Question

I’m developing an app for Mac OS, which includes a cross-platform lib in C++. There’s a macro defined as follows:

#define MY_GET(DataType,DataName,PtrFunName,DefaultVaule) \
DataType Get##DataName() \
{ \
    DataType dataTem = (DefaultVaule);\
    if (NULL == p) \
    { \
        return dataTem; \
    } \
    p->Get##PtrFunName(CComBSTR(L#DataName),&dataTem); \
    return dataTem; \
}

When compiling, the compiler generates the following error:

Use of undeclared identifier 'L'

Which is expanded from macro ‘MY_GET’. After searching for CComBSTR(L, I can find other usage of L”String”. So why is the L expanded from my macro is undefined while other L are compiled successfully.

Is L”String” legal in Objective-C?

Answer 1

I seems that you need the preprocessor “token concatenation” operator ## here:

CComBSTR(L ## #DataName)

instead of

CComBSTR(L#DataName)

The following code in an Objective-C file compiles and produces the wchar_t string L"abc":

#define LL(x) L ## #x
wchar_t *s = LL(abc); // expands to: L"abc"

I don’t know if other compilers behave differently, but the Apple LLVM 4.1 compiler does not a allow a space between L and the string:

#define LL(x) L#x
wchar_t *s = LL(abc); // expands to: L "abc"
// error: use of undeclared identifier 'L'