libcruft-util/except.hpp

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/*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/.
*
* Copyright 2018 Danny Robson <danny@nerdcruft.net>
*/
#pragma once
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#include <utility>
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#include <iosfwd>
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#include <string>
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namespace cruft {
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/// A base exception class for all cruft libraries.
///
/// There is deliberately no `what` method as it's not always acceptable
/// to store a string that we can return a pointer to. Instead we rely on
/// the `describe` method to output to a std::ostream. The user can wrangle
/// that into a std::string if they need one.
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class error {
public:
virtual ~error () = default;
virtual std::ostream& describe (std::ostream&) const = 0;
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};
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/// A mixin error class that can be used when inheriting from
/// `cruft::error`.
///
/// It accepts a single std::string, saving it so that it can be passed
/// through to a std::ostream later on.
///
/// By setting `BaseT` to a locally useful error class we can still
/// maintain an appropriate exception hierarchy for a given module.
/// eg, pass `foo::error` (which derives eventually from `cruft::error`)
/// in the module `foo`.
///
/// \tparam BaseT The desired base class.
template <typename BaseT>
class string_error : public BaseT {
public:
template <typename... Args>
string_error (std::string const &_message, Args &&...args)
: BaseT (std::forward<Args> (args)...)
, m_message (_message)
{ ; }
std::ostream&
describe (std::ostream &os) const override
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{
return os << m_message;
}
private:
std::string m_message;
};
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/// Use `error::describe` to render the supplied error object to a
/// std::ostream.
std::ostream&
operator<< (std::ostream &os, error const&);
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}