C++: using std::wstring in API function

Question

I'm using the SHGetSpecialFolderLocation API function. My application is set to "Use Unicode Character Set".

Here's what I have so far:

int main ( int, char ** )
{
    LPITEMIDLIST pidl;
    HRESULT hr = SHGetSpecialFolderLocation(NULL, CSIDL_PERSONAL, &pidl);


    /* Confused at this point */
    wstring wstrPath;

    wstrPath.resize ( _MAX_PATH );
    BOOL f = SHGetPathFromIDList(pidl, wstrPath.c_str () );
    /* End confusion */

The error I'm getting is:

error C2664: 'SHGetPathFromIDListW' : cannot convert parameter 2 from 'const wchar_t *' to 'LPWSTR'

Can someone help? What's the proper C++ way to do this?

Thanks!

Not really related to the question, but SHGetSpecialFolderLocation/SHGetSpecialFolderPath is obsolete. Microsoft suggests SHGetKnownFolderPath (msdn.microsoft.com/en-us/library/bb762188.aspx), or, if you want to stay compatible with Windows XP, SHGetFolderPath (msdn.microsoft.com/en-us/library/bb762181.aspx). — Philipp
– Philipp, Commented Jun 29, 2010 at 6:56

Matthew Flaschen · Accepted Answer · 2010-06-29 01:41:01Z

6

The second parameter is an out parameter, so you can't just pass c_str (which is const) directly. It would probably be simplest just to do:

wchar_t wstrPath[MAX_PATH];
BOOL f = SHGetPathFromIDList(pidl, wstrPath);

MAX_PATH is currently 260 characters.

answered Jun 29, 2010 at 1:41

Matthew Flaschen

286k53 gold badges523 silver badges554 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

stinky472 Over a year ago

+1. one could also use std::vector<wchar_t> for this and use &v[0], but when it's easy enough to allocate the char buffer on the stack like this with a MAX_PATH we can work with, it's both the simplest and fastest solution.

Billy ONeal Over a year ago

@Stinky: You can just do &wstrPath[0] as well. No reason you're forced to use a vector over a string here.

stinky472 Over a year ago

@Billy true but if you can create it on the stack, I say might as well do it. Otherwise we give more fuel to C trolls claiming C++ is inherently slower than C. It's not like you lose RAII benefits unless it needs to be heap-allocated. I don't see vector as a replacement for stack-allocated arrays of fixed sizes or global fixed-size arrays to store in the data segment. Boost.Array is a legitimate alternative.

stinky472 Over a year ago

@Billy sorry I forgot what I originally wrote and misunderstood what you meant. wstring would also work if we're going with a heap-based solution, yes.

Aaron Klotz · Accepted Answer · 2010-06-29 01:45:41Z

1

wstring::c_str() returns const wchar_t* and is read only. LPWSTR is not a const type, and that parameter is an out parameter. You will need to allocate the buffer yourself. You could do something like this:

wchar_t buf[MAX_PATH] = {0};
BOOL f = SHGetPathFromIDList( pidl, buf );
wstring wstrPath = buf;

answered Jun 29, 2010 at 1:45

Aaron Klotz

11.6k1 gold badge31 silver badges22 bronze badges

2 Comments

Aaron Klotz Over a year ago

It initializes all the elements of the buf array to zero. If the length of the initializer list is smaller than the size of the array, the remaining elements are initialized to zero, so initializing the first element to zero in effect zeros out the entire buf array.

Billy ONeal Over a year ago

So does empty braces {} for the same reason. (Resurrecting a very old thread, I know, feel free to ignore :) )

Community · Accepted Answer · 2017-05-23 12:09:44Z

1

You can get address of 1st array item in basic_string as pointer to writable string data. Although C++ standard does not guarantee that this block of memory must be continuous this is safe in all known implementations (How bad is code using std::basic_string as a contiguous buffer).

std::wstring path(_MAX_PATH, L'\0');
BOOL f = SHGetPathFromIDList(pidl, &path[0]);

edited May 23, 2017 at 12:09

CommunityBot

11 silver badge

answered Jun 29, 2010 at 6:48

Sergey

6471 gold badge9 silver badges16 bronze badges

1 Comment

MikeOnline Over a year ago

This is well documented in an MSDN Magazine article C++ - Using STL Strings at Win32 API Boundaries under topic A Shortcut for the Output Case: Working in Place with std::wstring. A couple other approaches that do not use std::wstring are presented as well. This article was peer reviewed by VC++ contributor Stephan T. Lavavej. As of C++11x, the memory within a std::basic_string which holds the data buffer is guaranteed to be contiguous.

Billy ONeal · Accepted Answer · 2014-12-15 18:09:15Z

1

std::basic_string::c_str() returns a constant buffer to it's memory. If you want to modify the string, you'd have to do something like this:

wstring wstrPath;
wstrPath.resize( MAX_PATH );
BOOL f = SHGetPathFromIDList(pidl, &wstrPath[0]);
wstrPath.erase(
   std::find(wstrPath.begin(), wstrPath.end(), L'\0'), wstrPath.end()
); //Throw away unused buffer space

EDIT: This should also work if you're not afraid of C libraries (though I've not tested it like I've tested the implementation above):

wstring wstrPath;
wstrPath.resize( MAX_PATH );
BOOL f = SHGetPathFromIDList(pidl, &wstrPath[0]);
wstrPath.resize(wcslen(wstrPath.c_str()));

edited Dec 15, 2014 at 18:09

answered Jun 29, 2010 at 1:44

Billy ONeal

107k61 gold badges329 silver badges566 bronze badges

Comments

zdan · Accepted Answer · 2010-06-29 01:46:05Z

0

wstring::c_str() does not let you modify its internal buffer in this way. Your easiest workaround is to create a wchar_t buffer yourself, and the pass that to the wstring constructor:

wchar_t buf[MAX_PATH];
BOOL f = SHGetPathFromIDList(pidl, buf );
wstring wstrPath(buf);

answered Jun 29, 2010 at 1:46

zdan

29.6k7 gold badges62 silver badges73 bronze badges

1 Comment

Billy ONeal Over a year ago

The only downside here is that you end up copying the buffer twice.

Collectives™ on Stack Overflow

C++: using std::wstring in API function

5 Answers 5

4 Comments

2 Comments

1 Comment

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

2 Comments

1 Comment

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related