Recommendation for moving / shifting vector members?

[Linda A. Walsh]

I note that the 'shift' function isn't implemented for the vector
template as for valarray.

But I have a case where I want to use the  vector extensibility to hold
a variable number of items up to some max (something valarray can't do
w/no capacity()/reserve() functionality).

Is there a "memmove" type function or a "shift" type function in the C++
library to work on the valid members of a vector. 

Ex: Elements 0..N-1 => 1->N (nothing extending into the capacity, but just
moving the "valid" members).

I can think of multiple ways to do this in different containers
I.e. manually store indexes into a valarray to represent the
"valid" bounds of the sub members, or using some other container,
but was wondering if the C++ library had some way of doing it.

I'm not sure if memmove on the members is supported or not?
Are they supposed to be contiguous in memory and memmove is
'fine' -- and I'm overthinking this, or is there better
solution?  It seems like moving the backing store of the
container (i.e. memmove), could potentially break the object
paradigm and be unsupported, at least in some cases (if in any?)

Thanks & it sure would have been nice if 'shift' worked on vectors
as well as valarrays... (sigh)...