What's the fastest QVector operation

Hello!

In copying some large data (lets say from an array) to a QVector, what should be faster?

Option 1:

Code:

---

for (int aaa = 0; aaa < SIZE_TO_COPY; aaa++)
{
    myVector.append(myArray[aaa]);
}

---

Option 2:

Code:

---

const int startPoint = myVector.size();
const int lastPos = startPoint + SIZE_TO_COPY;
myVector.resize(lastPos);  //(myVector.size() + SIZE_TO_COPY)
 
for (int aaa = startPoint, bbb = 0; aaa < lastPos; aaa++, bbb++)
{
    myVector.replace(aaa,myArray[bbb]);
}

---

Modifier 1: Include myVector.reserve(myVector.size() + SIZE_TO_COPY); before the loop in the first option or before the resize() in the second option.
Modifier 2: Use foreach(...) instead of for(...).

Thanks,

Momergil

Option 2 is faster than option 1.

You should probably also benchmark using an explicit memcpy from the source array to the target array.

Cheers,
_

Provided that the data type stored can be copied using memcpy.

Quote:

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Originally Posted by wysota

Provided that the data type stored can be copied using memcpy.

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Yeah, for my particular situation that inspired me to do this question, I'ld say memcpy is not allowed (the vector is a vector of double, and the array is an array of short). But in any case, how could I do a memcpy from an array to a QVector if their types were compatible? memcpy(&myVector,myArray,sizeof(myArray)) doesn't seem that would work.

You can safely copy doubles from one vector to another using memcpy. You cannot copy classes that have non-trivial constructors. memcpy() accepts addresses where it should copy to/from.

Code:

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memcpy(myVector.data(), myArray, length_of_array);

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