How to convert string to wchar* in unicode - c++

Question

I'm trying to define a function like this:
#ifdef _UNICODE
LPCTSTR A2T(const string& str);
#else
#define A2T
#endif
If my project is in Ansi, than A2T(str) is str itself. When my project is in unicode A2T(str) return a LPCTST type

Answer 1

When UNICODE is defined, LPCTSTR is an alias for const wchar_t*, otherwise it is an alias for const char*.
Your current macro returns const wchar_t* for Unicode, but returns std::string for Ansi. That doesn't make sense. You wouldn't be able to use A2T() consistently everywhere LPCTSTR is expected. The code would not even compile for Ansi since a std::string cannot be assigned directly to a char*. For Unicode, the code would compile, but you would have a memory leak since a conversion from std:string to wchar_t* requires a dynamic memory allocation that you have to free eventually.
A better option is to have A2T() return std::wstring for Unicode, and std::string for Ansi:
#ifdef UNICODE
std::wstring A2T(const string& str)
{
std::wstring result;
// do the conversion from Ansi to Unicode as needed...
// MultiByteToWideChar(), std::wstring_convert, etc...
return result;
}
#else
std::string A2T(const string& str)
{
return str;
}
#endif
Alternatively:
std::basic_string<TCHAR> A2T(const string& str)
{
#ifdef UNICODE
std::wstring result;
// do the conversion from Ansi to Unicode as needed...
// MultiByteToWideChar(), std::wstring_convert, etc...
return result;
#else
return str;
#endif
}
Either way, you get the automatic memory management you need after conversion, and you can use A2T() consistently in both Ansi and Unicode (when passing the return value of A2T(str) to a LPCTSTR, you can use A2T(str).c_str()).
Or, you could simply forget writing your own function and just use the existing A2CT() function or CA2CT class that is already available in MFC/ATL:
ATL and MFC String Conversion Macros