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Solved
memcpy vector to array in QT
Posted on 2011-03-07
Hi everybody,
I have a small problem here... I need to copy the contents of a vector to a certain position in an array. However, memcpy returns the error "invalid conversion from int to void*"
Here's an example of what I'm trying to do:
QVector<qint16> Vector1(1024);
QVector<qint16> Vector2(1024);
QVector<qint16> Vector3(1024);
int HArr[1024][1024];
memcpy(HArr[0][0], &Vector1[0], 1024);
memcpy(HArr[0][1], &Vector2[0], 1024);
memcpy(HArr[0][2], &Vector3[0], 1024);
Can someone show me how to do this correctly ?
Thanks,
Peter
I have a small problem here... I need to copy the contents of a vector to a certain position in an array. However, memcpy returns the error "invalid conversion from int to void*"
Here's an example of what I'm trying to do:
QVector<qint16> Vector1(1024);
QVector<qint16> Vector2(1024);
QVector<qint16> Vector3(1024);
int HArr[1024][1024];
memcpy(HArr[0][0], &Vector1[0], 1024);
memcpy(HArr[0][1], &Vector2[0], 1024);
memcpy(HArr[0][2], &Vector3[0], 1024);
Can someone show me how to do this correctly ?
Thanks,
Peter
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14 Comments
Active 3 days ago
Just make that
See also http://doc.qt.nokia.com/latest/qvector.html and http://doc.qt.nokia.com/latest/qvector.html#constData in particular.
memcpy(HArr[0][0], Vector1.constData(), 1024);
memcpy(HArr[0][1], Vector2.constData(), 1024);
memcpy(HArr[0][2], Vector3.constData(), 1024);
See also http://doc.qt.nokia.com/latest/qvector.html and http://doc.qt.nokia.com/latest/qvector.html#constData in particular.
Active 1 day ago
I'm not sure what the internal state of a QVector is so you might have to play with this some more, but the error that you got was to do with the first argument. Instead of ...
memcpy(HArr[0][0], &Vector1[0], 1024);
memcpy(HArr[0][1], &Vector2[0], 1024);
memcpy(HArr[0][2], &Vector3[0], 1024);
try...
memcpy(HArr[0], &Vector1[0], 1024);
memcpy(HArr[1], &Vector2[0], 1024);
memcpy(HArr[2], &Vector3[0], 1024);
memcpy(HArr[0][0], &Vector1[0], 1024);
memcpy(HArr[0][1], &Vector2[0], 1024);
memcpy(HArr[0][2], &Vector3[0], 1024);
try...
memcpy(HArr[0], &Vector1[0], 1024);
memcpy(HArr[1], &Vector2[0], 1024);
memcpy(HArr[2], &Vector3[0], 1024);
Active 3 days ago
Ooops, correction, you need to take the size of the data type into account - also make sure that you are using a target array data tape that matches qint16, i.e.
QVector<qint16> Vector1(1024);
QVector<qint16> Vector2(1024);
QVector<qint16> Vector3(1024);
short int HArr[1024][1024]; // 'short int' as the equivalent to 'quint16'
memcpy(HArr[0][0], Vector1.constData(), 1024 * sizeof(quint16));
memcpy(HArr[0][1], Vector2.constData(), 1024 * sizeof(quint16));
memcpy(HArr[0][2], Vector3.constData(), 1024 * sizeof(quint16));
Active today
the memcpy takes a pointer as first argument and not an int. also the second index runs faster than second. so it should be
if (sizeof(quint16) == sizeof(short int))
{
memcpy(&HArr[0][0], Vector1.constData(), 1024 * sizeof(quint16));
memcpy(&HArr[1][0], Vector2.constData(), 1024 * sizeof(quint16));
memcpy(&HArr[2][0], Vector3.constData(), 1024 * sizeof(quint16));
}
Sara
if (sizeof(quint16) == sizeof(short int))
{
memcpy(&HArr[0][0], Vector1.constData(), 1024 * sizeof(quint16));
memcpy(&HArr[1][0], Vector2.constData(), 1024 * sizeof(quint16));
memcpy(&HArr[2][0], Vector3.constData(), 1024 * sizeof(quint16));
}
Sara
Active 3 days ago
>>"invalid conversion from int to void*"
Aaah, I think everyone missed that little detail - you are not using the target arrays correctly, that should be
Aaah, I think everyone missed that little detail - you are not using the target arrays correctly, that should be
QVector<qint16> Vector1(1024);
QVector<qint16> Vector2(1024);
QVector<qint16> Vector3(1024);
short int HArr[1024][1024]; // 'short int' as the equivalent to 'quint16'
memcpy(HArr[0], Vector1.constData(), 1024 * sizeof(quint16));
memcpy(HArr[1], Vector2.constData(), 1024 * sizeof(quint16));
memcpy(HArr[2], Vector3.constData(), 1024 * sizeof(quint16));
Active today
thanks.
the last code of jkr also works cause &HArr[0][0] and HArr[0] both point to first element of the first row of HArr.
did you notice the
if (sizeof(quint16) == sizeof(short int))
i added?
the point is that it is dangerous when using memcpy to calculate the size to copy from the elements of the right side when the left side elements have a different type.
a short int not necessarily must have 16 bits but could grow in future. so if a system would use a 64-bit int, it very likely would make short int 32-bit.
Sara
the last code of jkr also works cause &HArr[0][0] and HArr[0] both point to first element of the first row of HArr.
did you notice the
if (sizeof(quint16) == sizeof(short int))
i added?
the point is that it is dangerous when using memcpy to calculate the size to copy from the elements of the right side when the left side elements have a different type.
a short int not necessarily must have 16 bits but could grow in future. so if a system would use a 64-bit int, it very likely would make short int 32-bit.
Sara
Active 1 day ago
I think both jkr and I deserve some of the points (in addition to sarabande) as jkr was the first person to get the correct pointer to copy from (http:#a35061677) and the correct size to copy (http:#a35061699) and I was the first person to get the correct destination pointer to copy to (http:#a35061698). Sara had the idea about checking for the size of the data items (http:#a35068086)
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