From bb26ca86ad350f2562b51284e7c631b1e4f77106 Mon Sep 17 00:00:00 2001 From: Kitsune Ral Date: Thu, 18 May 2017 03:37:06 +0900 Subject: util.h: lookup() uses forwarding refs; added Dispatch/dispatch and REGISTER_ENUM The Dispatch<> template and dispatch(), a facility function for it, simplify dispatching to functions that have different signatures that still can be converted to the same std::function<> type. The case in point is in event.cpp; Event::fromJson calls make() that always returns the type we need; however, once we have several possible base types (Event, RoomEvent, StateEvent), we'd have to either write a specific make() incarnation for each of them, or mess with function return type conversions. Dispatch<> helps to keep the code clean. REGISTER_ENUM is a cross-Qt versions approach to dumping enumeration values to qDebug() and the likes. --- util.h | 144 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 131 insertions(+), 13 deletions(-) (limited to 'util.h') diff --git a/util.h b/util.h index 79f76860..f2a7b267 100644 --- a/util.h +++ b/util.h @@ -18,6 +18,11 @@ #pragma once +#include +#include + +#include + namespace QMatrixClient { /** @@ -71,14 +76,19 @@ 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 varargs list - * (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. + * 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. Note that pointers to methods of different - * classes are of different object types, in particular. + * 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. lookup(parameters...)). 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: @@ -91,7 +101,7 @@ namespace QMatrixClient * } * * The implementation is based on tail recursion; every recursion step - * removes 2 arguments (match and option). There's no selector value for the + * 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 * @@ -99,20 +109,128 @@ namespace QMatrixClient * (the first parameter) - most likely it won't do what you expect because * of shallow comparison. */ + template + ValueT lookup(SelectorT/*unused*/, ValueT&& fallback) + { + return std::forward(fallback); + } + template - ValueT lookup(SelectorT selector, KeyT key, ValueT value, Ts... remainingMapping) + ValueT lookup(SelectorT&& selector, KeyT&& key, ValueT&& value, Ts&&... remainder) { if( selector == key ) - return value; + return std::forward(value); // Drop the failed key-value pair and recurse with 2 arguments less. - return lookup(selector, remainingMapping...); + return lookup(std::forward(selector), + std::forward(remainder)...); } - template - ValueT lookup(SelectorT/*unused*/, ValueT fallback) + /** + * 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, 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) + */ + template + class Dispatch { - return fallback; + // We take a chapter from functional programming here: 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. For + // each of these overloads, we'd have to carefully retrieve the list + // of parameters and cover up possible reference-vs-value + // incompatibilities. Instead, 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. + using fn_t = std::function; + public: + explicit Dispatch(ArgTs&&... args) + : boundArgs([=](fn_t &&f) { + return f(std::forward(args)...); + }) + { } + + template + 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). + // 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. + return boundArgs( + lookup(std::forward(lookupParams)...)); + } + + private: + std::function boundArgs; + }; + + /** + * Dispatch a set of parameters to one of a set of functions, depending on + * a selector value + * + * Use dispatch(parameters).to(lookup parameters) + * 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 + Dispatch dispatch(ArgTs&& ... args) + { + return Dispatch(std::forward(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 -- cgit v1.2.3 From f51d9ba6c4aa7d682eac7aae49a025721d298045 Mon Sep 17 00:00:00 2001 From: Kitsune Ral Date: Mon, 22 May 2017 10:10:05 +0900 Subject: Dispatch<>: Allow nullptr for a function; special-case GCC 4.8 --- util.h | 50 +++++++++++++++++++++++++++++++++----------------- 1 file changed, 33 insertions(+), 17 deletions(-) (limited to 'util.h') diff --git a/util.h b/util.h index f2a7b267..c9e2d1c9 100644 --- a/util.h +++ b/util.h @@ -150,12 +150,18 @@ namespace QMatrixClient * \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 +#else template +#endif class Dispatch { - // We take a chapter from functional programming here: Dispatch<> - // uses a function that in turn accepts a function as its argument. - // The sole purpose of the outer function (initialized by + // 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() @@ -164,21 +170,25 @@ namespace QMatrixClient // 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. For - // each of these overloads, we'd have to carefully retrieve the list - // of parameters and cover up possible reference-vs-value - // incompatibilities. Instead, 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. - using fn_t = std::function; + // 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; + explicit Dispatch(ArgT&& arg) + : boundArgs([=](fn_t &&f) { return f(std::move(arg)); }) + { } +#else + using fn_t = std::function; explicit Dispatch(ArgTs&&... args) - : boundArgs([=](fn_t &&f) { - return f(std::forward(args)...); - }) + : boundArgs([=](fn_t &&f) { return f(std::move(args)...); }) { } +#endif template ResultT to(LookupParamTs&&... lookupParams) @@ -190,12 +200,18 @@ namespace QMatrixClient // 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(std::forward(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. - return boundArgs( - lookup(std::forward(lookupParams)...)); + 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: -- cgit v1.2.3