std::allocator::construct

The function constructs an object of member type value_type (an alias of allocator's template parameter) using its copy constructor to initialize its value to a copy of val, as if the following code was used:
new ((void*)p) value_type (val);

The function constructs an object of type U in-place by forwarding its arguments to the appropriate constructor, as if the following code was used:
::new ((void*)p) U (forward<Args>(args)...);

p

Pointer to a location with enough storage space to contain an element of type value_type.
pointer is a member type (defined as an alias of T* in std::allocator<T>).

val

Value to initialize the construced element to.
const_reference is a member type (defined as an alias of T& in std::allocator<T>).

p

Pointer to a location with enough storage space to contain an element of type U.

args

Arguments forwarded to the constructor.
Args is a list of zero or more types.