Barracuda - 3 months ago 22

C++ Question

I'm trying to convert a byte array to integer:

`QByteArray b = QByteArray::fromHex("00008000");`

quint32 result = b[3];

result += b[2] << 8;

result += b[1] << 16;

result += b[0] << 24;

but I'm getting

`4294934528`

`32768`

Answer

`QByteArray`

is an array of `char`

s. Apparently, `char`

on your platform is signed and 8-bit wide. Thus, your problem can be distilled to:

```
char c = 0x80;
quint32 = c << 8;
```

The standard mandates that:

N4606 ยง 4.8 [conv.integral] / 3

If the destination type is signed, the value is unchanged if it can be represented in the destination type; otherwise, the value is implementation-defined.

In this case (as usual on 2's complement systems), `0x80`

is mapped to `std::numeric_limits<char>::min()`

== `-128`

, which is only logical because they share the same underlying bit pattern.

Now, `-128 << 8`

is defined as -128 * 2^{8}, which is -32768.

Finally, conversion from `-32768`

to 32 bit unsigned integer is well defined and yields `4294934528`