如何在不分配更多存储空间的情况下从数组初始化向量？

Question

The direct way to initialize a vector from an array seems to be:

int sizeArr; int * array = getArray(sizeArr);
std::vector<int> vec(array, array+sizeArr);

Here, I am getting the array from a function which allocates the space in memory and sets sizeArr by reference. {start edit} Unfortunately, the function is not written by me and I need to deal with C style array then convert it to a vector somehow. (If possible efficiently). {end edit}

When I initialize vec , obviously I am allocating space for it separately. If I have no intention of using the data using array anymore, is it possible to somehow "move" the data pointed by array to the vector vec and not allocate any space for it separately?

Answer 1

If the caller doesn't own the data (ie is not in charge of deleting it), then the easiest option might be to write a container-like wrapper:

template <typename T>
struct array_wrapper
{
  typedef T* iterator;
  typedef const T* const_iterator;
  array_wrapper(T* begin, T* end) : data_(begin), size_(end-begin) {}
  iterator begin() { return data_; }
  iterator end() { return data_ + size_; }
  const_iterator begin() const { return data_; }
  const_iterator end() const { return data_ + size_; }
  size_t size() const { return size_; }
  T& operator[](size_t i) { return _data[i]; }
  const T& operator[](size_t i) const { return _data[i]; }
  void swap(array_wrapper& other) { 
    std::swap(size_, other.size_);
    std::swap(data_, other.data_);
  }
 private:
  T* data_;
  size_t size_;
};

then

array_wrapper<int> vec(array, array+sizeArr);

This has a lot of the functionality of an std::vector , but does not own the underlying data, and supports no operations that would result in resizing or re-allocating.

Answer 2

Why do you even allocate the memory with a pointer for int? There is a very high chance, that you have no reason for doing this.

If you have a compiler which assists c++11 it should be possible to just initialize the vector with std::vector vec{array}; Or if you dont need the array anymore, just delete it and push references of the array to the vector