overriding operator== error

From:

linq936@hotmail.com

Newsgroups:

comp.lang.c++

Date:

14 Jun 2006 11:08:20 -0700

Message-ID:

<1150308500.735722.317720@u72g2000cwu.googlegroups.com>

Hi,
  I have this piece code,

   struct TriStr {
      MyString str1;
      MyString str2;
      MyString str3;

      TriStr(MyString s1, MyString s2, MyString s3){
         this->str1 = s1;
         this->str2 = s2;
         this->str3 = s3;
      }

      TriStr& operator=(const TriStr& rhs){
         this->str1 = rhs.str1;
         this->str2 = rhs.str2;
         this->str3 = rhs.str3;
         return *this;
      }

      bool operator==(const TriStr& rhs){
         return (this->str1.IEquals(rhs.str1)
                 && (this->str2.IEquals(rhs.str2))
                 && (this->str3.IEquals(rhs.str3)));
      }
   };

      bool operator==(TriStr& rhs){
         return (this->str1.IEquals(rhs.str1)
                 && (this->str2.IEquals(rhs.str2))
                 && (this->str3.IEquals(rhs.str3)));
      }

  I compile it using VC7 compiler on Windows, I get a lot of error
message, please see the messages at the end of this post.

  I then find that if I add "const" to operator== argument like this,

      bool operator==(const TriStr& rhs){
         return (this->str1.IEquals(rhs.str1)
                 && (this->str2.IEquals(rhs.str2))
                 && (this->str3.IEquals(rhs.str3)));
      }

  Then the compiler is happy.

  But I am wondering why is that?

Compile error:

c:\STL\export\stl\_algobase.c(106) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
        c:\STL\export\stl\_algobase.c(203) : see reference to function
template instantiation '_RandomAccessIter _STL
::__find<_InputIter,_Tp>(_RandomAccessIter,_RandomAccessIter,const _Tp
&,const _STL::random_access_iterator_tag &)' being compiled
        with
        [
            _RandomAccessIter=_STL::vector<Test::TriStr>::iterator,
            _InputIter=_STL::vector<Test::TriStr>::iterator,
            _Tp=Test::TriStr
        ]
        ../s\test.c(72) : see reference to function template
instantiation '_InputIter _STL::find<_STL::vector<_Tp>::iterator,MdtA
ddrGen::TriStr>(_InputIter,_InputIter,const _Tp &)' being compiled
        with
        [
            _InputIter=_STL::vector<Test::TriStr>::iterator,
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(109) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(112) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(115) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(121) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(124) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]
c:\STL\export\stl\_algobase.c(127) : error C2678: binary '==' : no
operator found which takes a left-hand operand of
type '_STL::vector<_Tp>::value_type' (or there is no acceptable
conversion)
        with
        [
            _Tp=Test::TriStr
        ]