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#include <iostream>
#include <functional>
 
class EG
{
   template<class T>
   std::function<void(T)> f;
 
   template<class T>
   void func(std::function<void(T)>&& arg)
   {
      if(f != nullptr)
      {
         f();
         std::cout << "f != nullptr" << std::endl;
      }
      f.swap(arg);
   }
 
};
 
int main(void)
{
EG eg;
eg.func([](std::string s){});
eg.func([](int i){});
 
return 0;

prog.cpp:7:28: error: data member ‘f’ cannot be a member template
prog.cpp: In member function ‘void EG::func(std::function<void(T)>&&)’:
prog.cpp:12:10: error: ‘f’ was not declared in this scope
prog.cpp:17:7: error: ‘f’ was not declared in this scope
prog.cpp: In function ‘int main()’:
prog.cpp:25:28: error: no matching function for call to ‘EG::func(main()::<lambda(std::string)>)’
prog.cpp:25:28: note: candidate is:
prog.cpp:10:9: note: template<class T> void EG::func(std::function<void(T)>&&)
prog.cpp:10:9: note:   template argument deduction/substitution failed:
prog.cpp:25:28: note:   ‘main()::<lambda(std::string)>’ is not derived from ‘std::function<void(T)>’
prog.cpp:26:20: error: no matching function for call to ‘EG::func(main()::<lambda(int)>)’
prog.cpp:26:20: note: candidate is:
prog.cpp:10:9: note: template<class T> void EG::func(std::function<void(T)>&&)
prog.cpp:10:9: note:   template argument deduction/substitution failed:
prog.cpp:26:20: note:   ‘main()::<lambda(int)>’ is not derived from ‘std::function<void(T)>’
prog.cpp:28:9: error: expected ‘}’ at end of input

Standard output is empty