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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. enum class kTypeFlag { PARENTONE, PARENTTWO, CHILD };
  5.  
  6. class ParentOne {
  7. public:
  8.     ParentOne() { }
  9.     virtual ~ParentOne() { }
  10.     virtual size_t byte_size() const = 0;
  11. };
  12.  
  13. class ParentTwo {
  14. public:
  15.     ParentTwo(uint32_t id) : id_(id) { }
  16.     virtual ~ParentTwo() { }
  17.     uint32_t id() const { return id_; }
  18. private:
  19.     const uint32_t id_;
  20. };
  21.  
  22. class Child : public ParentOne, public ParentTwo {
  23. public:
  24.     Child(uint32_t id) : ParentOne(), ParentTwo(id) { }
  25.     virtual ~Child() { }
  26.     size_t byte_size() const override { return sizeof(*this); }
  27. };
  28.  
  29. namespace details
  30. {
  31.     template<typename To, kTypeFlag>
  32.     struct cex {To* operator()() {
  33.     	static_assert(!std::is_same<To, To>::value, "invalid flag to Convert");} };
  34.  
  35.     template<typename To>
  36.     struct cex<To, kTypeFlag::PARENTONE> { To* operator()(void*p) {
  37.     	return static_cast<To*>(static_cast<ParentOne*>(p)); } };
  38.  
  39.     template<typename To>
  40.     struct cex<To, kTypeFlag::PARENTTWO> { To* operator()(void*p) {
  41.     	return static_cast<To*>(static_cast<ParentTwo*>(p)); } };
  42.  
  43.     template<typename To>
  44.     struct cex<To, kTypeFlag::CHILD>     { To* operator()(void*p) {
  45.     	return static_cast<To*>(static_cast<Child*>(p)); } };
  46. };
  47.  
  48. template <typename T, kTypeFlag Flag>
  49. T* ConvertEx (void* pointer)
  50. {
  51. 	return details::cex<T, Flag>()(pointer);
  52. }
  53.  
  54. int main()
  55. {
  56.     Child* child = new Child(5);
  57.     printf("byte_size: %zu\n", child->byte_size());
  58.     printf("id: %u\n", child->id());
  59.  
  60.     ParentOne* p1 = ConvertEx<ParentOne, kTypeFlag::CHILD>(child);
  61.     ParentTwo* p2 = ConvertEx<ParentTwo, kTypeFlag::CHILD>(child);
  62.     printf("byte_size: %zu\n", p1->byte_size());
  63.     printf("id: %u\n", p2->id());
  64.  
  65.  
  66. 	return 0;
  67. }
Success #stdin #stdout 0s 3460KB
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Standard input is empty
stdout
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byte_size: 12
id: 5
byte_size: 12
id: 5
https://ideone.com/24iaNv
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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