c++ - How to defined constructor outside of template class -

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• why can templates implemented in header file? 13 answers

i linker error if define constructor\destructor of template class outside class. not allowed? use visual studio 2010.

error 1>main.obj : error lnk2019: unresolved external symbol "public: __thiscall tree::~tree(void)" (??1?$tree@h@@qae@xz) referenced in function _main

1>main.obj : error lnk2019: unresolved external symbol "public: __thiscall tree::tree(void)" (??0?$tree@h@@qae@xz) referenced in function _main

in .h file

template <class t> class tree{ public:     tree(void);     ~tree(void);     t x; }; 

in .cpp file

#include "tree.h"  template <class t> tree<t>::tree(void){ }  template <class t> tree<t>::~tree(void){ } 

in main.cpp file

#include "tree.h" int main(){     tree<int> t;     return 0; } 

templates need declared , implemented in file include. cannot separate template class declaration , implementation , include header file.

with templates, class not compiled until it's used. there no such thing compiled template class can linked against. each time use template, has compiled different type. , since compiler not have access implementation, not know how compile it...