比较对象对 [英] Comparing pairs of objects

问题描述

这个简单的代码使用VC ++ 10进行编译和运行：

  #include     stdafx.h 
   #include   <   string  >  
   #include   <   iostream  >  
  
 使用 命名空间标准; 
 
  class  Foo 
 {
  public ：
 Foo（）; 
 Foo（ int  i，string s）{data = make_pair（i，s）; }; 
  bool   const   operator  = =（ const  Foo& f）{ return  f.data == data; }; 
  private ：
 pair< int，string>数据; 
  int  stuff; 
}; 
 
  int  _tmain（ int  argc，_TCHAR * argv []）
 {
 Foo f1（ 10 ，  XYZ）; 
 Foo f2（ 10 ，  xyz ）; 
 cout<< （（f1 == f2）？  equal：  不等于）<< ENDL; 
  return   0 ; 
} 

但是，如果我改变了Foo类，那么这对现在包括我的两个类：

  #include     stdafx.h 
   #include   < span class =code-keyword><   string  >  
   #include   <   iostream  >  
  
 使用 命名空间标准; 
 
  class  Bar 
 {
  public ：
 Bar（）; 
 Bar（字符串，字符串）; 
  bool   const   operator  = =（ const  Bar& b）{ return  b.s1 == s1&& b.s2 == s2; }; 
  private ：
 string s1; 
 string s2; 
  int  k; 
}; 
 
  class  Foo 
 {
  public ：
 Foo（）; 
 Foo（Bar b1，Bar b2）{data = make_pair（b1，b2）; }; 
  bool   const   operator  = =（ const  Foo& f）{ return  f.data == data; }; 
  private ：
 pair< Bar，Bar>数据; 
  int  stuff; 
}; 
 
  int  _tmain（ int  argc，_TCHAR * argv []）
 {
 Bar b1（  abc，  xyz）; 
 Bar b2（  abc，   xyz）; 
 Foo f1（b1，b2）; 
 Foo f2（b1，b2）; 
 cout<< （（f1 == f2）？  equal：  不等于）<< ENDL; 
  return   0 ; 
} 

它不会编译而且我得到：

 1> ---- -  Build build：Project：pair_test，Configuration：Debug Win32 ------ 
 1> pair_test.cpp 
 1> c：\program files\microsoft visual studio 10.0 \vc\include\utility（305）：错误C2678：二进制'=='：找不到左边的操作符'const Bar'类型的手操作数（或没有可接受的转换）
 1> c：\program files\microsoft visual studio 10.0 \vc\include\exception（470）：可能是'bool std :: operator ==（const std :: _ Exception_ptr&，const std :: _ Exception_ptr& ;）'
 1> c：\program files\microsoft visual studio 10.0 \vc\include\exception（475）：或'bool std :: operator ==（std :: _ Null_type，const std :: _ Exception_ptr&）'
 1> c：\program files\microsoft visual studio 10.0 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ b $ b 1> c：\program files\microsoft visual studio 10.0 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ ）'
 1> c：\program files\microsoft visual studio 10.0 \vc\include\system_error（416）：或'bool std :: operator ==（const std :: error_condition&，const std :: error_code& ）'
 1> c：\documents and settings \charles \ my documents \visual studio 2010 \projects\pair_test\pair_test.cpp（12）：或'const bool Bar :: operator ==（const Bar&） '
 1>在尝试匹配参数列表'（const Bar，const Bar）'
 1> c：\documents and settings \charles \ my documents \visual studio 2010 \projects\pair_test\pair_test.cpp（24）：参见函数模板实例化'bool std :: operator ==< Bar，Bar>（const std :: pair< _Ty1，_Ty2>&，const std :: pair< _Ty1，_Ty2>&）'正在编译
 1> 
 1> [
 1> _Ty1 = Bar，
 1> _Ty2 = Bar 
 1> ] 
 1> c：\program files \ microsoft visual studio 10.0 \vc\include\utility（305）：错误C2678：二进制'=='：找不到左侧的操作符'const Bar'类型的操作数（或没有可接受的转换）
 1> c：\program files\microsoft visual studio 10.0 \vc\include\exception（470）：可能是'bool std :: operator ==（const std :: _ Exception_ptr&，const std :: _ Exception_ptr& ;）'
 1> c：\program files\microsoft visual studio 10.0 \vc\include\exception（475）：或'bool std :: operator ==（std :: _ Null_type，const std :: _ Exception_ptr&）'
 1> c：\program files\microsoft visual studio 10.0 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ b $ b 1> c：\program files\microsoft visual studio 10.0 \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ ）'
 1> c：\program files\microsoft visual studio 10.0 \vc\include\system_error（416）：或'bool std :: operator ==（const std :: error_condition&，const std :: error_code& ）'
 1> c：\documents and settings \charles \ my documents \visual studio 2010 \projects\pair_test\pair_test.cpp（12）：或'const bool Bar :: operator ==（const Bar&） '
 1>尝试匹配参数列表'（const Bar，const Bar）'

看起来我在这里遗漏了一些明显的东西，但搜索并没有多大帮助。我是否必须为pair< bar，>？明确定义==运算符？如果它知道如何比较Bar对象，那么泛型==运算符应该正常工作，我认为。

解决方案

你的比较运算符没有定义为const，而是Foo比较operator提供const参数。因此找不到匹配项。试试这个：

  class  Bar 
 {
  public ：
 ... 
  bool   const   operator  ==（ const  Bar& b） const  {返回 b.s1 == s1&& b.s2 == s2; 
  //  在这里观看这个添加的const ...... ^  
 }; 
  class  Foo 
 {
  public ：
。 .. 
  bool   const   operator  ==（ const  Foo& f） const  { return  f.data == data; }; 

注意返回类型后的 const 是返回类型的限定符，而不是函数！

比较对模板的对象

  private ：
 pair< Bar，Bar>数据; 

in

  bool  < span class =code-keyword> const   operator  ==（ const  Foo& f ）{ return  f.data == data; }; 

毫无意义。你需要比较那对中的每个成员。也许这适合你：

  bool   const   operator  ==（ const  Foo& f）{返回（f.data.first == data.first）&& （f.data.second == data.second）; }; 

This simple code compiles and runs fine using VC++ 10:

#include "stdafx.h"
#include <string>
#include <iostream>

using namespace std;

class Foo
{
public:
	Foo();
	Foo(int i, string s) { data = make_pair(i, s); };
	bool const operator==(const Foo& f) { return f.data == data; };
private:
	pair<int, string> data;
	int stuff;
};

int _tmain(int argc, _TCHAR* argv[])
{
	Foo f1(10, "xyz");
	Foo f2(10, "xyz");
	cout << ((f1 == f2) ? "equal" : "not equal") << endl;
	return 0;
}

However, if I change the Foo class so that the pair now consists of two of my classes:

#include "stdafx.h"
#include <string>
#include <iostream>

using namespace std;

class Bar
{
public:
	Bar();
	Bar(string, string);
	bool const operator==(const Bar& b) { return b.s1 == s1 && b.s2 == s2; };
private:
	string s1;
	string s2;
	int k;
};

class Foo
{
public:
	Foo();
	Foo(Bar b1, Bar b2) { data = make_pair(b1, b2); };
	bool const operator==(const Foo& f) { return f.data == data ; };
private:
	pair<Bar, Bar> data;
	int stuff;
};

int _tmain(int argc, _TCHAR* argv[])
{
	Bar b1("abc", "xyz");
	Bar b2("abc", "xyz");
	Foo f1(b1, b2);
	Foo f2(b1, b2);
	cout << ((f1 == f2) ? "equal" : "not equal") << endl;
	return 0;
}

it will not compile and I get:

1>------ Build started: Project: pair_test, Configuration: Debug Win32 ------
1>  pair_test.cpp
1>c:\program files\microsoft visual studio 10.0\vc\include\utility(305): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'const Bar' (or there is no acceptable conversion)
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(470): could be 'bool std::operator ==(const std::_Exception_ptr &,const std::_Exception_ptr &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(475): or       'bool std::operator ==(std::_Null_type,const std::_Exception_ptr &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(481): or       'bool std::operator ==(const std::_Exception_ptr &,std::_Null_type)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\system_error(408): or       'bool std::operator ==(const std::error_code &,const std::error_condition &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\system_error(416): or       'bool std::operator ==(const std::error_condition &,const std::error_code &)'
1>          c:\documents and settings\charles\my documents\visual studio 2010\projects\pair_test\pair_test.cpp(12): or       'const bool Bar::operator ==(const Bar &)'
1>          while trying to match the argument list '(const Bar, const Bar)'
1>          c:\documents and settings\charles\my documents\visual studio 2010\projects\pair_test\pair_test.cpp(24) : see reference to function template instantiation 'bool std::operator ==<Bar,Bar>(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)' being compiled
1>          with
1>          [
1>              _Ty1=Bar,
1>              _Ty2=Bar
1>          ]
1>c:\program files\microsoft visual studio 10.0\vc\include\utility(305): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'const Bar' (or there is no acceptable conversion)
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(470): could be 'bool std::operator ==(const std::_Exception_ptr &,const std::_Exception_ptr &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(475): or       'bool std::operator ==(std::_Null_type,const std::_Exception_ptr &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\exception(481): or       'bool std::operator ==(const std::_Exception_ptr &,std::_Null_type)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\system_error(408): or       'bool std::operator ==(const std::error_code &,const std::error_condition &)'
1>          c:\program files\microsoft visual studio 10.0\vc\include\system_error(416): or       'bool std::operator ==(const std::error_condition &,const std::error_code &)'
1>          c:\documents and settings\charles\my documents\visual studio 2010\projects\pair_test\pair_test.cpp(12): or       'const bool Bar::operator ==(const Bar &)'
1>          while trying to match the argument list '(const Bar, const Bar)'

It looks like I'm missing something obvious here, but searching hasn't helped much. Do I have to explicitly define the == operator for pair<bar,>? The generic == operator should work fine if it knows how to compare Bar objects, I would think.

解决方案

Your comparison operators are not defined const, but the Foo comparison operator provides const arguments. Therefore no match is found. Try this:

class Bar
{
public:
    ...
    bool const operator==(const Bar& b) const { return b.s1 == s1 && b.s2 == s2;
    //  watch this added const here ......^
};
class Foo
{
public:
	...
	bool const operator==(const Foo& f) const { return f.data == data ; };

Note that the const after the return type is a qualifier to the return type, not to the function!

you compare the object of the pair template

private:
    pair<Bar, Bar> data;

in

bool const operator==(const Foo& f) { return f.data == data ; };

that makes no sense. You got to compare each member of that pair. Maybe that fits for you:

bool const operator==(const Foo& f) { return (f.data.first == data.first) && (f.data.second == data.second) ; };

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