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  1. #include <iostream>
  2. #include <list>
  3.  
  4. namespace NS1 
  5. {
  6.     template <class T>
  7.         class Base
  8.         {   
  9.         protected:
  10.             Base()
  11.             {   
  12.                 instances.push_back(static_cast<T*>(this));
  13.             }   
  14.         private:
  15.             static std::list<T*> instances;
  16.         };  
  17.  
  18.     namespace NS1_1
  19.     {   
  20.         template <class U>
  21.             class Intermediary : public Base<U>
  22.         {   
  23.         protected:
  24.             Intermediary()
  25.                 :Base<U>()
  26.             {   
  27.             }   
  28.         };  
  29.  
  30.         class Derived : public Intermediary<Derived>
  31.         {   
  32.         public:
  33.             Derived()
  34.                 :Intermediary<Derived>()
  35.             {   
  36.             }   
  37.         };  
  38.     }   
  39. }
  40.  
  41. template <> std::list<NS1::NS1_1::Derived*> NS1::Base<NS1::NS1_1::Derived>::instances;
  42.  
  43. int main()
  44. {
  45.     NS1::NS1_1::Derived d;
  46.     return 0;
  47. }
  48.  
Compilation error #stdin compilation error #stdout 0s 0KB
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stdin
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Standard input is empty
compilation info 
/home/peCbZi/cc4wDaLr.o: In function `main':
prog.cpp:(.text+0x5c): undefined reference to `NS1::Base<NS1::NS1_1::Derived>::instances'
collect2: ld returned 1 exit status
stdout
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Standard output is empty
https://ideone.com/U7cAf
language:
C++ (gcc 8.3)
created:
14 years ago
visibility:
public

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