Invalid use of incomplete type for partial template specialization c++ -

i trying specialize class method foo(). works full template specialization. however, not work partial template specialization.

here example code compiles fine on gcc , clang :

#include <iostream> #include <string>  template <typename key, typename value> struct simplekey {     key   key;     value value;     void foo() const {          std::cout << "base" << std::endl;      } };  /* // uncomment , won't work ! template<typename key> void simplekey<key, std::string>::foo() const {     std::cout << "partial" << std::endl;  } */  template<> void simplekey<int, std::string>::foo() const {     std::cout << "full" << std::endl;  }   int main() {     simplekey<double, std::string> key1{1.0,"key1"};     key1.foo();     simplekey<int, std::string> key2{1,"key2"};     key2.foo(); } 

the error on clang , gcc when uncommenting relevant code :

error: invalid use of incomplete type ‘struct simplekey >’ void simplekey::foo() const {

what should partial template specialization work "minimal" efforts ?

you can explicitly specialize member function of particular implicit instantiation of class template. not allowed partial specializations. if don't want write full partial specialization, can consider using tag dispatch:

private: void foo(std::true_type /*value_is_string*/) const { /* "partial" */ } void foo(std::false_type /*value_is_string*/) const { /* "base" */ }  public: void foo() const { return foo(std::is_same<value, std::string>()); } 

or refactoring foo() base class template partially specialize.