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902 :: Moving something on the heap

• andrea (orginal) [2015-02-20T12:52:45+01:00] view original

  I have a silly question, but I have mostly avoided using ref until now, and only at this point I realize I do not know how to move a value to the heap and get a reference to that value.

  For instance, consider the following:

  type Foo = object
    x: int
  
  proc getFoo(): Foo = Foo(x: 15)
  
  proc getRefFoo(): ref Foo =
    let x: ref Foo = getFoo()
    x
  
  echo getRefFoo.x
  Run

  I assumed that inside getFoo, Nim would create the Foo on the heap (because it has to be returned) and hence I would be able to get a reference to it. But the line

  let x: ref Foo = getFoo()
  Run

  does not compile, saying type mismatch: got (Foo) but expected 'ref Foo'. I have tried

  let x: ref Foo = ref getFoo()
  Run

  which is not even syntactically valid, and

  let x: ref Foo = addr getFoo()
  Run

  which fails because addr gives an untraced pointer.

  So, if I have a proc that returns a Foo and I need - say - to store it into an object that expects a field of type ref Foo, how do I do this (possibly copying the value if it is not already on the heap)?

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• dom96 (orginal) [2015-02-20T13:39:32+01:00] view original

  The following will work:

  
  type Foo = object
    x: int
  
  proc getFoo(): Foo = Foo(x: 15)
  
  proc getRefFoo(): ref Foo =
    new result
    result[] = getFoo()
  
  echo getRefFoo().x
  Run

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• andrea (orginal) [2015-02-20T14:04:53+01:00] view original

  Great, thank you!

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