Описать класс для хранения строк

#include <cstddef>
#include <cstdio>
#include <cstring>
#include <exception>
 
class String {
 public:
 
  class WrongCharacterExcpetion : public std::exception {
   public:
    WrongCharacterExcpetion(const size_t position, const char symbol) throw()
        : position_(position), symbol_(symbol) {
      sprintf(error_string_, "Incorrect symbol '%c' at position %d.\n",
              symbol_, position_);
    }
    virtual const char* what() const throw() {
      return error_string_;
    };
    size_t position() const {
      return position_;
    }
    char symbol() const {
      return symbol_;
    }
  private:
    size_t position_;
    char symbol_;
    char error_string_[80];
  };
 
  String() : size_(0), data_(NULL) {}
  explicit String(const char* string)
      : size_(0), data_(NULL) {
    SetString(string);
  }
  ~String() {
    delete data_;
  }
  void SetString(const char* new_string) {
    size_t new_string_length = strlen(new_string);
    char *string_buffer = new char[new_string_length + 1];
    size_t position = 0;
    while (new_string[position] != '\0') {
      if (new_string[position] >= '0' && new_string[position] <= '9') {
        string_buffer[position] = new_string[position];
      } else {
        delete string_buffer;
        throw WrongCharacterExcpetion(position, new_string[position]);
      }
      ++position;
    }
    string_buffer[position] = new_string[position];
    size_ = new_string_length;
    delete data_;
    data_ = string_buffer;
  }
  size_t Length() const { return size_; }
  char *CString() const { return data_; }
 private:
  size_t size_;
  char *data_;
};
 
int main(int argc, char *argv[]) {
  String a("12345");
 
  try {
    String b("125a12");
  } catch (String::WrongCharacterExcpetion &exception_rised) {
    printf("%s\n", exception_rised.what());
  }
  return 0;
}