c++ rvalue reference and const qualifier

Question

Among the many benefits of const qualification is to make an API more understandable, example:

template int function1(T const& in);
// clea         


        

Answer 1

You could say this:

template 
typename std::enable_if::value, int>::type
function(T && in)
{
   // ...
}

where you have something like:

template  struct immutable
: std::integral_constant::value> {};

template  struct immutable
: std::true_type {};

This way, the template will only be usable if the universal reference is either a const-reference (so T = U const &) or an rvalue-reference (so T is not a reference).

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That said, if the argument is not going to be changed, you could just use T const & and be done with it, since there's nothing to be gained from binding mutably to temporary values.