6 ms·

The question isn’t whether there’s some way to do arithmetic on raw pointers, but whether you can take a pointer to a subobject and expect the GC to keep the ob

• pjmlp 4y ago
  For .NET, https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/fixed-statement https://docs.microsoft.com/en-us/dotnet/csharp/language-refe... For Java, https://docs.oracle.com/en/java/javase/18/docs/specs/jni/design.html#implementing-local-references https://docs.oracle.com/en/java/javase/18/docs/specs/jni/des... https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/misc/Unsafe.java https://github.com/openjdk/jdk/blob/master/src/java.base/sha... And as bonus since you didn't ask for it, Eiffel https://www.eiffel.org/doc/solutions/CECIL_-_Eiffel_to_C https://www.eiffel.org/doc/solutions/CECIL_-_Eiffel_to_C
  • ErikCorry 4y ago
    The Java link doesn't describe a way to have a pointer to a subobject. In Java you must always point at the start of an object. In Go, objects are segregated by allocation size, so the GC can round subobject pointers to keep the surrounding object alive.
    • aardvark179 4y ago
      Although it isn't visible from the language level the JVM internally does have "derived pointers" which are normally stored on the stack and point to an offset within an object. The GC can figure this out because it has a map for what the stack entries mean.
      • moonchild 4y ago
        'The' JVM can internally have whatever it likes; so long as it is not exposed at the language level, nothing prevents an implementation from being devised which does not have such things internally, and which has a treadmill.
        • pjmlp 4y ago
          JNI and Unsafe also exist, it isn't only about the language alone.
      • ErikCorry 4y ago
        Oh cool, I didn't realize the stack maps were so advanced. Now I'm trying to imagine which optimizations made this necessary/useful.
• Someone 4y ago
  Java used to do something like that for strings, where taking a substring of a string created an object that pointed into the original string, but that only was for strings, and didn’t expose the pointer, so the implementation could keep a pointer to the original string around for the GC to use. C# has Span<T>, nowadays, but is guaranteed to only live on the stack, making it easier for the GC to keep the object it points into alive while the Span lives (https://docs.microsoft.com/en-us/dotnet/api/system.span-1?view=net-6.0 https://docs.microsoft.com/en-us/dotnet/api/system.span-1?vi...)
• twic 4y ago
  Java doesn't even have subobjects in this sense, so you're quite right, you can't do this!
  • pjmlp 4y ago
    It has JNI, Unsafe, and incoming, Panama. All provide ways to shoot yourself if not used properly.