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  1. #include <iostream>
  2. #include <string>
  3. #include <memory>
  4. #include <utility>
  5. #include <cassert>
  6.  
  7. #define GENERATE_META(type)                 \
  8.     public:                                 \
  9.     static const MetaInfo& StaticMeta()     \
  10.     {                                       \
  11.         static MetaInfo meta{#type}; \
  12.         return meta;                        \
  13.     }
  14.  
  15. struct MetaInfo
  16. {
  17.     const std::string _id;
  18.  
  19.     explicit MetaInfo( std::string ti )
  20.         : _id{std::move( ti )}{}
  21.  
  22.     bool operator==(const MetaInfo& rhs) const
  23.     {
  24.         return _id == rhs._id;
  25.     }
  26.  
  27.     bool operator!=(const MetaInfo& rhs) const
  28.     {
  29.         return !operator==(rhs);
  30.     }
  31. };
  32.  
  33. // Inheritance is just for example
  34. // In real project code is generated by tools
  35. struct Base
  36. {
  37. protected:
  38.     Base() = default;
  39. };
  40.  
  41. struct MyType1 : Base
  42. {
  43.     GENERATE_META(MyType1)
  44.     int _dummy = 1;
  45. };
  46.  
  47. struct MyType2 : Base
  48. {
  49.     GENERATE_META(MyType2)
  50.     float _dummy = 2.f;
  51. };
  52.  
  53. struct MyType3 : Base
  54. {
  55.     GENERATE_META(MyType3)
  56.     std::string _dummy = "gg";
  57. };
  58.  
  59.  
  60. class Table
  61. {
  62. public:
  63.     template< typename T >
  64.     static Table Create()
  65.     {
  66.         auto data = std::make_shared<T>();
  67.         auto meta = &T::StaticMeta();
  68.         return { data, meta };
  69.     }
  70.  
  71.     const std::shared_ptr<Base>& Data() const{ return _data; }
  72.     const MetaInfo& Meta() const{ return *_meta; }
  73.  
  74. private:
  75.     Table( std::shared_ptr<Base> data, const MetaInfo * meta )
  76.         : _data{std::move(data)}
  77.         , _meta{meta}
  78.     {}
  79.  
  80. private:
  81.     std::shared_ptr<Base> _data;
  82.     const MetaInfo * const _meta = nullptr;
  83. };
  84.  
  85. template< typename T >
  86. void Print( const Table& table )
  87. {
  88.     assert( table.Meta() == T::StaticMeta() );
  89.     const auto& data = *static_cast<T*>(table.Data().get());
  90.     std::cout << table.Meta()._id << " -> " << data._dummy << "\n";
  91. }
  92.  
  93. /////////////////////////////////////////////////////////////////////
  94.  
  95. template< typename T >
  96. bool CheckAndPrint( const Table& table )
  97. {
  98.     if ( table.Meta() == T::StaticMeta() )
  99.     {
  100.         Print<T>( table );
  101.         return true;
  102.     }
  103.  
  104.     return false;
  105. }
  106.  
  107. template< typename T, typename ...Tables >
  108. bool Validate( const Table& table )
  109. {
  110.     if ( CheckAndPrint<T>( table ) )
  111.         return true;
  112.  
  113.     return Validate<Tables...>(table);
  114. }
  115.  
  116.  
  117. int main()
  118. {
  119.     const auto t1 = Table::Create<MyType1>();
  120.     const auto t2 = Table::Create<MyType2>();
  121.     const auto t3 = Table::Create<MyType3>();
  122.  
  123.     const auto r1 = Validate<MyType1, MyType2>(t1);
  124.     assert( r1 );
  125.     const auto r2 = Validate<MyType1, MyType2>(t2);
  126.     assert( r2 );
  127.     const auto r3 = Validate<MyType1, MyType2>(t3);
  128.     assert( !r3 );
  129.  
  130.     return 0;
  131. }
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In instantiation of ‘bool Validate(const Table&) [with T = MyType2; Tables = {}]’:
prog.cpp:113:31:   required from ‘bool Validate(const Table&) [with T = MyType1; Tables = {MyType2}]’
prog.cpp:123:50:   required from here
prog.cpp:113:31: error: no matching function for call to ‘Validate(const Table&)’
     return Validate<Tables...>(table);
            ~~~~~~~~~~~~~~~~~~~^~~~~~~
prog.cpp:108:6: note: candidate: template<class T, class ... Tables> bool Validate(const Table&)
 bool Validate( const Table& table )
      ^~~~~~~~
prog.cpp:108:6: note:   template argument deduction/substitution failed:
prog.cpp:113:31: note:   couldn't deduce template parameter ‘T’
     return Validate<Tables...>(table);
            ~~~~~~~~~~~~~~~~~~~^~~~~~~
stdout
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Standard output is empty
https://ideone.com/24vOZu
language:
C++14 (gcc 8.3)
created:
6 years ago
visibility:
secret

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