Meaning of double bracket "[[foo()]] type name;" syntax in c++?

Question

In this article about avoiding False Sharing, the following code snipped for alignment is presented:

// C++ (using C++0x alignment syntax)
template<typename T>
struct cache_line_storage {
   [[ align(CACHE_LINE_SIZE) ]] T data;
   char pad[ CACHE_LINE_SIZE > sizeof(T)
        ? CACHE_LINE_SIZE - sizeof(T)
        : 1 ];
};

What is the meaning of line 4? I've never seen this double bracket syntax before.

Answer 1

That is the attribute specifier syntax. It was introduced as a unified syntax to access what were formerly compiler-specific extensions (now some are standardized).

In this case the code is telling the compiler to align data to CACHE_LINE_SIZE bytes.