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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <memory>
#include <string>
#include <vector>
namespace mongo {
/**
* Provides a mechanism for responding to an active exception with
* a dynamic registry of handlers for exception types.
*
* A handler is selected by probing the handlers in LIFO order until
* one of them accepts the exception. That handler invokes its registered
* callback, passing the exception along with any extra parameters to
* `doCatch(...)`. The types of these extra parameters are the `Args...` of
* `DynamicCatch<Args...>`.
*
* DynamicCatch<A0, A1> dc;
* dc.addCatch<Ex0>(f0);
* dc.addCatch<Ex1>(f1);
* dc.addCatch<Ex2>(f2);
* dc.doCatch(a0, a1);
*
* Is roughly equivalent to:
*
* try { throw; }
* catch (const Ex2& ex) { f2(ex, a0, a1); }
* catch (const Ex1& ex) { f1(ex, a0, a1); }
* catch (const Ex0& ex) { f0(ex, a0, a1); }
*/
template <typename... Args>
class DynamicCatch {
public:
/**
* Add a probe for exception type `Ex`. If an exception `const Ex& ex` is
* caught by `doCatch(...args)`, then `f(ex, ...args)` will be invoked.
*/
template <typename Ex, typename F>
void addCatch(F f) {
_handlers.push_back(std::make_unique<Handler<Ex, F>>(std::move(f)));
}
/**
* May only be called while an exception is active. Visits each handler
* starting from the back, until one accepts the exception. If no handler
* accepts the active exception, it will be rethrown by this function.
*/
void doCatch(Args... args) {
for (auto iter = _handlers.rbegin(); iter != _handlers.rend(); ++iter)
if ((*iter)->tryRun(args...))
return;
throw; // uncaught
}
private:
/** A type-erased exception handler. */
struct AbstractHandler {
virtual ~AbstractHandler() = default;
/**
* Handlers must try to rethrow and catch the active exception. If it
* is caught, they may apply the given `args` to some action.
*
* Returns true if the exception was accepted by this handler.
*/
virtual bool tryRun(Args... args) const = 0;
};
/**
* Handler that invokes `f(ex, args...)` if an exception `ex` of type `Ex`
* is active.
*/
template <typename Ex, typename F>
struct Handler : AbstractHandler {
Handler(F f) : f(std::move(f)) {}
bool tryRun(Args... args) const override {
try {
throw;
} catch (const Ex& ex) {
f(ex, args...);
return true;
} catch (...) {
return false;
}
}
F f;
};
std::vector<std::unique_ptr<AbstractHandler>> _handlers;
};
} // namespace mongo
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