Using std::vector as view on to raw memory

Question

I\'m using a external library which at some point gives me a raw pointer to an array of integers and a size.

Now I\'d like to use std::vector to access and

Answer 1

The problem is that std::vector has to make a copy of the elements from the array you initialize it with as it has the ownership of the objects it contains.

To avoid this, you can use a slice object for an array (i.e., similar to what std::string_view is to std::string). You could write your own array_view class template implementation whose instances are constructed by taking a raw pointer to an array's first element and the array length:

#include 

template
class array_view {
   T* ptr_;
   std::size_t len_;
public:
   array_view(T* ptr, std::size_t len) noexcept: ptr_{ptr}, len_{len} {}

   T& operator[](int i) noexcept { return ptr_[i]; }
   T const& operator[](int i) const noexcept { return ptr_[i]; }
   auto size() const noexcept { return len_; }

   auto begin() noexcept { return ptr_; }
   auto end() noexcept { return ptr_ + len_; }
};

array_view doesn't store an array; it just holds a pointer to the beginning of the array and the length of that array. Therefore, array_view objects are cheap to construct and to copy.

Since array_view provides the begin() and end() member functions, you can use the standard library algorithms (e.g., std::sort, std::find, std::lower_bound, etc.) on it:

#define LEN 5

auto main() -> int {
   int arr[LEN] = {4, 5, 1, 2, 3};

   array_view av(arr, LEN);

   std::sort(av.begin(), av.end());

   for (auto const& val: av)
      std::cout << val << ' ';
   std::cout << '\n';
}

Output:

1 2 3 4 5

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Use std::span (or gsl::span) instead

The implementation above exposes the concept behind slice objects. However, since C++20 you can directly use std::span instead. In any case, you can use gsl::span since C++14.