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Using std::basic_string the extensible way

Problem:
Using std::basic_string the extensible way
I'm writing a class that contains a string. But I decided to make it
more extensible and turn it into a class template:
template<typename CharT>
class My
{
     std::basic_string<CharT> text;
};

How do I operate on such a string, so that it will work for any valid
instantiation of basic_string? For example, I'd like to append some text
to the end of the string or check if the first letter is lowercase.
Currently I'm doing:
     text += "some other text";
For wstring I should probably do:
     test += L"some other text";
How do I do it in a generic way?
Solution:
That depends on the meaning of these literals.
Are these only some selected literals of the
corresponding character type with a special
meaning? If so, than a character-depending
policy/traits class would be reasonable, which
would be explicitely specialized for the corresponding
value, e.g.
template <typename CharT> 
class MyTraits; 
template 
<> 
struct MyTraits
<char> 
  
static const char* append() 
    
return "some other text"
  }

}

template 
<> 
struct MyTraits
<wchar_t> 
  
static const wchar_t* append() 
    
return L"some other text"
  }
 
}

template
<typename CharT> 
class My 

   std::basic_string
<CharT> text; 
   typedef MyTraits
<CharT> Traits; 
   
void foo() 
      text 
+= Traits::append(); 
   }
 
}


Otherwise you could only declare the member
functions in the My class template and explicitely
specialize them for the corresponding character
type:
template<typename CharT> 
class My 

   std::basic_string
<CharT> text; 
public
   
void foo(); // Only declared, not defined 
}

template
<> 
void My<char>::foo() 
  text 
+= "some other text"
}
 
template
<> 
void My<wchar_t>::foo() 
  text 
+= L"some other text"
}
 

In both cases some preprocessor magic
can help reducing the redundancies.
----------------------------------------------------------------------------
How about writing a helper function to convert a narrow string literal
to an arbitrary basic_string<T>: 
   // A template to 'widen' a string literal 
   
template < class CharT > 
   std::basic_string
<CharT> widen( char const* str ) 
     std::basic_stringstream
< CharT > s; 
     s 
<< str; return s.str(); 
   }
 
Then you can 
do 
   test 
+= widen<char>"foo bar" ); 
or 
   test 
+= widen<wchar_t>"foo bar" ); 

as appropriate.
You could even make it a constructor, but it's probably best to make it
explicit in the code when you want this widening.

posted on 2007-04-25 23:39 丑石 阅读(790) 评论(0)  编辑 收藏 引用 所属分类: C++ problem and solution


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