In addition to the well-known literals, like numerical constants (with or
without suffixes), character constants and string (textual) literals, bf(C++)
also supports
    emi(user-defined literals), also known as emi(extensible literals). 

A user-defined literal is defined by a function (see also section ref(UDL))
that must be defined at namespace scope. Such a function is called a
        i(literal operator).
    A literal operator cannot be a class member function.  The names of a
literal operator must start with an
    hi(identifier: initial underscore) underscore, and a literal operator is
used (called) by em(suffixing) its name (including the underscore) to the
argument that must be passed to it . Assuming tt(_NM2km) (nautical mile to km)
is the name of a literal operator, then it could be called as tt(100_NM2km),
producing, e.g., the value 185.2.

Using tt(Type) to represent the return type of the literal operator
its generic declaration looks like this:
        verb(    Type operator "" _identifier(parameter-list);)

The blank space trailing the empty string is required. The parameter lists
of literal operators can be:
    itemization(
    itt(unsigned long long int). It is used as, e.g., tt(123_identifier). The
        argument to this literal operator can be decimal constants,
        binary constants (initial i(0b)), octal constants (initial 0) and
        hexadecimal constants (initial 0x);
    itt(long double). It is used as, e.g., tt(12.25_NM2km);
    itt(char const *text). The tt(text) argument is an NTBS. It is
        used as, e.g., tt(1234_pental). The argument must em(not) be given
        double quotes, and must represent a numeric constant, as also expected
        by literal operators defining  tt(unsigned long long int)
        parameters.
    itt(char const *text, size_t len). Here, the compiler determines tt(len)
        as if it had called tt(strlen(text)). It is used as, e.g.,
        tt("hello"_nVowels);
    itt(wchar_t const *text, size_t len), same as the previous one, but
        accepting a string of tt(wchar_t) characters. It is used as, e.g.,
        tt(L"1234"_charSum);
    itt(char16_t const *text, size_t len), same as the previous one, but
        accepting a string of tt(char16_t) characters. It is used as, e.g.,
        tt(u"utf 16"_uc);
    itt(char32_t const *text, size_t len), same as the previous one, but
        accepting a string of tt(char32_t) characters. It is used as, e.g.,
        tt(U"UTF 32"_lc);
    )
    If literal operators are overloaded the compiler will pick the literal
operator requiring the least `effort'. E.g., 120 is processed by a literal
operator defining an tt(unsigned long long int) parameter and not by its
overloaded version, defining a tt(char const *) parameter. But if overloaded
literal operators exist defining tt(char const *) and tt(long double)
parameters then the operator defining a tt(char const *) parameter is used
when the argument 120 is provided, while the operator defining a tt(long
double) parameter is used with the argument 120.3.

    A literator operator can define any return type. Here is
an example of a definition of the tt(_NM2km) literal operator:
        verb(    double operator "" _NM2km(char const *nm)
    {
        return std::stod(nm) * 1.852;
    }

    double value = 120_NM2km;   // example of use)

Of course, the argument could also have been a tt(long double)
constant. Here's an alternative implementation, explicitly expecting a tt(long
double):

        verb(    double constexpr operator "" _NM2km(long double nm)
    {
        return nm * 1.852;
    }

    double value = 450.5_NM2km;   // example of use)

A numeric constant can also be processed completely at compile-time.
Section ref(UDL) provides the details of this type of literal operator.

    Arguments to literal operators are themselves always constants. A literal
operator like tt(_NM2km) cannot be used to convert, e.g., the value of a
variable. A literal operator, although it is defined as a function, cannot be
called like a function. The following examples therefore
result in compilation errors:
        verb(    double speed;

    speed_NM2km;        // no identifier 'speed_NM2km'
    _NM2km(speed);      // no function _NM2km
    _NM2km(120.3);      // no function _NM2km)

