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

C++20's std::span

If you are able to use C++20, you could use std::span which is a pointer - length pair that gives the user a view into a contiguous sequence of elements. It is some sort of a std::string_view, and while both std::span and std::string_view are non-owning views, std::string_view is a read-only view.

From the docs:

The class template span describes an object that can refer to a contiguous sequence of objects with the first element of the sequence at position zero. A span can either have a static extent, in which case the number of elements in the sequence is known and encoded in the type, or a dynamic extent.

So the following would work:

#include 
#include 
#include 

int main() {
    int data[] = { 5, 3, 2, 1, 4 };
    std::span s{data, 5};

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

    for (auto const i : s) {
        std::cout << i << "\n";
    }

    return 0;
}

Check it out live

Since std::span is basically pointer - length pair, you can use in a following manner too:

size_t size = 0;
int *data = get_data_from_library(size);
std::span s{data, size};

Note: Not all compilers support std::span. Check compiler support here.

UPDATE

If you are not able to use C++20, you could use gsl::span which is basically the base version of the C++ standard's std::span.

C++11 solution

If you are limited to C++11 standard, you can try implementing your own simple span class:

template
class span {
   T* ptr_;
   std::size_t len_;

public:
    span(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];
    }

    std::size_t size() const noexcept {
        return len_;
    }

    T* begin() noexcept {
        return ptr_;
    }

    T* end() noexcept {
        return ptr_ + len_;
    }
};

Check out C++11 version live