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10294 :: Wrapping C++ classes and constructors lead me to compilation errors global vs local, stable vs devel

• giaco (orginal) [2023-06-23T18:58:46+02:00] view original

  Would you explain to me what's happening here? I'm having different compiler behaviours when intdefine OPT = 1, 2 , 3 or 4

  I don't get if MyClass() default constructor is a legal C++ call or not if MyClass(short a) constructor has been defined. I also don't get the differences between local vs global context (case OPT = 2 vs 3)

  case OPT = 4 seems a fixed bug stable vs devel, but not sure

  thanks

  {.emit: """
  class MyClass {
      public:
          short attr1;
          MyClass(short a) {
            attr1 = a;
          }
  };
  """.}
  
  type
    MyClass {.importcpp: "MyClass".} = object
      attr1: cshort
  
  proc constructMyClass(a: cshort): MyClass {.importcpp: "MyClass(@)", constructor.}
  
  const OPT {.intdefine.}: int = 1
  
  # stable 1.6.12: WORKS
  # devel 1.9.5: WORKS
  when OPT == 1:
    proc main =
      var obj = constructMyClass(42)
      assert obj.attr1 == 42
    main()
  
  # stable 1.6.12: WORKS
  # devel 1.9.5: WORKS
  when OPT == 2:
    var obj = MyClass()
    obj.attr1 = 42
    assert obj.attr1 == 42
  
  # stable 1.6.12: error: no matching function for call to ‘outer::inner::MyClass::MyClass()’
  # devel 1.9.5: error: no matching function for call to ‘outer::inner::MyClass::MyClass(<brace-enclosed initializer list>)’
  when OPT == 3:
    proc main =
      var obj = MyClass()
    main()
  
  # stable 1.6.12: error: no matching function for call to ‘outer::inner::MyClass::MyClass()’
  # devel 1.9.5: WORKS
  when OPT == 4:
    let obj = constructMyClass(42)
    assert obj.attr1 == 42
  
  Run

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• giaco (orginal) [2023-06-23T19:21:56+02:00] view original

  been talking with @jmgomez on IRC about this, he proposed to move some insights here

  talking about case OPT = 2

  
  18:54 <FromDiscord> <jmgomez> It works by coincidence really
  18:54 <FromDiscord> <jmgomez> `N_LIB_PRIVATE MyClass objbugg_6 = {0}`
  18:55 <FromDiscord> <jmgomez> it's calling the constructor you defined without intending to. If you add another parameter probably it will fail
  18:57 <FromDiscord> <jmgomez> You can ask in #internals or forums or in gh issues if it's the intended behaviors. Im not familiar with how the logic of `{}` is expected to work in Nim
  ...
  19:08 <FromDiscord> <jmgomez> There is no way the Nim compiler knows that you deleted the constructor
  Run

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• sls1005 (orginal) [2023-06-24T06:30:20+02:00] view original

  According to https://en.cppreference.com/w/cpp/language/default_constructor, a default constructor (one that without parameters) is automatically generated only when there's no user-defined constructor. Since you have defined a constructor taking one argument, you should not be able to use MyClass() within a proc unless by explictly defining (in C++) a constructor that takes no argument. However, according to https://en.cppreference.com/w/cpp/language/initialization, a non-local variable may be zero-initialized when constant initialization (one that requires a constructor) is not available, so you could still use MyClass() outside procs.

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• giaco (orginal) [2023-06-24T13:50:05+02:00] view original

  This clearly explains case OPT = 2 and 3!

  Thanks a lot, I didn't know about non-local variables that can be zero-initialized when constant initialization is not available

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