Question

Consider the following code:

template <int dim> struct vec {     vec normalize(); };   template <> struct vec<3> {     vec cross_product(const vec& second);     vec normalize(); };  template <int dim> vec<dim> vec<dim>::normalize() {     // code to normalize vector here     return *this; }  int main() {     vec<3> direction;     direction.normalize(); } 

Compiling this code produces the following error:

1>main.obj : error LNK2019: unresolved external symbol ‘public: struct vec<3> __thiscall vec<3>::normalize(void)’ (?normalize@?$vec@$02@@QAE?AU1@XZ) referenced in function _main

Answer 1

You can’t 🙂 What you want is to specialize the member functions instead:

template <int dim> struct vec {     // leave the function undefined for everything except dim==3     vec cross_product(const vec& second);     vec normalize(); };  template<> vec<3> vec<3>::cross_product(const vec& second) {     // ... }  template <int dim> vec<dim> vec<dim>::normalize() {     // code to normalize vector here     return *this; } 

Another, slightly more complicated solution is to use boost::enable_if:

template <int dim> struct vec {     // function can't be called for dim != 3. Error at compile-time     template<int dim1>     typename boost::enable_if_c< dim == dim1 && dim1 == 3, vec<dim1> >::type      cross_product(const vec<dim1>& second) {         // ...     }     vec normalize();      // delegate to the template version     void without_params() {         // delegate         this->without_params<dim>();     }  private:     // function can't be called for dim != 3. Error at compile-time     template<int dim1>     typename boost::enable_if_c< dim == dim1 && dim1 == 3 >::type      without_params() {         // ...     }    };  template <int dim> vec<dim> vec<dim>::normalize() {     // code to normalize vector here     return *this; } 

That will cause a compile time error if cross_product is called for any dim != 3. Note that that ‘trick’ only works for functions with parameters, since only then the template parameter can be auto-deduced. For cases without parameters, i have provided a function without_parameters above :).