Convert QString to wchar_t*

[Mr Gisa]

I did it like this:

    // MAX_FILENAME_SIZE = 2048
    wchar_t filenameW[MAX_FILENAME_SIZE];
    int length = filename.left(MAX_FILENAME_SIZE - 1).toWCharArray(filenameW);
    filenameW[length] = '\0';

Any tips for improvement?

[VRonin]

Since you have to use a container anyway, instead of a raw array I'd just use std::wstring:

QString testStr("Test");
std::wstring  testWstring  = testStr.toStdWString();
functionThatTakesWcharPointer(testWstring.c_str());

[Mr Gisa]

@VRonin c_str returns a const right? It's not a const that it asks for, so I'm getting an error saying that can't convert from const wchar_t* to wchar_t*.

... I just put a const_cast<wchar_t *> and it compiled but the string isn't right, it's like this:

[Devopia53]

@Mr-Gisa

Just use like this:

QString qString("Test");
wchar_t *wc = reinterpret_cast<wchar_t *>(qString.data())
const wchar_t *cwc = reinterpret_cast<const wchar_t *>(qString.utf16());

[aha_1980]

Hi @Devopia53,

Your code assumes that wchar_t is 16 bit wide. That is not always the case, see the already mentioned docs.

[VRonin]

A function that takes non-const wchar_t* is very dodgy! it should have no way of knowing the maximum allocated memory to that pointer/array so it is either allocating it internally (hence our discussion has been pointless so far) or that function is terribly designed and it's just a ticking time bomb.
Can I ask what that function does?

[Mr Gisa]

@VRonin It's not a function actually, sorry for the bad information, it's a struct.

struct RAROpenArchiveDataEx
{
  char         *ArcName;
  wchar_t      *ArcNameW;
  unsigned int  OpenMode;
  unsigned int  OpenResult;
  char         *CmtBuf;
  unsigned int  CmtBufSize;
  unsigned int  CmtSize;
  unsigned int  CmtState;
  unsigned int  Flags;
  UNRARCALLBACK Callback;
  LPARAM        UserData;
  unsigned int  Reserved[28];
};

ArcNameW
Input parameter which should point to zero terminated Unicode string containing the archive name or NULL if Unicode name is not specified.

[VRonin]

Who has the responsibility of managing memory allocation of those struct members?

[Mr Gisa]

I couldn't find the docs online.

RAROpenArchiveEx pretty much.

You can download the dlls, examples and the docs on the official website: https://www.rarlab.com/rar/UnRARDLL.exe

[VRonin]

Ok, got it now, looks like it is used-as-const so the const_cast<wchar_t *>(testWstring.c_str()) should work. The string you see in the debugger might just be because you are trying to read the variable past its lifetime, I wouldn't worry about it

[Mr Gisa]

@VRonin It's not working. After calling RAROpenArchiveEx it writes to RAROpenArchiveDataEx.OpenResult and the result should be ERAR_SUCCESS but it's returning ERAR_EOPEN and isn't right. For some reason it's only working the way I did before:

    // MAX_FILENAME_SIZE = 2048
    wchar_t filenameW[MAX_FILENAME_SIZE];
    int length = filename.left(MAX_FILENAME_SIZE - 1).toWCharArray(filenameW);
    filenameW[length] = '\0';

[JonB]

@Mr-Gisa said in Convert QString to wchar_t*:

ERAR_EOPEN

I don't know what state your code is in. But ERAR_EOPEN is some kind of "file is already open" error on a RAR archive, and it hardly seems likely that could be generated as a direct result of something about converting strings, does it to you?

[Mr Gisa]

@JonB The code I posted worked because it's converting the path for the rar file correctly, altough when I try the alternatives here I get a weird string and not a correct path/rar file.

[VRonin]

@Mr-Gisa said in Convert QString to wchar_t*:

I get a weird string

I suspect you are holding a pointer to a temporary variable. If you are doing something like this, you are doing it wrong:

void fillName(RAROpenArchiveDataEx* rar, const QString& fileName){
rar->ArcNameW = const_cast<wchar_t *>(fileName.toStdWString().c_str());
}

As the temporary std::wstring will be destroyed and rar->ArcNameW will contain garbage

[Mr Gisa]

bool RAR::open(RAR::OpenMode mode)
{
    RAROpenArchiveDataEx openArchiveData;
    memset(&openArchiveData, 0, sizeof(openArchiveData));

    openArchiveData.ArcName = 0;

    auto archive = const_cast<wchar_t *>(mArchive.toStdWString().c_str());

    openArchiveData.ArcNameW = archive;
    openArchiveData.OpenMode = mode;
    openArchiveData.CmtBuf = 0;
    openArchiveData.CmtBufSize = 0;
    openArchiveData.Callback = 0;
    openArchiveData.UserData = 0;

    mHandle = RAROpenArchiveEx(&openArchiveData);
    mResult = openArchiveData.OpenResult;

    if (mResult != ERAR_SUCCESS) {
        return false;
    }

    scan();

    return true;
}

[VRonin]

@Mr-Gisa said in Convert QString to wchar_t*:

auto archive = const_cast<wchar_t *>(mArchive.toStdWString().c_str());

Boom! exactly as I supected!

auto archiveWString = mArchive.toStdWString();
auto archive = const_cast<wchar_t *>(archiveWString.c_str());

[Mr Gisa]

@VRonin said in Convert QString to wchar_t*:

auto archiveWString = mArchive.toStdWString();
auto archive = const_cast<wchar_t *>(archiveWString.c_str());

Yay now its working. Thx.