.. _more-structures-const-parameters:

``const`` parameters
--------------------

You might have noticed that the parameters for ``after`` and ``add_time``
are being passed by reference. Since these are pure functions, they do
not modify the parameters they receive, so I could just as well have
passed them by value.

The advantage of passing by value is that the calling function and the
callee are appropriately encapsulated—it is not possible for a change in
one to affect the other, except by affecting the return value.

On the other hand, passing by reference usually is more efficient,
because it avoids copying the argument. Furthermore, there is a nice
feature in C++, called ``const``, that can make reference parameters
just as safe as value parameters.


.. index::
   single: constant reference
   single: constant reference parameter

If you are writing a function and you do not intend to modify a
parameter, you can declare that it is a **constant reference
parameter**. The syntax looks like this:

::

   void print_time (const time& time) ...
   time add_time (const time& t1, const time& t2) ...

I've included only the first line of the functions.
If you tell the compiler that you don't intend to change a parameter,
it can help remind you.
If you try to change one, you should get a compiler error, 
or at least a warning.

.. tb-match::
   :name: const_parameters_1

   Match the action to a benefit of its use.

   passing by value
      the calling function and the callee are appropriately encapsulated—it is not possible for a change in one to affect the other, except by affecting the return value.
   passing by reference
      more efficient, because it avoids copying the argument

.. tb-choice::
   :name: const_parameters_2

   Which statement does NOT describe the ``const`` as a parameter feature in C++?

   - [ ] Makes reference parameters just as safe as value parameters.

     This is true.
   - [x] Can be used when you intend to modify a parameter.

     It is used when you are writing a function and you do not intend to modify a parameter.
   - [ ] The syntax can look like this: void print_time (const time& time)...

     This is true