C++ MFC how to compare LPCTSTR in a if statement?

Question

I have the following code:

LPCTSTR strPermission = Method();

if (strPermission == L"0")
{
    return true;
}
else
{
    return false;
}

While debugging I can see that strPermission does equal "0", yet when I compare it like in the if statement it always returns false.

The only thing I can think of is that it is comparing the memory address of the variable rather than the variable value.

How do I compare strPermission to L"0" so that it would return true if strPermission equals "0".

Thank you!

LPCTSTR is either `const char*` or `const wchar_t*`, so think about it. Also, shouldn't you be checking for NULL or the null terminator and not the string literal "0"? — Jesse Good, Mar 20 '12 at 23:25
You can use C-style string-comparison functions, but use the version for TCHAR, which is [tcscmp](http://msdn.microsoft.com/en-us/library/e0z9k731(v=vs.80).aspx). — chris, Mar 20 '12 at 23:26
Oops, I meant `_tcscmp`, if you tried the first one to some disappointment. — chris, Mar 20 '12 at 23:37
possible duplicate of [How to compare pointer to strings in C](http://stackoverflow.com/questions/3663668/how-to-compare-pointer-to-strings-in-c) — Mahmoud Al-Qudsi, Mar 20 '12 at 23:44
Close, but these data types are a bit more specific. That is the main issue here though. — chris, Mar 20 '12 at 23:46
May one ask what the circumstances are that force you to support Windows 9x (by using the T stuff)? — Cheers and hth. - Alf, Mar 20 '12 at 23:53
For me I create a CString variable and use `.Compare()` to compare `LPCTSTR` — Deqing, Oct 11 '16 at 08:40

score 4 · Accepted Answer · answered Mar 20 '12 at 23:26

4

You'll need to use a C runtime library function. strcmp compares ANSI strings, wcscmp compares UNICODE strings.

You use it like this:

bool match = wcscmp(strPermission, L"0") == 0;

answered Mar 20 '12 at 23:26

AVIDeveloper

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score 2 · Answer 2 · answered Mar 20 '12 at 23:25

2

You can't compare C-style strings like that in C or C++. Check out this C FAQ question & answer.

The function you're looking for is called lstrcmp.

answered Mar 20 '12 at 23:25

Carl Norum

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Mooing Duck · Answer 3 · 2012-03-20T23:42:31.960

LPCTSTR is a pointer to an array of const wchar_t. strPermission points to the first character of the array. L"0" is a string literal, which is an array of const wchar_t, which decays to a pointer of const wchar_t. But the pointers are not equal, they point to different arrays. This is why we invented C++. Please use it.

std::wstring strPermission = Method();
return (strPermission == L"0"); //works like magic!

or, if Method is returning something you have to retain, at least do this

std::unique_ptr<wchar_t[]> strPermission = Method();
return (std::wcscmp(strPermission.get(), L"0")==0); 
//wcscmp is for comparing strings, but returns 0 if they're equal.

Also, are you sure that strPemission points to an array that contains a zero character followed by a null character? If not, and you're not using wstring, then you also have to check that it points at an array

if (strPermission)
     //do stuff
else
     //its a NULL pointer.

I have been prodded by chris to point out that the type of LPCTSTR actually depends on the compiler options. I can tell by your code that you're coding with _UNICODE set, which makes it a const wchar_t*, but if you want to be able to compile with other options (I can't think of a good reason to do so) you should use _tcscmp to compare, have the literals as _T("0") and they'll be arrays of TCAHR. For the strings, you'll have to add a typedef somewhere:

#ifdef _UNICODE
    typedef std::string std::tstring 
    //you'll probably have to add more t helper functions here
#else
    typedef std::string std::wstring
    //you'll probably have to add more t helper functions here
#endif

If you want to be certain that your code is always _UNICODE (which is what I do), explicitly call MethodW() instead of Method(). (There is also a corresponding MethodA(), but not much reason to call it).

There's also a UNICODE macro, but it should always be the same as the _UNICODE macro. (Never define these yourself, they belong in the project options)

The thing is that although you can assume LPCTSTR is a LPCWSTR because of the L"0" in the question, it is technically a `const TCHAR *`. — chris, Mar 20 '12 at 23:35
@chris: technically it is, but I ignored that. I'll add a comment. — Mooing Duck, Mar 20 '12 at 23:36

C++ MFC how to compare LPCTSTR in a if statement?

3 Answers3

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