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  1. module test;
  2.  
  3. import std.stdio;
  4.  
  5. struct Result(T) 
  6. {
  7.     @property static ref Result!T fail(int errorCode) 
  8.     { 
  9.         static Result!T result;
  10.         result.errorCode = errorCode;
  11.         return result;
  12.     }    
  13.  
  14.     @property ref Result!T ok(T init) 
  15.     {
  16.         errorCode = 0;
  17.         __hiddenMember = init;
  18.         return this;
  19.     }
  20.  
  21.     int errorCode = -1;
  22.     auto opCast(T = bool)() { return errorCode == 0; }
  23.     T __hiddenMember;
  24.     alias __hiddenMember this;
  25. }
  26.  
  27. struct Coords { int x, y, z; }
  28.  
  29. Result!Coords getCoords(bool isTrue)
  30. {
  31.     typeof(return) result;
  32.  
  33.     if (isTrue)
  34.         return result.ok(Coords(1, 2, 3));
  35.     else
  36.         return result.fail(-4);
  37. }
  38.  
  39. void main()
  40. {
  41.     Coords coords1 = getCoords(true);
  42.     assert(coords1 == Coords(1, 2, 3));
  43.  
  44.     auto result = getCoords(false);
  45.     if (!result)  // calls opCast!bool
  46.         writefln("Error code: %s", result.errorCode);
  47.     else
  48.         coords1 = result;  // uses 'alias this' to assign the field
  49.  
  50.     Coords coords2;
  51.     auto status2 = getCoords(true);
  52.     if (status2)  // calls opCast!bool
  53.         coords2 = status2;  // uses 'alias this' to assign the field
  54.     assert(coords2 == Coords(1, 2, 3));
  55.  
  56.     Coords coords3;
  57.     if (auto status3 = getCoords(true))  // calls opCast!bool, and if true returns 'Result!Coords' struct
  58.         coords3 = status3;  // uses 'alias this' to assign the field
  59.     assert(coords3 == Coords(1, 2, 3));
  60. }
  61.  
  62.  
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https://ideone.com/qy8i3
language:
D (dmd 2.085.0)
created:
12 years ago
visibility:
public

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