The function template capitalize(), implemented as follows, works with strings of any type of characters. It does not modify the input string but creates a new string. To do so, it uses an std::stringstream. It iterates through all the characters in the input string and sets a flag indicating a new word to true every time a space or punctuation is encountered. Input characters are transformed to uppercase when they represent the first character in a word and to lowercase otherwise:

template <class Elem>
using tstring = std::basic_string<Elem, std::char_traits<Elem>, 
                                  std::allocator<Elem>>;
template <class Elem>
using tstringstream = std::basic_stringstream<
   Elem, std::char_traits<Elem>, std::allocator<Elem>>;

template <class Elem>
tstring<Elem> capitalize(tstring<Elem> const & text)
{
   tstringstream<Elem> result;
   bool newWord = true;
   for (auto const ch : text)
   {
      newWord = newWord || std::ispunct(ch) || std::isspace(ch);
      if (std::isalpha(ch))
      {
         if (newWord)
         {
            result << static_cast<Elem>(std::toupper(ch));
            newWord = false;
         }
         else
            result << static_cast<Elem>(std::tolower(ch));
      }
      else result << ch;
   }
   return result.str();
}

In the following program you can see how this function is used to capitalize texts:

int main()
{
   using namespace std::string_literals;
   assert("The C++ Challenger"s ==
          capitalize("the c++ challenger"s));
   assert("This Is An Example, Should Work!"s == 
          capitalize("THIS IS an ExamplE, should wORk!"s));
}