type_erasure/example/print_sequence.cpp

// Boost.TypeErasure library
//
// Copyright 2011 Steven Watanabe
//
// 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)
//
// $Id$

//[print_sequence

/*`
    (For the source of this example see
    [@boost:/libs/type_erasure/example/print_sequence.cpp print_sequence.cpp])

    This example defines a class hierarchy that allows a sequence
    to be formatted in several different ways.  We'd like to be
    able to handle any sequence and any stream type, since the
    range formatting is independent of the formatting of
    individual elements.  Thus, our interface needs to look
    something like this:

    ``
        class abstract_printer {
        public:
            template<class CharT, class Traits, class Range>
            virtual void print(std::basic_ostream<CharT, Traits>& os, const Range& r) const = 0;
        };
    ``

    Unfortunately, this is illegal because a virtual function
    cannot be a template.  However, we can define a
    class with much the same behavior using Boost.TypeErasure.
*/

#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/iterator.hpp>
#include <boost/type_erasure/operators.hpp>
#include <boost/type_erasure/tuple.hpp>
#include <boost/type_erasure/same_type.hpp>
#include <boost/range/begin.hpp>
#include <boost/range/end.hpp>
#include <boost/range/iterator.hpp>
#include <iostream>
#include <iomanip>
#include <vector>

using namespace boost::type_erasure;

struct _t : placeholder {};
struct _iter : placeholder {};
struct _os : placeholder {};

template<class T, class U = _self>
struct base_and_derived
{
    static T& apply(U& arg) { return arg; }
};

namespace boost {
namespace type_erasure {

template<class T, class U, class Base>
struct concept_interface<base_and_derived<T, U>, Base, U> : Base
{
    operator typename rebind_any<Base, const T&>::type() const
    {
        return call(base_and_derived<T, U>(), const_cast<concept_interface&>(*this));
    }
    operator typename rebind_any<Base, T&>::type()
    {
        return call(base_and_derived<T, U>(), *this);
    }
};

}
}

// abstract_printer - An abstract base class for formatting sequences.
class abstract_printer {
public:
    // print - write a sequence to a std::ostream in a manner
    //   specific to the derived class.
    //
    // Requires: Range must be a Forward Range whose elements can be
    //   printed to os.
    template<class CharT, class Traits, class Range>
    void print(std::basic_ostream<CharT, Traits>& os, const Range& r) const {
        // Capture the arguments
        typename boost::range_iterator<const Range>::type
            first(boost::begin(r)),
            last(boost::end(r));
        tuple<requirements, _os&, _iter, _iter> args(os, first, last);
        // and forward to the real implementation
        do_print(get<0>(args), get<1>(args), get<2>(args));
    }
    virtual ~abstract_printer() {}
protected:
    // define the concept requirements of the arguments of
    // print and typedef the any types.
    typedef boost::mpl::vector<
        base_and_derived<std::ios_base, _os>,
        ostreamable<_os, _t>,
        ostreamable<_os, const char*>,
        forward_iterator<_iter, const _t&>,
        same_type<_t, forward_iterator<_iter, const _t&>::value_type>
    > requirements;
    typedef boost::type_erasure::any<requirements, _os&> ostream_type;
    typedef boost::type_erasure::any<requirements, _iter> iterator_type;
    // do_print - This method must be implemented by derived classes
    virtual void do_print(
        ostream_type os, iterator_type first, iterator_type last) const = 0;
};

// separator_printer - writes the elements of a sequence
//   separated by a fixed string.  For example, if
//   the separator is ", " separator_printer produces
//   a comma separated list.
class separator_printer : public abstract_printer {
public:
    explicit separator_printer(const std::string& sep) : separator(sep) {}
protected:
    virtual void do_print(
        ostream_type os, iterator_type first, iterator_type last) const {
        if(first != last) {
            os << *first;
            ++first;
            for(; first != last; ++first) {
                os << separator.c_str() << *first;
            }
        }
    }
private:
    std::string separator;
};

// column_separator_printer - like separator_printer, but
//   also inserts a line break after every n elements.
class column_separator_printer : public abstract_printer {
public:
    column_separator_printer(const std::string& sep, std::size_t num_columns)
      : separator(sep),
        cols(num_columns)
    {}
protected:
    virtual void do_print(
        ostream_type os, iterator_type first, iterator_type last) const {
        std::size_t count = 0;
        for(; first != last; ++first) {
            os << *first;
            boost::type_erasure::any<requirements, _iter> temp = first;
            ++temp;
            if(temp != last) {
                os << separator.c_str();
            }
            if(++count % cols == 0) {
                os << "\n";
            }
        }
    }
private:
    std::string separator;
    std::size_t cols;
};

// aligned_column_printer - formats a sequence in columns
//   reading down.  For example, given the sequence
//   { 1, 2, 3, 4, 5 }, aligned_column_printer might print
//   1   4
//   2   5
//   3
class aligned_column_printer : public abstract_printer {
public:
    aligned_column_printer(std::size_t column_width, std::size_t num_columns)
      : width(column_width),
        cols(num_columns)
    {}
protected:
    virtual void do_print(
        ostream_type os, iterator_type first, iterator_type last) const
    {
        if(first == last) return;
        std::vector<iterator_type> column_iterators;

        // find the tops of the columns
        std::size_t count = 0;
        for(iterator_type iter = first; iter != last; ++iter) {
            ++count;
        }
        std::size_t rows = (count + cols - 1) / cols;
        count = 0;
        for(iterator_type iter = first; iter != last; ++iter) {
            if(count % rows == 0) {
                column_iterators.push_back(iter);
            }
            ++count;
        }

        iterator_type last_col = column_iterators.back();

        // print the full rows
        while(column_iterators.back() != last) {
            for(std::vector<iterator_type>::iterator
                iter = column_iterators.begin(),
                end = column_iterators.end(); iter != end; ++iter)
            {
                static_cast<std::ios_base&>(os).width(width);
                os << **iter;
                ++*iter;
            }
            os << "\n";
        }

        // print the rows that are missing the last column
        column_iterators.pop_back();
        if(!column_iterators.empty()) {
            while(column_iterators.back() != last_col) {
                for(std::vector<iterator_type>::iterator
                    iter = column_iterators.begin(),
                    end = column_iterators.end(); iter != end; ++iter)
                {
                    static_cast<std::ios_base&>(os).width(width);
                    os << **iter;
                    ++*iter;
                }
                os << "\n";
            }
        }
    }
private:
    std::size_t width;
    std::size_t cols;
};

int main() {
    int test[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    separator_printer p1(",");
    p1.print(std::cout, test);
    std::cout << std::endl;
    column_separator_printer p2(",", 4);
    p2.print(std::cout, test);
    std::cout << std::endl;
    aligned_column_printer p3(16, 4);
    p3.print(std::cout, test);
}

//]