c++ - cannot convert 'const char*' to 'LPCWSTR {aka const wchar_t*}'

Question

I'm getting an error in my C++ code that I can't quite make sense of. The stripped down code bits are here:

RS232Handle=OpenRS232("COM1", 9600);

HANDLE OpenRS232(const char* ComName, DWORD BaudRate)
{
    ComHandle=CreateFile(ComName, GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
}

I get the following error:

error: cannot convert 'const char*' to 'LPCWSTR {aka const wchar_t*}' for argument '1' to 'void* CreateFileW(LPCWSTR, DWORD, DWORD, LPSECURITY_ATTRIBUTES, DWORD, DWORD, HANDLE)'
     ComHandle=CreateFile(ComName, GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);

The code was taken from VS code and I am now using Qt creator.

How can I fix this issue? Thanks!

Answer 1

The Windows CreateFile function is actually a macro that expands to one of:

• CreateFileA, which takes a file path of type const char*
• CreateFileW, which takes a file path of type const wchar_t*.

(The same is true for most of the functions in the Windows API that take a string.)

You're declaring the parameter const char* ComName, but apparently compiling with UNICODE defined, so it's calling the W version of the function. There's no automatic conversion from const wchar_t* to const char*, hence the error.

Your options are to:

1. Change the function parameter to a UTF-16 (const wchar_t*) string.
2. Keep the char* parameter, but have your function explicitly convert it to a UTF-16 string with a function like MultiByteToWideChar.
3. Explicitly call CreateFileA instead of CreateFile.
4. Compile your program without UNICODE, so that the macros expand to the A versions by default.
5. Kidnap a prominent Microsoft developer and force him to read UTF-8 Everywhere until he agrees to have Windows fully support UTF-8 as an “ANSI” code page, thus freeing Windows developers everywhere from this wide-character stuff.

Edit: I don't know if a kidnapping was involved, but Windows 10 1903 finally added support for UTF-8 as an ANSI code page.