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67 lines
3.3 KiB
Plaintext
67 lines
3.3 KiB
Plaintext
[/
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Copyright 2011 - 2020 John Maddock.
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Copyright 2013 - 2019 Paul A. Bristow.
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Copyright 2013 Christopher Kormanyos.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt).
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]
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[section:logged_adaptor logged_adaptor]
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`#include <boost/multiprecision/logged_adaptor.hpp>`
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namespace boost{ namespace multiprecision{
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template <class Backend>
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void log_postfix_event(const Backend& result, const char* event_description);
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template <class Backend, class T>
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void log_postfix_event(const Backend& result1, const T& result2, const char* event_description);
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template <class Backend>
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void log_prefix_event(const Backend& arg1, const char* event_description);
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template <class Backend, class T>
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void log_prefix_event(const Backend& arg1, const T& arg2, const char* event_description);
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template <class Backend, class T, class U>
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void log_prefix_event(const Backend& arg1, const T& arg2, const U& arg3, const char* event_description);
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template <class Backend, class T, class U, class V>
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void log_prefix_event(const Backend& arg1, const T& arg2, const U& arg3, const V& arg4, const char* event_description);
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template <Backend>
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class logged_adaptor;
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template <class Number>
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using logged_adaptor_t = number<logged_adaptor<typename Number::backend_type>, Number::et>;
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}} // namespaces
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The `logged_adaptor` type is used in conjunction with `number` and some other backend type: it acts as a thin wrapper around
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some other backend to class `number` and logs all the events that take place on that object. Before any number operation takes
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place, it calls `log_prefix_event` with the arguments to the operation (up to 4), plus a string describing the operation.
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Then after the operation it calls `log_postfix_event` with the result of the operation, plus a string describing the operation.
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Optionally, `log_postfix_event` takes a second result argument: this occurs when the result of the operation is not a `number`,
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for example when `fpclassify` is called, `log_postfix_event` will be called with `result1` being the argument to the function, and
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`result2` being the integer result of `fpclassify`.
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The default versions of `log_prefix_event` and `log_postfix_event` do nothing, it is therefore up to the user to overload these
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for the particular backend being observed.
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This type provides `numeric_limits` support whenever the template argument Backend does so.
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Template alias `logged_adaptor_t` can be used as a shortcut for converting some instantiation of `number<>` to its logged equivalent.
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This type is particularly useful when combined with an interval number type - in this case we can use `log_postfix_event`
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to monitor the error accumulated after each operation. We could either set some kind of trap whenever the accumulated error
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exceeds some threshold, or simply print out diagnostic information. Using this technique we can quickly locate the cause of
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numerical instability in a particular routine. The following example demonstrates this technique in a trivial algorithm
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that deliberately introduces cancellation error:
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[logged_adaptor]
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When we examine program output we can clearly see that the diameter of the interval increases after each subtraction:
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[logged_adaptor_output]
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[endsect] [/section:logged_adaptor logged_adaptor]
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