Strange declaration with using in C++

Question

On a piece of code in a previous question in stackoverflow I saw this, strange to me, declaration with using:

template <std::size_t SIZE> 
class A
{
public:
  ...
  using const_buffer_t = const char(&)[SIZE];
  ...
};

Could someone please address the following questions:

1. What type it represents?
2. Where do we need such kind of declarations?

Answer 1

That's a type alias, a new syntax available since c++11.

What you're actually doing is typedefing the type of an array

const_buffer_t 

will be an array of const char with length = SIZE

Answer 2

That using declaration is a new syntax introduced in C++11; it introduces a type alias, specifying that const_buffer_t is now an alias for the type const char(&)[SIZE]. In this respect, this use of using is substantially identical to a typedef (although using type aliases are more flexible).

As for the actual type we are talking about (const char(&)[SIZE]), it's a reference to an array of size SIZE; references to array are rarely used, but can have their use:

• if in some function you want to enforce receiving a reference to an array of a specific size instead of a generic pointer, you can do that with array references (notice that even if you write int param[5] in a function declaration it's parsed as int *);
• the same holds for returing references to array (documenting explicitly that you are returning a reference to an array of a specific size);
• more importantly, if you want to allocate dynamically "true" multidimensional arrays (as opposed to either an array of pointers to monodimensional array or a "flat array" with "manual 2d addressing") you have to use them.

See also the array FAQ, where much of this stuff is explained in detail.