How to cheaply assign C-style array to std::vector?

Question

Currently I do the following:

// float *c_array = new float[1024];

void Foo::foo(float *c_array, size_t c_array_size) {
  //std::vector<float> cpp_array;

  cpp_array.assign(c_array, c_array + c_array_size);
  delete [] c_array;
}

How can I optimize this assigning? I would like not to perform elementwise copy but just swap pointers.

Answer 1

The current std::vector doesn't provide any capability or interface to take ownership of previously allocated storage. Presumably it would be too easy to pass a stack address in by accident, allowing more problems than it solved.

If you want to avoid copying into a vector, you'll either need to use vectors through your entire call chain, or do it the C way with float[] the entire time. You can't mix them. You can guaranteed that &vec[0] will be equivalent to the C-array though, fully contiguous, so using vector in the whole program may be feasible.

Answer 2

Currently, the std::vector interface does not possess the capacity to move from or swap with anything except another std::vector.

Answer 3

The only way to do it would be to create a custom allocator.

1. Write an allocator class that you can initialise with your array.

2. Instantiate the vector with the allocator as an argument.

Answer 4

Unlikely it's possible - it's quite dangerous, because std::vector doesn't know how the memory was allocated and how it should be freed.

If it's possible, you may replace original allocation with creation of std::vector of correct size. It uses contiguous memory area, so it can replace manually allocated buffer.