How to convert a pointer value to QString?

Question

for debug purposes I often output pointer values (mostly this) to qDebug:

qDebug("pointer of current object = 0x%08x",this);, using "%08x" as format string and simply passing this as a parameter.

How can I convert the pointer value to a QString?

This is what I got so far:

char p = (char)this;
return QString("0x%1").arg(p, 8, '0');

But the compiler doesn't seem to figure out what to do with that value. Is casting to char correct in this case? Or what would be a safer way to do this?

Using Visual C++ with Qt 4.7.4.

EDIT

Using qulonglong as suggested

qulonglong p = (qulonglong)this;
return QString("0x%1").arg(p, 8, '0');

yields in a compiler error message (error C2666).

Converting the pointer to a char will produce wrong result, as characters are only eight bits. Try `qulonglong` instead (or `qulong` if you are on a 32-bit system). — Some programmer dude, Jan 16 '12 at 15:17
What do you think `qDebug` is doing when you pass `this` as a parameter with the format specifier in that literal string? — AJG85, Jan 16 '12 at 15:23
Sincerely: I have not much of a clue. I know that "%08x" represents an 8-bit hexadecimal value. — Martin Hennings, Jan 16 '12 at 15:26
In the format string `"%08x"`, the "8" doesn't mean it's 8 bits, it means that the width will be 8 characters. The "0" means that if the value is not enough for 8 characters, it will be padded with the character '0'. I suggest you read more about `printf` and its formatting. — Some programmer dude, Jan 16 '12 at 15:46
Thanks for that hint. The "x" actually makes the output hexadecimal. The "8 bit" remark was nonsense, sorry. "%08x" will output the number as zero-padded, 8-digit hex number, which represents 32 bit. So I guess for an x64 pointer this would read "%016x"? — Martin Hennings, Jan 16 '12 at 15:56
Prefer C++ casts over c-style casts: `static_cast(this)` etc. — AJG85, Jan 16 '12 at 16:06
Better use: `qDebug("pointer of current object = %p", this);` or `qDebug() << "current object = " << this;` — quinmars, Jan 17 '12 at 00:24

Ignitor · Accepted Answer · 2022-11-29T13:44:04.270

25

Using QString::arg():

MyClass *ptr = new MyClass();
QString ptrStr = QString( "0x%1" ).arg( reinterpret_cast<quintptr>(ptr), 
                    QT_POINTER_SIZE * 2, 16, QChar('0') );

It will use the correct type and size for pointers (quintptr and QT_POINTER_SIZE) and will always prefix "0x".

Notes:
To prefix the value with zeros, the fourth parameter needs to be QChar('0').
To output the correct number of digits, QT_POINTER_SIZE needs to be doubled (because each byte needs 2 hex digits).

edited Nov 29 '22 at 13:44

answered May 15 '13 at 15:05

Ignitor

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Use quintptr instead? – Peter Tseng Aug 20 '13 at 23:16

Kamil Klimek · Answer 2 · 2012-01-20T14:35:11.000

14

Why not simply use QString & QString::sprintf ( const char * cformat, ... )

QString t;
// QString::sprintf adds 0x prefix
t.sprintf("%08p", ptr);

edited Jan 20 '12 at 14:35

answered Jan 16 '12 at 16:34

Kamil Klimek

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Actually, `QString().sprintf("%08p", ptr)` seems to be the simplest way. The format `p` creates the '0x' itself. – Martin Hennings Jan 20 '12 at 12:55
Forgot that Qt does it right way ;) not sure but in vc printf implementation there is no 0x prefix – Kamil Klimek Jan 20 '12 at 14:34
2

Qt 5 recommends [not to use `QString::sprintf()` and `QString::asprintf()` in new Qt code](http://doc.qt.io/qt-5/qstring.html#asprintf). – Ignitor Feb 21 '17 at 09:08
Sure, but answer was about Qt 4.7 – Kamil Klimek Feb 21 '17 at 09:10
Worked in Qt 3.3.8 as well. – jaba Jun 20 '17 at 13:54
this is now `QString str; str.asprintf("%p",ptr);` – Chris May 10 '20 at 19:48
asprintf is not recommended anymore https://doc.qt.io/qt-5/qstring.html#asprintf – pktiuk Oct 23 '22 at 15:28

score 8 · Answer 3 · edited May 23 '17 at 12:34

QTextStream appears to offer the functionality you seek, and operates simply on a void*.

const void * address = static_cast<const void*>(ptr);
QString addressString;
QTextStream addressStream (&addressString);
addressStream << address;
qDebug() << addressString;

Unlike the other approaches, it does not reference notions like the particular number "8" or casting to "qlonglong". Seems less worrisome, and is much like the prescribed method for std::string (though without getting std::string conversions involved)

score 2 · Answer 4 · answered Jan 16 '12 at 15:44

2

You could do an intermediate step with sprintf:

QString pointer_to_qstring(void *ptr)
{
    char temp[16];
    sprintf(temp, "0x%08p", ptr);
    return QString(static_cast<char *>(temp));
}

Or through ostringstream:

QString pointer_to_qstring(void *ptr)
{
    std::ostringstream oss;
    oss << std::setw(8) << std::setfill('0') << std::hex << ptr:
    return QString(oss.str().c_str());
}

answered Jan 16 '12 at 15:44

Some programmer dude

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You don't need the `static_cast` or the `c_str()` I don't believe. – AJG85 Jan 16 '12 at 16:06

score 1 · Answer 5 · answered Nov 09 '17 at 09:17

1

For me mostly it is enough to see if the pointer is the nullptror if it has the same value as an other pointer. For those cases it is enough to convert the pointer to a number and then use QString::number()

Cat * cat = new Cat();
qDebug()<<QString::number((std::uintptr_t)(cat));;

answered Nov 09 '17 at 09:17

newandlost

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Your answer's output isn't hexadecimal (use argument 16 in `QString::number()`), not starting with `0x` and not zero-padded (use `QString("0x%1").arg(pointer, length, 16, QChar('0'))`, as in @Ignitor s answer). – Martin Hennings Nov 09 '17 at 09:41
(I agree that using padded hexadecimal output is not neccessary to compare pointer values. But in hex comparing is easier because the strings are shorter. And my debugger (VS2012) always displays addresses in hex.) – Martin Hennings Nov 09 '17 at 09:44
That is what I am saying. For me it is not important to get in hex format and I wonder if the one that asked that questions needs the hex format at all. – newandlost Nov 09 '17 at 09:44

How to convert a pointer value to QString?

5 Answers5