Question

I used array to store the data, but I replaced with vector, so I would like to replace all the c operators with c++ operators. I used memcpy to copy one memory blocks

for (i = 0; i < rows_; i++)
    memcpy((T *) &tmp.data_[cols_ * i], (T *) &a.data_[cols_ * (2 * i + 1)], rows_ * sizeof(T));

It’s also working with vectors, I just want to know is there an equivalent function in c++?

I tried the copy:

std::copy(tmp.data_[cols_ * i], tmp.data_[cols_ * i+rows], a.data_[cols_ * (2 * i + 1)]);

but I’m receiving the following error:

error: invalid use of member function (did you forget the ‘()’ ?)

For example:

I have an 2xnxn size array and I’m using the for cycle to make an nxn array instead. for example I have 1 2 3 4 5 6 7 8, my new array has to be the following: 3 4 7 8. I used memcpy to achieve this, but I don’t know how can I achieve in c++

Answer 1

Use std::copy or std::vector::assign if you copy from array to vector

  int from_array[10] = {1,2,3,4,5,6,7,8,9,10};

  std::vector<int> to_vector;

  int array_len = sizeof(from_array)/sizeof(int);
  to_vector.reserve(array_len);
  std::copy( from_array, from_array+10, std::back_inserter(to_vector)); 
  or C++11
  std::copy( std::begin(from_array), std::end(from_array), std::back_inserter(to_vector));   

  std::vector<int> to_vector2;
  to_vector2.reserve(array_len);
  to_vector2.assign(from_array, from_array + array_len);

if copy from vector to vector

   std::vector<int> v1;
   std::vector<int> v2;
   v2 = v1; // assign operator = should work

if you don’t need to keep v1, std::swap also works

v2.swap(v1);

Update:

  const int M = 2;
  const int N = 4;
  int from_array[M][N] = {{1,2,3,4},{5,6,7,8}};

  std::vector<int> to_vector;
  to_vector.reserve(N);
  int start=2;
  int end = 4;
  for (int i=0; i<M; i++)
  {
    std::copy( from_array[i]+start, from_array[i]+end, std::back_inserter(to_vector)); 
  }