Correct way to convert a QString to a const wchar_t* with L prefix

robro

Hello,

I try to convert a QString to a const wchar_t* with L prefix.

Is the following try correct?

void qStringToWChar(QString string)
{
    //Required is a const wchar_t* from a QString (Output shall be like: const wchar_t* array = L"string_content";
    const wchar_t * array;
    array = new wchar_t[string.length() + 1];
    string.toWCharArray(array);
    array[string.length()] = 0;
    qDebug() << "Output as wchar: " << array;

    delete[] array;
}

I am wondering whether the "L" before the string needs to be included somewhere.
The system is windows 10 x64.

Thank you very much :-)

aha_1980

Hi @robro,

Indeed the L prefix is only needed if you specify the constant in source code. Your code looks good so far.

However if you only need a read access to the QString contents and only need that on Windows, then maybe you don't need to copy the data:

http://doc.qt.io/qt-5/qstring.html#utf16

const wchar_t *array = (const wchar_t *)string.utf16();

Note that this only works if you don't modify the string while using the array pointer.

Regards

robro

Thank you very much! :-)
The read access is sufficient for me, so I will take your example.

EDIT:
@aha_1980
I thought it would work, but it seems I have another problem.

I am using an API which has the data type SI_WC (wchar_t).

If I try that:

SI_WC myConvertedString[] = (SI_WC[])myQString.utf16();

Then I get the error: C2440 const ushort* cannot be converted to SI_WC[].

How can this be solved?

Thanks!

JonB

@robro
I don't know whether your intention was to copy the string to an array, but you can't do that with a cast. And QString::utf16() returns a const pointer. So I'm guessing you want:

const SI_WC *myConvertedString = (const SI_WC *)myQString.utf16();

You might not (depending on the definition of wchar_t) need the cast, i.e.:

const SI_WC *myConvertedString = myQString.utf16();

may work.

But from the name of the variable, are you wanting actually to copy that string into a buffer where you're going to "convert" (alter) it, or what?

I am wondering: are you really wanting to use http://doc.qt.io/qt-5/qstring.html#toWCharArray by any chance?

aha_1980

Hi @robro,

what is the definition of SI_WC? It needs to be compatible to wchar_t aka unsigned short (on Windows, Unix also has a wchar_t which is 4 byte).

Two further notes:

in C [] and * are interchangeable, so your code could be written as const SI_WC *myConvertedString = (const SI_WC *)myQString.utf16();
if a function takes a non const pointer, it is not save to give a const pointer, as utf16() does. The function may want to modify the data given by the pointer (called pass by reference)

robro

Works fine! :-)
Thanks a lot!

Sprezzatura

Here are a couple of variations that worked for me in Visual Studio 2017:

diac = QStringLiteral("ÀÉ");
uint chin[] = {0x4E83, 0};        // L'亃'
diac2 = QString::fromUcs4(chin);	
wchar_t chinW[] = {L'亃', 0};
wchar_t *chinW2 = L"亃";
diac3 = QString::fromWCharArray(chinW2);   // also chinW
diac += diac2 + diac3;

Sorry I just realized you want to go the other way.