模板和重载操作符<< [英] Templates and overloadering operator <<

问题描述

我尝试重载<<运算符，但编译器给我一个链接器错误。目标是能够将解除引用的基类指针发送到std :: cout，使得导出的运算符<

这可能吗？

  class IBase 
 {
 public：
 IBase（）{}; 
 virtual〜IBase（）{}; 
}; 
 
 template< typename T> 
 class Derived 
：public IBase 
 {
 public：
 Derived（T data）; 
 friend std :: ostream& operator<<（std :: ostream& os，const Derived< T>& dt）; 
 private：
 T data_; 
}; 
 
 template< typename T> 
 Derived< T> :: Derived（T data）
：IBase（），
 data_（data）
 {
} 
 
 template< typename T> 
 std :: ostream& operator<<<（std :: ostream& os，const Derived< T& dt）
 {
 os< dt.data_; 
 return os; 
} 
 
 
 int _tmain（int argc，_TCHAR * argv []）
 {
 
 
 //问题1 
 Derived< int> der（234）; 
 std :: cout<< der; 
 
 //问题2 
 // IBase * base = new Derived< int>（5）; 
 // std :: cout<< * base 
} 
  

这里有错误：

> >

&，class Derived const&）（?? 6 @ YAAAV？$ basic_ostream @ DU？$ char_traits @ D @ std @@@ std @@ AAV01 @ ABV？$ Derived @ H @@@ Z）

p>

致命错误LNK1120：1未解析的外部

 模板< typename T1> 
 friend std :: ostream& operator<<（std :: ostream& os，const Derived< T1>& dt）; 
  

I'm trying to overload the << operator for a class template, but the compiler gives me a linker error. The goal is to be able to send a de-referenced base class pointer to std::cout so that the derived operator<< gets called.

Is this possible?

class IBase
{
public:
    IBase() {};
    virtual ~IBase() {};
};

template <typename T>
class Derived
    : public IBase
{
public:
    Derived(T data);
    friend std::ostream& operator<<(std::ostream& os, const Derived<T>& dt);
private:
    T data_;
};

template <typename T>
Derived<T>::Derived(T data)
    : IBase(),
      data_(data)
{
}

template <typename T>
std::ostream& operator<<(std::ostream& os, const Derived<T>& dt)
{
    os << dt.data_;
    return os;
}


int _tmain(int argc, _TCHAR* argv[])
{


    // Question 1
    Derived<int> der(234);
    std::cout << der;

    // Question 2
    //IBase* base = new Derived<int>(5);
    // std::cout << *base
}

Here are the errors:

error LNK2001: unresolved external symbol "class std::basic_ostream > & __cdecl operator<<(class std::basic_ostream

&,class Derived const &)" (??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$Derived@H@@@Z)

and

fatal error LNK1120: 1 unresolved externals

解决方案

You need to declarate friend operator as a template too

template<typename T1>
friend std::ostream& operator<<(std::ostream& os, const Derived<T1>& dt);

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