okay, I'm getting the same warnings as you now. I needed to build with clang. For some reason, GNU isn't picking up all of them.

It seems to have something to do with how NULL is being interpreted.

[sezero]

Well, the gcc I used is 4.9 which is pretty old. And I believe that the warning should actually be really about using actual 0 instead of NULL, maybe that's why your clang didn't warn?

To me, this patch is blah (sorry, I already don't like c++, much less using newer bloated standards of it.)

[myQwil]

I think there's an example that illustrates the problem pretty well:

#include <iostream>

struct B {};

struct A
{
    operator B*() {return 0;}
    operator bool() {return true;}
};

int main()
{
    A a;
    B* pb = 0;
    typedef void* null_ptr_t;
    null_ptr_t null = 0;

    std::cout << "(a == pb): " << (a == pb) << std::endl;
    std::cout << "(a == 0): " << (a == 0) << std::endl; // no warning
    std::cout << "(a == NULL): " << (a == NULL) << std::endl; // warns sometimes
    std::cout << "(a == null): " << (a == null) << std::endl;
}

(a == pb): 1
(a == 0): 0
(a == NULL): 0
(a == null): 1

My understanding is that it looks at 0 and NULL and thinks "These are more similar to booleans than pointers, so I'm going to convert a to a boolean."

At some point, we start getting into the realm of ambiguity and it's probably better if we're just really explicit about our intentions with the code.

@sezero I understand that some individuals may have reservations about adopting newer features and standards in programming languages, but the introduction of nullptr in C++11 was driven by the desire to improve code safety, clarity, and expressiveness.

Consistently using nullptr throughout a codebase can improve code readability and maintainability. It establishes a clear convention and avoids mixing different null representation styles, such as NULL and 0, which can lead to confusion and inconsistencies.