Error compiling Function templates with C++ [duplicate]

Question

Possible Duplicate:
Why can templates only be implemented in the header file?

I'm getting some errors in gcc and clang that I'm not familiar with. First off, here's the code in question.

WaypointTree.h:

class WaypointTree {

 private:
  Waypoint *head = nullptr;
  Waypoint *current = nullptr;
  template<class Function>
    void each_recur(Waypoint *current, Function f);

 public:
  template<class Function>
    void foreach(Function f);
  void add(Waypoint *wp, Waypoint *parent = nullptr);
  bool isLast(Waypoint *wp);
  Waypoint *next();
  Waypoint *getHead() { return head; }
  void reverse() {}
};

WaypointTree.cpp:

. . .

template<class Function>
void WaypointTree::foreach(Function f) {
  each_recur(head, f);
}

. . .

Flock.cpp:

. . .

_waypoints->foreach([] (Waypoint *wp) {
        glPushMatrix(); {
          glTranslatef(wp->getX(), wp->getY(), wp->getZ());
          glutSolidSphere(0.02, 5, 5);
        } glPopMatrix();
      });

. . .

With clang, I get the following warning:

lib/WaypointTree.h:17:7: warning: function 'WaypointTree::foreach<<lambda at lib/Flock.cpp:128:22> >' has internal linkage but is not defined
        void foreach(Function f);
             ^
lib/Flock.cpp:128:14: note: used here
        _waypoints->foreach([] (Waypoint *wp) {
                    ^

Followed by the following linker error:

lib/Flock.cpp:128: error: undefined reference to 'void WaypointTree::foreach<Flock::render(std::vector<Boid*, std::allocator<Boid*> >)::$_3>(Flock::render(std::vector<Boid*, std::all
ocator<Boid*> >)::$_3)'

With gcc I get only the following error :

lib/WaypointTree.h:17:7: error: ‘void WaypointTree::foreach(Function) [with Function = Flock::render(std::vector<Boid*>)::<lambda(Waypoint*)>]’, declared using local type ‘Flock::ren
der(std::vector<Boid*>)::<lambda(Waypoint*)>’, is used but never defined [-fpermissive]

I'm not really sure where to begin debugging this, but it seems to be related to the templates and the lambdas.

Answer 1

You have to implement your template functions in the .h file, not in the .cpp

Here's some further reading