How do you copy the contents of an array to a std:

Question

I have an array of values that is passed to my function from a different part of the program that I need to store for later processing. Since I don't know how many times my function will be called before it is time to process the data, I need a dynamic storage structure, so I chose a std::vector. I don't want to have to do the standard loop to push_back all the values individually, it would be nice if I could just copy it all using something similar to memcpy.

Answer 1

Assuming you know how big the item in the vector are:

std::vector<int> myArray;
myArray.resize (item_count, 0);
memcpy (&myArray.front(), source, item_count * sizeof(int));

http://www.cppreference.com/wiki/stl/vector/start

Answer 2

Since I can only edit my own answer, I'm going to make a composite answer from the other answers to my question. Thanks to all of you who answered.

Using std::copy, this still iterates in the background, but you don't have to type out the code.

int foo(int* data, int size)
{
   static std::vector<int> my_data; //normally a class variable
   std::copy(data, data + size, std::back_inserter(my_data));
   return 0;
}

Using regular memcpy. This is probably best used for basic data types (i.e. int) but not for more complex arrays of structs or classes.

vector<int> x(size);
memcpy(&x[0], source, size*sizeof(int));

Answer 3

If all you are doing is replacing the existing data, then you can do this

std::vector<int> data; // evil global :)

void CopyData(int *newData, size_t count)
{
   data.assign(newData, newData + count);
}

Answer 4

avoid the memcpy, I say. No reason to mess with pointer operations unless you really have to. Also, it will only work for POD types (like int) but would fail if you're dealing with types that require construction.

Answer 5

In addition to the methods presented above, you need to make sure you use either std::Vector.reserve(), std::Vector.resize(), or construct the vector to size, to make sure your vector has enough elements in it to hold your data. if not, you will corrupt memory. This is true of either std::copy() or memcpy().

This is the reason to use vector.push_back(), you can't write past the end of the vector.

Answer 6

There have been many answers here and just about all of them will get the job done.

However there is some misleading advice!

Here are the options:

vector<int> dataVec;

int dataArray[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
unsigned dataArraySize = sizeof(dataArray) / sizeof(int);

// Method 1: Copy the array to the vector using back_inserter.
{
    copy(&dataArray[0], &dataArray[dataArraySize], back_inserter(dataVec));
}

// Method 2: Same as 1 but pre-extend the vector by the size of the array using reserve
{
    dataVec.reserve(dataVec.size() + dataArraySize);
    copy(&dataArray[0], &dataArray[dataArraySize], back_inserter(dataVec));
}

// Method 3: Memcpy
{
    dataVec.resize(dataVec.size() + dataArraySize);
    memcpy(&dataVec[dataVec.size() - dataArraySize], &dataArray[0], dataArraySize * sizeof(int));
}

// Method 4: vector::insert
{
    dataVec.insert(dataVec.end(), &dataArray[0], &dataArray[dataArraySize]);
}

// Method 5: vector + vector
{
    vector<int> dataVec2(&dataArray[0], &dataArray[dataArraySize]);
    dataVec.insert(dataVec.end(), dataVec2.begin(), dataVec2.end());
}

To cut a long story short Method 4, using vector::insert, is the best for bsruth's scenario.

Here are some gory details:

Method 1 is probably the easiest to understand. Just copy each element from the array and push it into the back of the vector. Alas, it's slow. Because there's a loop (implied with the copy function), each element must be treated individually; no performance improvements can be made based on the fact that we know the array and vectors are contiguous blocks.

Method 2 is a suggested performance improvement to Method 1; just pre-reserve the size of the array before adding it. For large arrays this might help. However the best advice here is never to use reserve unless profiling suggests you may be able to get an improvement (or you need to ensure your iterators are not going to be invalidated). Bjarne agrees. Incidentally, I found that this method performed the slowest most of the time though I'm struggling to comprehensively explain why it was regularly significantly slower than method 1...

Method 3 is the old school solution - throw some C at the problem! Works fine and fast for POD types. In this case resize is required to be called since memcpy works outside the bounds of vector and there is no way to tell a vector that its size has changed. Apart from being an ugly solution (byte copying!) remember that this can only be used for POD types. I would never use this solution.

Method 4 is the best way to go. It's meaning is clear, it's (usually) the fastest and it works for any objects. There is no downside to using this method for this application.

Method 5 is a tweak on Method 4 - copy the array into a vector and then append it. Good option - generally fast-ish and clear.

Finally, you are aware that you can use vectors in place of arrays, right? Even when a function expects c-style arrays you can use vectors:

vector<char> v(50); // Ensure there's enough space
strcpy(&v[0], "prefer vectors to c arrays");

Hope that helps someone out there!