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  1. #include <exception>
  2. #include <stdexcept>
  3. #include <cstddef>
  4. #include <cstdint>
  5. #include <cstdlib>
  6.  
  7. using error_type = std::exception_ptr;
  8.  
  9. struct ResultBase
  10. {
  11.     bool hasError() const
  12.     {
  13.         return *reinterpret_cast<const bool*>(this);
  14.     }
  15.  
  16.     std::exception_ptr error() const
  17.     {
  18.         return *reinterpret_cast<const error_type*>(
  19.             reinterpret_cast<const char*>(this)
  20.             + sizeof(std::max_align_t));
  21.     }
  22.  
  23. protected:
  24.     ResultBase() { }
  25. };
  26.  
  27. template <class T>
  28. struct Result : ResultBase
  29. {
  30.     Result(error_type error)
  31.         : mHasError(true) { new (&mError) error_type(error); }
  32.  
  33.     Result(T value)
  34.         : mHasError(false) { new (&mValue) T(value); }
  35.  
  36.     ~Result()
  37.     {
  38.         if (mHasError)
  39.             mError.~error_type();
  40.         else
  41.             mValue.~T();
  42.     }
  43.  
  44.     void setError(error_type error)
  45.     {
  46.         if (mHasError) {
  47.             mError = error;
  48.         } else {
  49.             mValue.~T();
  50.             new (&mError) error_type(error);
  51.             mHasError = true;
  52.         }
  53.     }
  54.  
  55.     void setValue(T value)
  56.     {
  57.         if (mHasError) {
  58.             mError.~error_type();
  59.             new (&mValue) T(value);
  60.             mHasError = false;
  61.         } else {
  62.             mValue = value;
  63.         }
  64.     }
  65.  
  66. private:
  67.     union {
  68.         bool mHasError;
  69.         std::max_align_t mAligner;
  70.     };
  71.     union {
  72.         error_type mError;
  73.         T mValue;
  74.     };
  75. };
  76.  
  77. static_assert(std::is_standard_layout<Result<int>>::value, "");
  78.  
  79. void check(bool condition)
  80. {
  81.     if (!condition) std::terminate();
  82. }
  83.  
  84. void f(const ResultBase& alias, Result<int>& r)
  85. {
  86.     check(!alias.hasError());
  87.  
  88.     r.setError(std::exception_ptr());
  89.     check(alias.hasError());
  90.     check(alias.error() == nullptr);
  91.  
  92.     r.setValue(1);
  93.     check(!alias.hasError());
  94.  
  95.     r.setError(std::make_exception_ptr(std::runtime_error("!")));
  96.     check(alias.hasError());
  97.     check(alias.error() != nullptr);
  98. }
  99.  
  100. int main()
  101. {
  102.     Result<int> r(0);
  103.     f(r, r);
  104. }
  105.  
Success #stdin #stdout 0s 3456KB
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https://ideone.com/GJaaYb
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
secret

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