Bind temporary rvalue to reference lvalue in std::vector constructors

Question

Until now, I thought that we cannot pass a temporary rvalue to a lvalue reference. But just recently, I took a closer look on the fill constructor of std::vector:

explicit vector (size_type n, const value_type& val = value_type(),
                 const allocator_type& alloc = allocator_type());

const value_type& val is a reference lvalue and = value_type() is a anonymous rvalue, in my understanding. Why is it possible? Isn't the temporary rvalue immediately destroyed, so that the reference has nothing to point to?

Answer 1

The relevant part of the standard is in [class.temporary] ( §12.2/5 in N4140):

A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full-expression containing the call.

So the temporary value_type() will be bound to val and will persist for the duration of the constructor.