What does T&& (double ampersand) mean in C++11?

Question

I\'ve been looking into some of the new features of C++11 and one I\'ve noticed is the double ampersand in declaring variables, like T&& var.

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Answer 1

The term for T&& when used with type deduction (such as for perfect forwarding) is known colloquially as a forwarding reference. The term "universal reference" was coined by Scott Meyers in this article, but was later changed.

That is because it may be either r-value or l-value.

Examples are:

// template
template foo(T&& t) { ... }

// auto
auto&& t = ...;

// typedef
typedef ... T;
T&& t = ...;

// decltype
decltype(...)&& t = ...;

More discussion can be found in the answer for: Syntax for universal references