Question

可能重复：
Why can templates only be implemented in the header file?

我在gcc和clang中遇到一些我不熟悉的错误。首先，这是有问题的代码。

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();
      });

。。

有了clang，我收到以下警告：

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) {
                    ^

后面是以下链接器错误：

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)'

使用gcc我只得到以下错误：

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]

我不确定从哪里开始调试，但它似乎与模板和lambdas有关。

Answer 1

您必须在.h文件中实现模板函数，而不是在.cpp

中

Here's some further reading

Answer 2

编译器需要WaypointTree.cpp中的定义才能生成_waypoints->foreach([] (Waypoint *wp)的代码。因此，您必须同时包含cpp或在头文件中定义foreach。

使用C ++编译函数模板时出错

2 个答案: