boost_1_45_0/libs/spirit/doc/karma/num_list.qbk

[/==============================================================================
    Copyright (C) 2001-2010 Hartmut Kaiser
    Copyright (C) 2001-2010 Joel de Guzman

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[/////////////////////////////////////////////////////////////////////////////]
[section:num_list Number List - Printing Numbers From a std::vector]

[heading Using the List Operator]

The C++ Standard library lacks an important feature, namely the support for
any formatted output of containers. Sure, it's fairly easy to write a custom 
routine to output a specific container, but doing so over and over again is 
tedious at best. In this section we will demonstrate some more of the 
capabilities of __karma__ for generating output from arbitrary STL containers. 
We will build on the example presented in an earlier section (see 
[link spirit.karma.tutorials.warming_up Warming Up]).

The full source code of the example shown in this section can be found here: 
[@../../example/karma/num_list2.cpp num_list2.cpp].

[import ../../example/karma/num_list2.cpp]

This time we take advantage of Karma's __karma_list__ operator. The semantics 
of the list operator are fully equivalent to the semantics of the sequence
we used before. The generator expression

    double_ << *(',' << double_)

is semantically equivalent to the generator expression

    double_ % ','

simplifying the overall code. The list operator's attribute is compatible with 
any STL container as well. For a change we use a `std::vector<double>`
instead of the `std::list<double>` we used before. Additionally, the routine
`generate_numbers` takes the container as a template parameter, so it will now 
work with any STL container holding `double` numbers.

[tutorial_karma_numlist2]

[note Despite the container being a template parameter, the __karma__ 
      formatting expression (`double_ % ','`) does not depend on the actual 
      type of the passed container. The only precondition to be met here is 
      that the elements stored in the container have to be convertible to 
      `double`.]

[heading Generate Output from Arbitrary Data]

The output routine developed above is still not generically usable for all types 
of STL containers and for arbitrary elements stored in them. In order to be 
usable the items stored in the container still need to be convertible to a 
`double`. Fortunately __karma__ is capable to output arbitrary 
data types while using the same format description expression. It implements 
the [karma_stream `stream`] generators which are able to consume any attribute
type as long as a matching standard streaming operator is defined. I.e. 
for any attribute type `Attrib` a function:

    std::ostream& operator<< (std::ostream&, Attrib const&);

needs to be available. The [karma_stream `stream`] generator will use the 
standard streaming operator to generate the output. 

The following example modifies the code shown above to utilize the 
[karma_stream `stream`] operator, which makes it compatible with almost any 
data type. We implement a custom data type `complex` to demonstrate this. The 
example shows how it is possible to integrate this (or any other) custom data 
type into the __karma__ generator framework.

[import ../../example/karma/num_list3.cpp]

This is the custom data structure together with the required standard streaming 
operator:

[tutorial_karma_numlist3_complex]

And this is the actual call to generate the output from a vector of those. This
time we interleave the generated output with newline breaks (see 
__karma_eol__), putting each complex number onto a separate line:

[tutorial_karma_numlist3]

The code shown is fully generic and can be used with any STL container as long 
as the data items stored in that container implement the standard streaming 
operator.

The full source code of the example presented in this section can be found here: 
[@../../example/karma/num_list3.cpp num_list3.cpp].

[endsect]

[/////////////////////////////////////////////////////////////////////////////]
[section:num_matrix Matrix of Numbers - Printing Numbers From a Matrix]

In this section we will discuss the possibilities of __karma__ when it comes to
generating output from more complex - but still regular - data structures.
For simplicity we will use a `std::vector<std::vector<int> >` as a poor 
man's matrix representation. But even if the data structure seems to be very 
simple, the presented principles are applicable to more complex, or custom
data structures as well. The full source code of the example discussed in this 
section can be found here: [@../../example/karma/num_matrix.cpp num_matrix.cpp].

[import ../../example/karma/num_matrix.cpp]

[endsect]