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  1.     #include <string>
  2.     #include <utility>
  3.     #include <functional>
  4.     #include <iostream>
  5.  
  6.     namespace detail
  7.     {
  8.         template<typename T, typename U, typename = void>
  9.         struct maybe_call_init
  10.         {
  11.             static void maybe_call(U* obj) { }
  12.         };
  13.  
  14.         template<typename T, typename U>
  15.         struct maybe_call_init<T, U,
  16.             decltype(T::Init(std::declval<U*>()), void(0))>
  17.         {
  18.             static void maybe_call(U* obj) { T::Init(obj); }
  19.         };
  20.     }
  21.  
  22.     template<template<typename> class T, typename U>
  23.     void maybe_call_init(T<U>* obj)
  24.     {
  25.          detail::maybe_call_init<U, T<U>>::maybe_call(obj);
  26.     }
  27.  
  28.     template<typename Traits>
  29.     class Test
  30.     {
  31.         public:
  32.             Test()
  33.             {
  34.                 maybe_call_init(this);
  35.             }
  36.  
  37.         public:
  38.             typename Traits::Type value;
  39.  
  40.         friend Traits;
  41.     };
  42.  
  43.     struct TestTraits
  44.     {
  45.         typedef std::string Type;
  46.     };
  47.  
  48.     struct TestTraitsInit
  49.     {
  50.         typedef int Type;
  51.  
  52.         static void Init(Test<TestTraitsInit>* obj)
  53.         {
  54.             obj->value = 42;
  55.         }
  56.     };
  57.  
  58.     int main()
  59.     {
  60.         Test<TestTraits> obj1;
  61.         std::cout << "Value of obj1: " << obj1.value << std::endl;
  62.         Test<TestTraitsInit> obj2;
  63.         std::cout << "Value of obj2: " << obj2.value << std::endl;
  64.     }
  65.  
Success #stdin #stdout 0s 2896KB
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stdin
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Standard input is empty
stdout
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Value of obj1: 
Value of obj2: 42
https://ideone.com/ULTiC2
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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