Question

I am comparing two strings, str1 and str2, using the string.compare function in #string. Is there a way to force the class to think that '-' is the equivalent of ' '. Looking at the member functions of char_traits I thought that the .assign would allow me to accomplish this but it is acting as if I am saying, str1='-'; or str1=' ';. I would prefer not to rewrite my own string handling class.

Answer 1

There are several possibilities, depending whether you want this behavior:

• once
• encoded in the class

If you want to behavior once: simply use your own (custom) algorithm:

bool isSpace(char i) { return i == '-' or i == ' '; }

int compare(std::string const& left, std::string const& right) {
  typedef std::string::const_iterator ConstIterator;
  typedef std::pair<ConstIterator, ConstIterator> Result;

  size_t const size = std::min(left.size(), right.size());

  Result const r = std::mismatch(left.begin(),
                                 left.begin() + size,
                                 right.begin(),
                                 [](char a, char b) {
                                   return a == b or (isSpace(a) and isSpace(b));
                                 });

  if (r.first == left.begin() + size) { // equal up til the end, shorter wins
    return left.size() < right.size() ? -1 :
          (left.size() == right.size() ? 0 : 1);
  }

  // not equal until the end
  return *r.first < *r.second ? -1 : 1;
}

If this behavior need be encoded within the class itself, you need to use basic_string and provide a custom trait class.

The traits class provides a static int compare ( const char_type* s1, const char_type* s2, size_t n); function that is used by std::string::compare under the hood.

So for example:

struct MyTraits: char_traits<char> // too lazy to reimplement everything
{
  static int compare(const char_type* s1, const char_type* s2, size_t n);
  // definition can be trivially derived from the above version
};

typedef std::basic_string<char, MyTraits> MyString;

Of course, MyString is then completely incompatible with other std::string.

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Frankly, if you can, simply “normalize” your string and decide whether you’ll use ‘-‘ or ‘ ‘. It will make your life easier.