模板类与友元函数， 重载运算符<<的问题

is02bing

链接提示有问题，请教

complex.h
----------------------------

1. 
   
2. #ifndef _COMPLEX_HEADER_
   
3. #define _COMPLEX_HEADER_
   
4. 
   
5. #include <iostream>
   
6. 
   
7. template<class Type>
   
8. class complex
   
9. {
   
10.         friend std::ostream& operator << (std::ostream& out, const complex<Type>& complex_num);
    
11. 
    
12.     private:
    
13.                 Type real;
    
14.                 Type image;
    
15.         public:
    
16.                 complex();
    
17.                 complex(complex& number);
    
18.                 complex(Type real, Type image);
    
19.                 void operator = (const complex& in_number);
    
20.                
    
21.         public:
    
22.                 complex& operator + (const complex& add) const;
    
23.                 Type get_real() const { return real; } ;
    
24.                 Type get_image() const { return image; } ;
    
25.                
    
26. };
    
27. 
    
28. template<class Type>
    
29. std::ostream&  operator << (std::ostream& os, const complex<Type>& complex_num);
    
30. 
    
31. template<class Type>
    
32. complex<Type>::complex()
    
33. {
    
34.         real = 0;
    
35.         image = 0;
    
36. }
    
37. 
    
38. template<class Type>
    
39. complex<Type>::complex(complex<Type>& number)
    
40. {
    
41.         real = number.real;
    
42.         image = number.image;
    
43. }
    
44. 
    
45. template<class Type>
    
46. complex<Type>::complex(Type in_real, Type in_image)
    
47. {
    
48.         real = in_real;
    
49.         image = in_image;
    
50. }
    
51. 
    
52. template<class Type>
    
53. complex<Type>& complex<Type>::operator + (const complex<Type>& add) const
    
54. {
    
55.         complex<Type> result;
    
56.         result.real = real + add.real;
    
57.         result.image = image + add.image;
    
58. 
    
59.         return result;
    
60. }
    
61. 
    
62. 
    
63. template<class Type>
    
64. void complex<Type>::operator = (const complex<Type>& in_number)
    
65. {
    
66.         real = in_number.real;
    
67.         image = in_number.image;
    
68. }
    
69. 
    
70. 
    
71. template<class Type>
    
72. std::ostream&  operator <<  ( std::ostream& os, const complex<Type>& complex_num)
    
73. {
    
74.         os<<complex_num.real;
    
75.         if (complex_num.image > 0) os<<'+';
    
76.         if (complex_num.image != 0) os<<complex_num.image<<"i";
    
77.         return os;
    
78. }
    
79. 
    
80. #endif //_COMPLEX_HEADER_
    

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complex.cpp
------------------------

1. 
   
2. #include <iostream>
   
3. #include "complex.h"
   
4. 
   
5. int main()
   
6. {
   
7.         complex<int> a(3,5);
   
8.         complex<int> b(7,8);
   
9.         complex<int> c;
   
10.        
    
11.         c = a + b;
    
12. 
    
13.         std::cout<<"c="<<c.get_real()<<"+"<<c.get_image()<<"i"<<std::endl;
    
14. 
    
15.         std::cout<<c;
    
16. 
    
17.         return 0;
    
18. }
    

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编译输出：
-----------------------------

1. 
   
2. waterzh@bing ~ $ g++ complex.cpp
   
3. complex.h:9: 警告：友元声明 ‘std::ostream& operator<<(std::ostream&, const complex<Type>&)’ 声明了一个非模板函数
   
4. complex.h:9: 警告：(如果这不是您原来的想法，请确定此函数模板已经声明过，并在这里的函数名后面添加 <>) -Wno-non-template-friend 禁用此警告
   
5. complex.h: In member function ‘complex<Type>& complex<Type>::operator+(const complex<Type>&) const [with Type = int]’:
   
6. complex.cpp:10:   instantiated from here
   
7. complex.h:54: 警告：返回了对局部变量的 ‘result’ 的引用
   
8. /tmp/ccQnC6Iu.o：在函数‘main’中：
   
9. complex.cpp:(.text+0x16c)：对‘operator<<(std::basic_ostream<char, std::char_traits<char> >&, complex<int> const&)’未定义的引用
   
10. collect2: ld 返回 1
    
11. 
    

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rickxbx

这是在做什么?

2. template<class Type>
3. std::ostream&  operator << (std::ostream& os, const complex<Type>& complex_num);

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is02bing

Post by rickxbx
这是在做什么?

1. 
   
2. template<class Type>
   
3. std::ostream&  operator << (std::ostream& os, const complex<Type>& complex_num);
   

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定义函数<<，有没有这两行结果都一样。
ld返回结果是连接类型没有匹配上，找不到

1. 
   
2. operator<<(std::basic_ostream<char, std::char_traits<char> >&, [b]complex<int>[/b] const&)
   

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请明示

pupilzeng

You should declare the friend template function before declaring it as a friend of the class, which leads to actually 3 steps for a friend template function declaration:

1. 
   
2. template <class Type> class complex;
   
3. template<class Type>
   
4. std::ostream&  operator << (std::ostream& os, const complex<Type>& complex_num);
   
5. 
   
6. template<class Type>
   
7. class complex
   
8. {
   
9.         friend std::ostream& operator << <>(std::ostream& out, const complex<Type>& complex_num);
   
10. };
    

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Note that there is a pair of angle brackets ('<>') after '<<'.
More information can be found in this link

is02bing

试过把<< 的重载声明放在类的实现前，编译提示<<函数中complex不是一个有效的类型，所以就把<<的声明放在类的实现后面， 但没有想到把complex的声明放在<< 前。

等错误转变成一个链接错误的时候，就错失方向了。一念之差。

谢谢。