Pointer arithmetic with array_view

Question

I've just seen the first part of Herb Sutter's "Writing Good C++14... By Default" (https://www.youtube.com/watch?v=hEx5DNLWGgA) and i have a (possible stupid) question regarding array_view.

The presented case was sending an array_view instead of a pointer and length in order to avoid pointer arithmetic, but how can array_view handle a case like this:

int vec[10];
func(vec+2, 5); //start from the 2nd element and process 5 of them

Does array_view support this kind of stuff or i just got the use-case wrong?

Answer 1

With string_view you can do this:

const char* str = "hello world!"; 
func(std::experimental::string_view(str + 2, 5));

That is use one of the constructors of the view to to build it from a substring.

So with array_view you'll probably be able to do this:

int vec[10];
func(std::experimental::array_view(vec + 2, 5));

Note: There doesn't seem to be any official array_view as of c++14.

Answer 2

First of all, you can get the slides from cppcon's Github repo.

As you can see in #8 and #10, you can write the following:

Run It Online !

// http://llvm.org/apt/
// sudo apt-get install clang-3.6 lldb-3.6 libc++-dev libc++abi-dev
// clang-3.6 -stdlib=libc++ -std=c++14 main.cpp -lc++ -lc++abi

#include <array>
#include <vector>
#include "array_view.h"  // https://github.com/Microsoft/GSL

void func(gsl::array_view<int> av)
{
    // ...
}

int main()
{
    {
        int vec[10];
        func(vec);
        //func(vec, 5);    // syntax error (func expects exactly 1 argument)
        func({vec, 5});    // notice the curly braces
        func({vec+2, 5});
    }

    {
        std::vector<int> vec;
        func(vec);
    }

    {
        size_t len = 10;
        int* p = new int[10];
        func({p,len});
        // remember to delete[] p
    }

    {
        std::array<int, 2> arr;
        func(arr);
    }
}

And that makes sense. If you look at array_view.h, you'll see all of array_view's constructors:

constexpr array_view(pointer ptr, bounds_type bounds)
constexpr array_view(T * const & data, size_type size)
constexpr array_view(T(&arr)[N], size_type size)
constexpr array_view (const std::array<std::remove_const_t<value_type>, N> & arr)
// ...