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!

Solution 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.

Solution 2

There are many ways of fixing this

1. Open the project properties, General/Character Set. This will be set to either Unicode or Multi byte character set. If you wish to use char* change from Unicode to MBCS. This will convert CreateFile to CreateFileW if Unicode is specified and CreateFileA if MBCS is specified.
2. Enclose all strings in _T() eg _T("COM1"). What this does is to compile the string as a char* if MBCS is specified, wchar_t if unicode is specified
3. Force all strings to be wide strings by prefixing with L eg L"COM1"

Note that in some error handling routine the strings are specifically MBCS

Solution 3

Try this:

RS232Handle=OpenRS232(L"COM1", 9600);
HANDLE OpenRS232(const wchar_t* ComName, DWORD BaudRate)
{
    ComHandle=CreateFileW(ComName, GENERIC_READ|GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
}

On Windows the wchar_t type is used to represent characters in UTF-16 encoding. This is what the Windows kernel uses internally and therefore modern versions of Visual C++ default to Unicode functions. If you insist on using the ANSI functions instead (thus going back to your original code), remove the L-prefix from the string "COM1" and change the call from CreateFileW to CreateFileA.

Most Windows API functions that deal with strings have both a W and an A version; the only exception that I am aware of is the function GetProcAddress which always takes an ANSI string regardless of whether you're working with ANSI or Unicode in your project.

Solution 4

const char* and const wchar_t* are two different types with no implicit conversion between them. Therefore, you need to perform the conversion to before passing the value on to the CreateFile function. Take a look at this answer for a possible conversion approach: https://stackoverflow.com/a/3074785/6710751

Alternatively you might just use the CreateFileA function instead of CreateFile, as was suggested by Ben Voigt.

View more solutions

Comments

[Ben Voigt]

Or just call CreateFileA which will do the conversion for you.

[Ben Voigt]

If your datatype is hardcoded as wchar_t*, use CreateFileW. The CreateFile function macro should only be used together with the TCHAR or LPCTSTR type macros.

[Ben Voigt]

#5 isn't a distinct option, it just lets #2, #3, and #4 work when the string contains extended characters.

[Striezel]

Thanks for the suggestion, I've added this to the answer.

[Govind Parmar]

@BenVoigt Expanded my answer

[Cypher]

Changing the Character Set in Visual C++ did the trick for me...