CPP Windows string conversions confusion CStringA and LPCWSTR - c++

Question

I need to use WinAPI function to restart a windows service, I am not familiar with strings in C++.
My function receive as parameter: const CStringA& serviceName:
bool MyClassName::RestartServer(const CStringA& serviceName)
When I obtain SC Handle via OpenService(..) I need to provide type LPCWSTR :
SC_HANDLE SHandle = OpenService(hSCManager, LPCWSTR serviceNameAsWideString, SC_MANAGER_ALL_ACCESS);
How do I convert CStringA to LPCWSTR?
I tried to following:
CA2W(serviceName, CP_UTF8);
CString str("MyServiceName"); CStringW strw(str); LPCWSTR ptr = strw;
Both did not work properly, they compiled, but when I tried to execute the code.
It failed to OpenService().
What worked:
LPCWSTR newString = serviceName.AllocSysString();
What am I missing here? Why 1 and 2 did not work? Why 3 worked?
How do I properly deallocate newString?

Answer 1

Your code requires a conversion because you are calling the TCHAR-based OpenService() macro, which maps to either OpenServiceW() or OpenServiceA() depending on whether UNICODE is defined:
__checkReturn
WINADVAPI
SC_HANDLE
WINAPI
OpenServiceA(
__in SC_HANDLE hSCManager,
__in LPCSTR lpServiceName,
__in DWORD dwDesiredAccess
);
__checkReturn
WINADVAPI
SC_HANDLE
WINAPI
OpenServiceW(
__in SC_HANDLE hSCManager,
__in LPCWSTR lpServiceName,
__in DWORD dwDesiredAccess
);
#ifdef UNICODE
#define OpenService OpenServiceW
#else
#define OpenService OpenServiceA
#endif // !UNICODE
In your case, UNICODE is clearly being defined in your project, so your code is really calling OpenServiceW(), which is why it expects an LPCWSTR as input.
Your RestartServer() method takes a CStringA (char-based ANSI) string as input, so you should use OpenServiceA() explicitly to match the same character type, no conversion needed:
bool MyClassName::RestartServer(const CStringA& serviceName)
{
...
SC_HANDLE SHandle = OpenServiceA(hSCManager, serviceName, SC_MANAGER_ALL_ACCESS);
...
}
Otherwise, if you are going to continue using TCHAR-based functionality in your code 1, then you should change your RestartServer() method to take a CString instead of a CStringA so it adopts the same ANSI/Unicode mapping that OpenService() does (and other TCHAR-based functions do), again avoiding a conversion:
1: which you should not do, since there is rarely a need to ever write code for Win9x/ME nowadays. Windows has been a Unicode-based OS since NT4.
bool MyClassName::RestartServer(const CString& serviceName)
If that is not an option for you, then CA2W() will work just fine:
bool MyClassName::RestartServer(const CStringA& serviceName)
{
USES_CONVERSION;
...
SC_HANDLE SHandle = OpenService(hSCManager, ATL::CA2W(serviceName), SC_MANAGER_ALL_ACCESS);
...
}
Though, you might consider just using CString internally and let it handle a conversion if needed:
bool MyClassName::RestartServer(const CStringA& serviceName)
{
...
SC_HANDLE SHandle = OpenService(hSCManager, CString(serviceName), SC_MANAGER_ALL_ACCESS);
...
}
Or, make the code conditional:
bool MyClassName::RestartServer(const CStringA& serviceName)
{
...
SC_HANDLE SHandle = OpenService(hSCManager,
#ifdef UNICODE
CStringW(serviceName)
#else
serviceName
#endif
, SC_MANAGER_ALL_ACCESS);
...
}

Answer 2

CStringA and CStringW have constructors taking both const char* and const wchar_t* C strings.
Write following:
CStringW serviceNameW( serviceName );
About AllocSysString, it creates a copy in BSTR, they’re more complex than C strings, they’re null-terminated too but they also have length at negative offset. If you want to do manual memory management, call SysFreeString on the pointer. Or if you want BSTR but don’t want manual memory management, use CComBSTR class.