Function template parameters are either template type parameters or template
non-type parameters (actually, a third type of template parameter exists, the
em(template template parameter), which is discussed in chapter
ref(ADVANCEDTEMPL) (section ref(TEMPTEMPPAR))).

    Class templates hi(class template: non-type parameter) may also define
non-type parameters. Like the function template non-type parameters they must
be (integral) constants whose values must be known at object instantiation
time.

    Different from function template non-type parameters the values of class
template non-type parameters are em(not) deduced by the compiler using
arguments passed to class template members.

Assume we extend our design of the class template tt(CirQue) so that it
defines a second (non-type) parameter tt(size_t Size). Our intent is to use
this tt(Size) parameter in the constructor defining an array parameter of
tt(Size) elements of type tt(Data).

The  tt(CirQue) class template now becomes (only showing the relevant
constructor):
        verb(    template <typename Data, size_t Size>
    class CirQue
    {
        // ... data members
        public:
            CirQue(Data const (&arr)[Size]);
            ...
    };

    template <typename Data, size_t Size>
    CirQue<Data, Size>::CirQue(Data const (&arr)[Size])
    :
        d_maxSize(Size),
        d_size(0),
        d_data(operator new(Size * sizeof(Data))),
        d_front(d_data),
        d_back(d_data),
    {
        std::copy(arr, arr + Size, back_inserter(*this));
    })

Unfortunately, this setup doesn't satisfy our needs as  the values of
template non-type parameters are not deduced by the compiler. When the
compiler is asked to compile the following tt(main) function it reports a
mismatch between the required and actual number of template parameters:
        verb(    int main()
    {
        int arr[30];

        CirQue<int> ap{ arr };
    }
    /*
        Error reported by the compiler:

        In function `int main()':
            error: wrong number of template arguments (1, should be 2)
            error: provided for `template<class Data, size_t Size>
                   class CirQue'
    */)

Defining tt(Size) as a non-type parameter having a default value doesn't
work either. The compiler always uses the default unless its value is
explicitly specified. Reasoning that tt(Size) can be 0 unless we need another
value, we might be tempted to specify tt(size_t Size = 0) in the template's
parameter type list.  Doing so we create  a mismatch between the default
value 0 and the actual size of the array tt(arr) as defined in the above
tt(main) function. The compiler, using the default value, reports:
        verb(    In instantiation of `CirQue<int, 0>':
    ...
    error: creating array with size zero (`0'))

So, although class templates may use non-type parameters they must always
be specified like type parameters when an object of that class is
defined. Default values can be specified for those non-type parameters causing
the compiler to use the default when the non-type parameter is left
unspecified.

    hi(template parameter: default value) Default template parameter values
(either type or non-type template parameters) may em(not) be specified when
defining template member functions. In general: function template definitions
(and thus: class template member functions) may not be given default template
(non) type arguments. If default template arguments are to be used for class
template members, they em(have) to be specified by the class interface.

    Similar to non-type parameters of function templates default argument
values for non-type class template parameters may only be specified as
constants:
    itemization(
    it() Global hi(global variable) variables have constant addresses, which
can be used as arguments for non-type parameters.
    it() Local hi(local variables) and i(dynamically allocated variables) have
addresses that are em(not) known by the compiler when the source file is
compiled. These addresses can therefore em(not) be used as arguments for
non-type parameters.
    it() Lvalue hi(lvalue transformations) transformations are allowed: if a
pointer is defined as a non-type parameter, an array name may be specified.
    it() hi(conversions) Qualification hi(qualification conversions)
conversions are allowed: a pointer to a non-const object may be used with a
non-type parameter defined as a tt(const) pointer.
    it() Promotions hi(promotions) are allowed: a constant of a `narrower'
data type may be used when specifying a default argument for a non-type
parameter of a `wider' type (e.g., a tt(short) can be used when an tt(int) is
called for, a tt(long) when a tt(double) is called for).
    it() Integral hi(integral conversions) conversions are allowed: if a
tt(size_t) parameter is specified, an tt(int) may be used as well.
    it() Variables cannot be used to specify template non-type parameters as
their values are not constant expressions. Variables defined using the
tt(const) modifier, however, may be used as their values never change.
    )

    Although our attempts to define a constructor of the class tt(CirQue)
accepting an array as its argument has failed so far, we're not yet out of
options. In the next section a method is described that em(does) allow us
to reach our goal.
