大家帮我看看，使用模板时的一个函数未定义的问题

[bad__ba]

#include< iostream >
using namespace std;

template< class T >
class ostream_iterator
{
public:
	ostream_iterator( ostream & os, const char * s ):strm( & os ), str( s ){}
	ostream_iterator & operator ++()
	{
		return * this;
	}
	ostream_iterator & operator ++( int )
	{
		return * this;
	}
	ostream_iterator & operator *()
	{
		return * this;
	}
	ostream_iterator & operator = ( const T & t )
	{
		* strm << t << str;
		return * this;
	}
private:
	ostream * strm;
	const char * str;
};

template< class T >
class istream_iterator
{
	friend int operator == ( istream_iterator< T > & p, istream_iterator< T > & q );
public:
	istream_iterator( istream & is ) : strm( & is ), full( 0 ), eof( 0 ){}
	istream_iterator() : strm( 0 ), full( 0 ), eof( 1 ){}
	istream_iterator & operator ++()
	{
		full = 0;
		return * this;
	}
	istream_iterator operator ++( int )
	{
		istream_iterator r = * this;
		full = 0;
		return r;
	}
	T operator *()
	{
		fill();
		return buffer;
	}
private:
	T buffer;
	istream *strm;
	int full;
	int eof;
	void fill()
	{
		if( ! full && ! eof )
		{
			if( * strm >> buffer )
			{
				full = 1;
			}
			else
			{
				eof = 1;
			}
		}
	}
};

template< class T>
int operator == ( istream_iterator< T > & p, istream_iterator< T > & q )
{
	if( p.eof  && q.eof )
		return 1;
	if( ! p.eof && ! q.eof )
		return( & p == & q );
	p.fill();
	q.fill();
	return ( p.eof == q.eof );
}

template< class T >
int operator != ( istream_iterator< T > & p, istream_iterator< T > & q )
{
	return !( p == q );
}

template< class P, class T >
void Copy( P start, P end, T t )
{
	while( start != end )
	{
		* t ++ = * start ++;
	}
}

int main( int argc, char * argv[] )
{
	ostream_iterator< int > output ( cout, "\n" );
	istream_iterator< int > input( cin );
	istream_iterator< int > eof;

	Copy( input, eof, output );
}

1>------ Rebuild All started: Project: testcpp, Configuration: Debug Win32 ------
1>Deleting intermediate and output files for project 'testcpp', configuration 'Debug|Win32'
1>Compiling...
1>test.cpp
1>Linking...
1>test.obj : error LNK2019: unresolved external symbol "int __cdecl operator==(class istream_iterator<int> &,class istream_iterator<int> &)" (??8@YAHAAV?$istream_iterator@H@@0@Z) referenced in function "int __cdecl operator!=<int>(class istream_iterator<int> &,class istream_iterator<int> &)" (??$?9H@@YAHAAV?$istream_iterator@H@@0@Z)
1>C:\Documents and Settings\zhao123\My Documents\Visual Studio 2008\Projects\testcpp\Debug\testcpp.exe : fatal error LNK1120: 1 unresolved externals
1>Build log was saved at "file://c:\Documents and Settings\zhao123\My Documents\Visual Studio 2008\Projects\testcpp\testcpp\Debug\BuildLog.htm"
1>testcpp - 2 error(s), 0 warning(s)
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========
有两个问题想请教
第一个就是上面警告说的那里

	ostream_iterator & operator ++( int )
	{
		return * this;
	}

这里返回值类型应该是一个ostream_iterator啊，那函数定义时候的ostream_iterator& 怎么理解？
第二个问题就是
unresolved external symbol "int __cdecl operator==(class istream_iterator<int> &,class istream_iterator<int> &)" (??8@YAHAAV?$istream_iterator@H@@0@Z) referenced in function "int __cdecl operator!=<int>(class istream_iterator<int> &,class istream_iterator<int> &)" (??$?9H@@YAHAAV?$istream_iterator@H@@0@Z)
我上面已经定义了==号了，为什么链接的时候又说没有这个符号的定义呢？

[guolihui112]

[Quote=引用 3 楼 namelij 的回复:]
我来告诉你最基本的原因
因为c++中，有个关键字 ostream_iterator,你的重名了，所以可能出错
[/Quote]

楼上说的不对 ，楼主有没有包含 ostream_iterator 的头文件，这不是问题所在。

主要是模板不是宏，宏的定义里可以包含前面没有声明的函数或变量。
模板必须声明。

1. istream_iterator 里用到了 operaotr == 函数，这个函数前边必须声明。
2. 若将 operator === 函数声明放到 istream_iterator 定义前。因为 operator == 函数又用到了 istream_iterator 类，所以还要在 operator == 前添加 istream_iterator 的前向声明

所以楼主的代码改动有二：
1. 在 istream_iterator 前添加如下两个声明：

template <class T> class istream_iterator;

template< class T>
int operator == ( istream_iterator< T > & p, istream_iterator< T > & q );

2. 因为 operator == 是模板函数， 所以将 istream_iterator 的 friend 声明改为如下：

    friend int operator == <T> ( istream_iterator< T > & p, istream_iterator< T > & q );

附上 gcc 的链接错误，可以清楚的看到什么错误了（行号可能与楼主代码有出入)：

tpl.cpp:36: warning: friend declaration 'int operator==(istream_iterator<T>&,
istream_iterator<T>&)' declares a non-template function

tpl.cpp:36: note: (if this is not what you intended, make sure the function temp
late has already been declared and add <> after the function name here)

C:\DOCUME~1\ADMINI~1\LOCALS~1\Temp\ccjFsnr7.o:tpl.cpp:(.text$_ZneIiEiR19istre
am_iteratorIT_ES3_[int operator!=<int>(istream_iterator<int>&, istream_ite
rator<int>&)]+0x14): undefined reference to `operator==(istream_iterator<int>
&, istream_iterator<int>&)'
collect2: ld returned 1 exit status

看第一行，

[珍惜生命远离CPP]

将operator==的实现放到 class里面你写friend的地方吧。
模板的友元函数不是这么写的

[珍惜生命远离CPP]

#include< iostream >
using namespace std;

template< class T >
class ostream_iterator
{
public:
    ostream_iterator( ostream & os, const char * s ): strm( & os ), str( s ) {}
    ostream_iterator & operator ++()
    {
        return * this;
    }
    ostream_iterator & operator ++( int )
    {
        return * this;
    }
    ostream_iterator & operator *()
    {
        return * this;
    }
    ostream_iterator & operator = ( const T & t )
    {
        * strm << t << str;
        return * this;
    }
private:
    ostream * strm;
    const char * str;
};

template< class T >
class istream_iterator
{
    friend int operator == <>( istream_iterator< T > & p, istream_iterator< T > & q );
public:
    istream_iterator( istream & is ) : strm( & is ), full( 0 ), eof( 0 ) {}
    istream_iterator() : strm( 0 ), full( 0 ), eof( 1 ) {}
    istream_iterator & operator ++()
    {
        full = 0;
        return * this;
    }
    istream_iterator operator ++( int )
    {
        istream_iterator r = * this;
        full = 0;
        return r;
    }
    T operator *()
    {
        fill();
        return buffer;
    }
private:
    T buffer;
    istream *strm;
    int full;
    int eof;
    void fill()
    {
        if ( ! full && ! eof )
        {
            if ( * strm >> buffer )
            {
                full = 1;
            }
            else
            {
                eof = 1;
            }
        }
    }
};

template< class T>
int operator == ( istream_iterator< T > & p, istream_iterator< T > & q )
{
    if ( p.eof  && q.eof )
        return 1;

    if ( ! p.eof && ! q.eof )
        return( & p == & q );

    p.fill();
    q.fill();
    return ( p.eof == q.eof );
}

template< class T >
int operator != ( istream_iterator< T > & p, istream_iterator< T > & q )
{
    return !( p == q );
}

template< class P, class T >
void Copy( P start, P end, T t )
{
    while ( start != end )
    {
        *( t++ ) = *( start++ );
    }
}

int main( int argc, char * argv[] )
{
    ostream_iterator< int > output ( cout, "\n" );
    istream_iterator< int > input( cin );
    istream_iterator< int > eof;

    Copy( input, eof, output );
}

[高性能架构探索]

我来告诉你最基本的原因
因为c++中，有个关键字 ostream_iterator,你的重名了，所以可能出错

[bad__ba]

我试了一下，应该是使用模板时，友元函数定义的形式不对，不应是这样：

friend int operator == ( istream_iterator< T > & p, istream_iterator< T > & q );

而应该在函数名后面加<>，像下面这样：

friend int operator == < T > ( istream_iterator< T > & p, istream_iterator< T > & q );

[bad__ba]

是声名