c++ - Singleton Design Pattern - Explicitly stating a Constructor outside the class -

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i trying implement singleton pattern assumption of using private constructor, private instance of class , public static method return instance. encountered error following code in visual studio

// singleton invoice #include <iostream> using namespace std;  class singleton { public:   //public static method return type of class access instance.   static singleton* getinstance(); private:   //private constructor   singleton();   //private static instance of class   static singleton* objsingleton; };  singleton* singleton::objsingleton = null;  singleton* singleton::getinstance() {   if (objsingleton == null) {     //lazy instantiation: if instance not needed never created     objsingleton = new singleton();     cout << "object created" << endl;   }   else   {     cout << "object created" << endl;   }   return objsingleton; }  int main() {   singleton::getinstance();   singleton::getinstance();   singleton::getinstance();   return 0; }

the error :

lnk2019 unresolved external symbol "private: __thiscall singleton::singleton(void)" (??0singleton@@aae@xz) referenced in function "public: static class singleton * __cdecl singleton::getinstance(void)" (?getinstance@singleton@@sapav1@xz)

then resolved error rewriting constructor outside class

singleton::singleton() { }

i know cause error , why constructor needs explicitly written outside class.

in class constructor declared, not defined. definition includes function body. doesn't matter whether define inline in class, or outside class (as did), 1 little difference definition in class it's implicitly inline.

in other news:

  • singletons improve on global variables avoiding e.g. static initialization order fiasco, have same problems respect invisible lines of communication , side effects. best avoided.

  • if don't need singleton persist after corresponding global variable destroyed, use simple meyers' singleton.

here's meyers' singleton:

class foo { private:     foo() {} public:     static auto instance()         -> foo&     {         static foo the_instance;         return the_instance;     } };

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