在模板外部重载模板类的输出流运算符

我想重载输出流运算符<<在模板类定义之外。

在模板类中实现它就可以了：

template
<typename T,int _MaxSize=10,template <class C> class Policy=NoCheck,typename Container=std::vector<T>>
class MyContainer : public Policy<T>
{
public:
  MyContainer():p(_MaxSize){};
  std::ostream& operator<<(MyContainer<T,_MaxSize,Policy,Container>& obj){ };
private:
  Container p;
};

但是当我尝试在模板类之外执行此操作时：

template
<typename T,int _MaxSize=10,template <class C> class Policy=NoCheck,typename Container=std::vector<T>>
class MyContainer : public Policy<T>
{
public:
  MyContainer():p(_MaxSize){};
  friend std::ostream& operator<<(std::ostream& out,MyContainer<T,_MaxSize,Policy,Container> obj);
private:
  Container p;
};

template
<typename T,int _MaxSize,template <class C> class Policy,typename Container>
std::ostream& operator<<(std::ostream& out,MyContainer<T,_MaxSize,Policy,Container> obj)
{
};

编译器抱怨：

warning: friend declaration ‘std::ostream& operator<<(std::ostream&, MyContainer<T, _MaxSize, Policy, Container>)’ declares a non-template function [-Wnon-template-friend]
tempstruct.cc:39:97: note: (if this is not what you intended, make sure the function template has already been declared and add <> after the function name here)

任何人都可以举一个简单的例子来说明输出流运算符如何<<在模板类之外定义？

在我在这里找到的相关帖子中，每个人都在模板类中执行此操作。

任何人都可以举一个简单的例子来说明如何在模板类之外定义输出流运算符

不，因为这并不简单。我可以举一个复杂的例子：

// Declare the class template, because we need it to declare the operator
template <typename,int,template <class C> class,typename> class MyContainer;

// Declare the operator, because (as the error says) templates must be declared
// in the namespace before you can declare them friends. At this point, we can't
// define the operator, since the class template is incomplete.
template 
<typename T,int _MaxSize,template <class C> class Policy,typename Container>
std::ostream& operator<<(std::ostream&,MyContainer<T,_MaxSize,Policy,Container>);

// Define the class template
template
<typename T,int _MaxSize=10,template <class C> class Policy=NoCheck,typename Container=std::vector<T>>
class MyContainer : public Policy<T>
{
public:
  MyContainer():p(_MaxSize){};

  // Include <> to indicate that this is the template
  friend std::ostream& operator<< <>(std::ostream& out,MyContainer<T,_MaxSize,Policy,Container> obj);
private:
  Container p;
};

// Finally define the operator
template
<typename T,int _MaxSize,template <class C> class Policy,typename Container>
std::ostream& operator<<(std::ostream& out,MyContainer<T,_MaxSize,Policy,Container> obj)
{
  // print some stuff
};

在我在这里找到的相关帖子中，每个人都在模板类中执行此操作。

我会那么做；这会简单得多。或者，更好的是，我会根据公共接口实现输出，假设它提供了对容器内容的足够访问权限。那么您就不需要朋友声明，因此也不需要前向声明。