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Diffstat (limited to 'lib/util.h')
| -rw-r--r-- | lib/util.h | 205 |
1 files changed, 205 insertions, 0 deletions
diff --git a/lib/util.h b/lib/util.h new file mode 100644 index 00000000..65de0610 --- /dev/null +++ b/lib/util.h @@ -0,0 +1,205 @@ +/****************************************************************************** + * Copyright (C) 2016 Kitsune Ral <kitsune-ral@users.sf.net> + * + * This library is free software; you can redistribute it and/or + * modify it under the terms of the GNU Lesser General Public + * License as published by the Free Software Foundation; either + * version 2.1 of the License, or (at your option) any later version. + * + * This library 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 GNU + * Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this library; if not, write to the Free Software + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA + */ + +#pragma once + +#include <QtCore/QMetaEnum> +#include <QtCore/QDebug> + +#include <functional> + +namespace QMatrixClient +{ + /** + * @brief Lookup a value by a key in a varargs list + * + * This function template takes the value of its first argument (selector) + * as a key and searches for it in the key-value map passed in + * a parameter pack (every next pair of arguments forms a key-value pair). + * If a match is found, the respective value is returned; if no pairs + * matched, the last value (fallback) is returned. + * + * All options should be of the same type or implicitly castable to the + * type of the first option. If you need some specific type to cast to + * you can explicitly provide it as the ValueT template parameter + * (e.g. <code>lookup<void*>(parameters...)</code>). Note that pointers + * to methods of different classes and even to functions with different + * signatures are of different types. If their return types are castable + * to some common one, @see dispatch that deals with this by swallowing + * the method invocation. + * + * Below is an example of usage to select a parser depending on contents of + * a JSON object: + * {@code + * auto parser = lookup(obj.value["type"].toString(), + * "type1", fn1, + * "type2", fn2, + * fallbackFn); + * parser(obj); + * } + * + * The implementation is based on tail recursion; every recursion step + * removes 2 arguments (match and value). There's no selector value for the + * fallback option (the last one); therefore, the total number of lookup() + * arguments should be even: selector + n key-value pairs + fallback + * + * @note Beware of calling lookup() with a <code>const char*</code> selector + * (the first parameter) - most likely it won't do what you expect because + * of shallow comparison. + */ + template <typename ValueT, typename SelectorT> + ValueT lookup(SelectorT/*unused*/, ValueT&& fallback) + { + return std::forward<ValueT>(fallback); + } + + template <typename ValueT, typename SelectorT, typename KeyT, typename... Ts> + ValueT lookup(SelectorT&& selector, KeyT&& key, ValueT&& value, Ts&&... remainder) + { + if( selector == key ) + return std::forward<ValueT>(value); + + // Drop the failed key-value pair and recurse with 2 arguments less. + return lookup<ValueT>(std::forward<SelectorT>(selector), + std::forward<Ts>(remainder)...); + } + + /** + * A wrapper around lookup() for functions of different types castable + * to a common std::function<> form + * + * This class uses std::function<> magic to first capture arguments of + * a yet-unknown function or function object, and then to coerce types of + * all functions/function objects passed for lookup to the type + * std::function<ResultT(ArgTs...). Without Dispatch<>, you would have + * to pass the specific function type to lookup, since your functions have + * different signatures. The type is not always obvious, and the resulting + * construct in client code would almost always be rather cumbersome. + * Dispatch<> deduces the necessary function type (well, almost - you still + * have to specify the result type) and hides the clumsiness. For more + * information on what std::function<> can wrap around, see + * https://cpptruths.blogspot.jp/2015/11/covariance-and-contravariance-in-c.html + * + * The function arguments are captured by value (i.e. copied) to avoid + * hard-to-find issues with dangling references in cases when a Dispatch<> + * object is passed across different contexts (e.g. returned from another + * function). + * + * \tparam ResultT - the desired type of a picked function invocation (mandatory) + * \tparam ArgTs - function argument types (deduced) + */ +#if __GNUC__ < 5 && __GNUC_MINOR__ < 9 + // GCC 4.8 cannot cope with parameter packs inside lambdas; so provide a single + // argument version of Dispatch<> that we only need so far. + template <typename ResultT, typename ArgT> +#else + template <typename ResultT, typename... ArgTs> +#endif + class Dispatch + { + // The implementation takes a chapter from functional programming: + // Dispatch<> uses a function that in turn accepts a function as its + // argument. The sole purpose of the outer function (initialized by + // a lambda-expression in the constructor) is to store the arguments + // to any of the functions later looked up. The inner function (its + // type is defined by fn_t alias) is the one returned by lookup() + // invocation inside to(). + // + // It's a bit counterintuitive to specify function parameters before + // the list of functions but otherwise it would take several overloads + // here to match all the ways a function-like behaviour can be done: + // reference-to-function, pointer-to-function, function object. This + // probably could be done as well but I preferred a more compact + // solution: you show what you have and if it's possible to bring all + // your functions to the same std::function<> based on what you have + // as parameters, the code will compile. If it's not possible, modern + // compilers are already good enough at pinpointing a specific place + // where types don't match. + public: +#if __GNUC__ < 5 && __GNUC_MINOR__ < 9 + using fn_t = std::function<ResultT(ArgT)>; + explicit Dispatch(ArgT&& arg) + : boundArgs([=](fn_t &&f) { return f(std::move(arg)); }) + { } +#else + using fn_t = std::function<ResultT(ArgTs...)>; + explicit Dispatch(ArgTs&&... args) + : boundArgs([=](fn_t &&f) { return f(std::move(args)...); }) + { } +#endif + + template <typename... LookupParamTs> + ResultT to(LookupParamTs&&... lookupParams) + { + // Here's the magic, two pieces of it: + // 1. Specifying fn_t in lookup() wraps all functions in + // \p lookupParams into the same std::function<> type. This + // includes conversion of return types from more specific to more + // generic (because std::function is covariant by return types and + // contravariant by argument types (see the link in the Doxygen + // part of the comments). + auto fn = lookup<fn_t>(std::forward<LookupParamTs>(lookupParams)...); + // 2. Passing the result of lookup() to boundArgs() invokes the + // lambda-expression mentioned in the constructor, which simply + // invokes this passed function with a set of arguments captured + // by lambda. + if (fn) + return boundArgs(std::move(fn)); + + // A shortcut to allow passing nullptr for a function; + // a default-constructed ResultT will be returned + // (for pointers, it will be nullptr) + return {}; + } + + private: + std::function<ResultT(fn_t&&)> boundArgs; + }; + + /** + * Dispatch a set of parameters to one of a set of functions, depending on + * a selector value + * + * Use <code>dispatch<CommonType>(parameters).to(lookup parameters)</code> + * instead of lookup() if you need to pick one of several functions returning + * types castable to the same CommonType. See event.cpp for a typical use case. + * + * \see Dispatch + */ + template <typename ResultT, typename... ArgTs> + Dispatch<ResultT, ArgTs...> dispatch(ArgTs&& ... args) + { + return Dispatch<ResultT, ArgTs...>(std::forward<ArgTs>(args)...); + } + + // The below enables pretty-printing of enums in logs +#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0)) +#define REGISTER_ENUM(EnumName) Q_ENUM(EnumName) +#else + // Thanks to Olivier for spelling it and for making Q_ENUM to replace it: + // https://woboq.com/blog/q_enum.html +#define REGISTER_ENUM(EnumName) \ + Q_ENUMS(EnumName) \ + friend QDebug operator<<(QDebug dbg, EnumName val) \ + { \ + static int enumIdx = staticMetaObject.indexOfEnumerator(#EnumName); \ + return dbg << Event::staticMetaObject.enumerator(enumIdx).valueToKey(int(val)); \ + } +#endif +} // namespace QMatrixClient + |