Subject: [Boost-commit] svn:boost r80604 - sandbox/type_erasure/libs/type_erasure/example
From: steven_at_[hidden]
Date: 2012-09-19 18:52:25
Author: steven_watanabe
Date: 2012-09-19 18:52:25 EDT (Wed, 19 Sep 2012)
New Revision: 80604
URL: http://svn.boost.org/trac/boost/changeset/80604
Log:
More description of the print_sequence example.
Text files modified:
sandbox/type_erasure/libs/type_erasure/example/print_sequence.cpp | 42 +++++++++++++++++++++++++++++++++++++--
1 files changed, 39 insertions(+), 3 deletions(-)
Modified: sandbox/type_erasure/libs/type_erasure/example/print_sequence.cpp
==============================================================================
--- sandbox/type_erasure/libs/type_erasure/example/print_sequence.cpp (original)
+++ sandbox/type_erasure/libs/type_erasure/example/print_sequence.cpp 2012-09-19 18:52:25 EDT (Wed, 19 Sep 2012)
@@ -14,8 +14,24 @@
(For the source of this example see
[@boost:/libs/type_erasure/example/print_sequence.cpp print_sequence.cpp])
- This example demonstrates using Boost.TypeErasure to implement a
- virtual "template" function.
+ 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>
@@ -61,9 +77,14 @@
}
}
+// abstract_printer - An abstract base class for formatting sequences.
class abstract_printer {
public:
- // Range must be a Forward Range whose elements can be printed to os.
+ // 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
@@ -76,6 +97,8 @@
}
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>,
@@ -85,10 +108,15 @@
> 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) {}
@@ -107,6 +135,8 @@
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)
@@ -134,6 +164,12 @@
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)