C++的std :: stringstream的运营商<<重载

Question

我有下面的类（prototipe）：

class Token 
{ 
public: 
    //members, etc. 
    friend std::stringstream& operator<< (std::stringstream &out, Token &t); 
}; 

且操作者实施这样的：

std::stringstream & operator<< (std::stringstream &out, Token &t) 
{ 
    out << t.getValue(); //class public method 
    return out; 
} 

现在，我想使用这样的：

std::stringstream out; 
Token t; 
//initialization, etc. 

out << t; 

而且VS给了我的错误，说是有敌不过< <运营商。我在错什么？

Answer 1

std::stringstream & operator<< (std::stringstream &out, Token &t) 

应该

std::ostream & operator<< (std::ostream &out, Token const &t)