Question

How can I compare a wstring, such as L"Hello", to a string? If I need to have the same type, how can I convert them into the same type?

Answer 1

Since you asked, here’s my standard conversion functions from string to wide string, implemented using C++ std::string and std::wstring classes.

First off, make sure to start your program with set_locale:

#include <clocale>

int main()
{
  std::setlocale(LC_CTYPE, "");  // before any string operations
}

Now for the functions. First off, getting a wide string from a narrow string:

#include <string>
#include <vector>
#include <cassert>
#include <cstdlib>
#include <cwchar>
#include <cerrno>

// Dummy overload
std::wstring get_wstring(const std::wstring & s)
{
  return s;
}

// Real worker
std::wstring get_wstring(const std::string & s)
{
  const char * cs = s.c_str();
  const size_t wn = std::mbsrtowcs(NULL, &cs, 0, NULL);

  if (wn == size_t(-1))
  {
    std::cout << "Error in mbsrtowcs(): " << errno << std::endl;
    return L"";
  }

  std::vector<wchar_t> buf(wn + 1);
  const size_t wn_again = std::mbsrtowcs(buf.data(), &cs, wn + 1, NULL);

  if (wn_again == size_t(-1))
  {
    std::cout << "Error in mbsrtowcs(): " << errno << std::endl;
    return L"";
  }

  assert(cs == NULL); // successful conversion

  return std::wstring(buf.data(), wn);
}

And going back, making a narrow string from a wide string. I call the narrow string “locale string”, because it is in a platform-dependent encoding depending on the current locale:

// Dummy
std::string get_locale_string(const std::string & s)
{
  return s;
}

// Real worker
std::string get_locale_string(const std::wstring & s)
{
  const wchar_t * cs = s.c_str();
  const size_t wn = std::wcsrtombs(NULL, &cs, 0, NULL);

  if (wn == size_t(-1))
  {
    std::cout << "Error in wcsrtombs(): " << errno << std::endl;
    return "";
  }

  std::vector<char> buf(wn + 1);
  const size_t wn_again = std::wcsrtombs(buf.data(), &cs, wn + 1, NULL);

  if (wn_again == size_t(-1))
  {
    std::cout << "Error in wcsrtombs(): " << errno << std::endl;
    return "";
  }

  assert(cs == NULL); // successful conversion

  return std::string(buf.data(), wn);
}

Some notes:

• If you don’t have std::vector::data(), you can say &buf[0] instead.
• I’ve found that the r-style conversion functions mbsrtowcs and wcsrtombs don’t work properly on Windows. There, you can use the mbstowcs and wcstombs instead: mbstowcs(buf.data(), cs, wn + 1);, wcstombs(buf.data(), cs, wn + 1);

In response to your question, if you want to compare two strings, you can convert both of them to wide string and then compare those. If you are reading a file from disk which has a known encoding, you should use iconv() to convert the file from your known encoding to WCHAR and then compare with the wide string.

Beware, though, that complex Unicode text may have multiple different representations as code point sequences which you may want to consider equal. If that is a possibility, you need to use a higher-level Unicode processing library (such as ICU) and normalize your strings to some common, comparable form.