C++11 has given us std::add_const; with C++17, we have a new structure - std::as_const(). The former just tacks a const before the type you provide it with. The second one is a proper (template of a) function, not type trait, which seems to do the same - except for when the type is an rvalue-reference, in which case it cannot be used.
I don't quite understand the motivation for providing std::as_const(). Why do we need it in addition to std::add_const?
"Need" is a strong word... std::as_const exists because it's useful, not strictly necessary. Since it's a function rather than a trait, we can use it to "add const" to actual values rather than to types.
More specifically: Suppose I have some variable my_value and I want to treat it as a const, but not copy it. Before C++17 I would need to write:
static_cast<const MyType&>(my_value)
and if I don't want to specify the type explicitly, it would be:
static_cast
<std::add_const_t<std::remove_reference_t<decltype(my_value)>> &>
(my_value)
or if you want to get down and dirty, and use C-style casting:
(const decltype(my_value) &) (my_value)
all of which are annoying and verbose.
Instead of these, with C++17 now write std::as_const(my_value) and that's all there is to it.
Notes:
This function is disabled for rvalue references even though it works just fine for them. The reason is to help you avoid inadvertantly keeping a reference to a temporary past its destruction. As @NicolBolas explains, if you write something like:
for(auto &x : std::as_const(returns_container())) { /* do stuff with x */ }
then the returned container's lifetime ends before the first iteration of the loop. Very easy to miss!
For additional (?) information, consult the official proposition of this utility function: P007R1, by Adam David Alan Martin and Alisdair Meredith.