Ichthyostega
5191073558
A simple yet weird workaround (and basically equivalent to our helper function) is to wrap the argument tuple itself into std::forward<Args> -- which has the effect of exposing RValue references to the forwarding function, thus silencing the compiler. I am not happy with this result, since it contradicts the notion of perfect forwarding. As an asside, the ressearch has sorted out some secondary suspicions.. - it is *not* the Varargs argument pack as such - it is *not* the VerbToken type as such The problem clearly is related to exposing tuple elements to a forwarding function.
143 lines
4.9 KiB
C++
143 lines
4.9 KiB
C++
/* try.cpp - for trying out some language features....
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* scons will create the binary bin/try
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*
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*/
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// 8/07 - how to control NOBUG??
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// execute with NOBUG_LOG='ttt:TRACE' bin/try
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// 1/08 - working out a static initialisation problem for Visitor (Tag creation)
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// 1/08 - check 64bit longs
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// 4/08 - comparison operators on shared_ptr<Asset>
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// 4/08 - conversions on the value_type used for boost::any
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// 5/08 - how to guard a downcasting access, so it is compiled in only if the involved types are convertible
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// 7/08 - combining partial specialisation and subclasses
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// 10/8 - abusing the STL containers to hold noncopyable values
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// 6/09 - investigating how to build a mixin template providing an operator bool()
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// 12/9 - tracking down a strange "warning: type qualifiers ignored on function return type"
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// 1/10 - can we determine at compile time the presence of a certain function (for duck-typing)?
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// 4/10 - pretty printing STL containers with python enabled GDB?
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// 1/11 - exploring numeric limits
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// 1/11 - integer floor and wrap operation(s)
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// 1/11 - how to fetch the path of the own executable -- at least under Linux?
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// 10/11 - simple demo using a pointer and a struct
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// 11/11 - using the boost random number generator(s)
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// 12/11 - how to detect if string conversion is possible?
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// 1/12 - is partial application of member functions possible?
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// 5/14 - c++11 transition: detect empty function object
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// 7/14 - c++11 transition: std hash function vs. boost hash
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// 9/14 - variadic templates and perfect forwarding
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// 11/14 - pointer to member functions and name mangling
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// 8/15 - Segfault when loading into GDB (on Debian/Jessie 64bit
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// 8/15 - generalising the Variant::Visitor
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// 1/16 - generic to-string conversion for ostream
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// 1/16 - build tuple from runtime-typed variant container
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// 3/17 - generic function signature traits, including support for Lambdas
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// 9/17 - manipulate variadic templates to treat varargs in several chunks
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// 11/17 - metaprogramming to detect the presence of extension points
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// 11/17 - detect generic lambda
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// 12/17 - investigate SFINAE failure. Reason was indirect use while in template instantiation
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// 03/18 - Dependency Injection / Singleton initialisation / double checked locking
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// 04/18 - investigate construction of static template members
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// 08/18 - Segfault when compiling some regular expressions for EventLog search
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// 10/18 - investigate insidious reinterpret cast
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// 12/18 - investigate the trinomial random number algorithm from the C standard lib
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// 04/19 - forwarding tuple element(s) to function invocation
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/** @file try.cpp
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* Research how to apply a tuple to a varargs function forwarder.
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* The recent standard library has a std::apply, which we can not yet use, unfortunately.
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* @note this research remains inconclusive. As far as I can see, the simplified setup
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* exactly mimics the problematic call situation; however, in the real use case,
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* we need to std::forward<Args> the argument tuple object field while here in
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* this simplified case, it compiles just fine without -- as it should after all,
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* since that is the whole point of perfect forwarding; std::get should expose
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* a LValue reference to the tuple element, and we pass that through a forwarding
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* function into the double dispatch to the receiving visitor.
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*/
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typedef unsigned int uint;
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#include "lib/format-cout.hpp"
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#include "lib/test/test-helper.hpp"
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#include "lib/util.hpp"
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#include "lib/meta/variadic-helper.hpp"
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#include <utility>
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#include <string>
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#include <tuple>
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using std::string;
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using std::tuple;
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#define SHOW_TYPE(_TY_) \
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cout << "typeof( " << STRINGIFY(_TY_) << " )= " << lib::meta::typeStr<_TY_>() <<endl;
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#define SHOW_EXPR(_XX_) \
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cout << "Probe " << STRINGIFY(_XX_) << " ? = " << _XX_ <<endl;
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template<typename FUN, typename...ARGS>
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void
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forwardInvoker (FUN& fun, ARGS&&... args)
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{
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cout << "forwardInvoker...\n"
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<< lib::test::showVariadicTypes(args...)
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<< endl;
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fun (std::forward<ARGS>(args)...);
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}
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template<typename FUN, typename...ARGS>
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struct Holder
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{
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using Tup = tuple<ARGS...>;
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Tup tup;
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Holder (Tup& tup)
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: tup{tup}
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{ }
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template<size_t...idx>
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void
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unpack_and_forward (FUN& fun, lib::meta::IndexSeq<idx...>)
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{
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cout << "unpack_and_forward...\n";
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SHOW_TYPE (Tup)
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forwardInvoker (fun, std::get<idx> (tup)...);
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}
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void
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applyTuple (FUN& fun)
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{
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cout << "applyTuple...\n";
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SHOW_TYPE (Tup)
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using SequenceIterator = typename lib::meta::BuildIdxIter<ARGS...>::Ascending;
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unpack_and_forward (fun, SequenceIterator());
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}
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};
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int
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main (int, char**)
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{
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auto tup = std::make_tuple(1,2,3);
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auto fun = [](int a, int b, int c)
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{
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cout << a<<"+"<<b<<"+"<<c<<"="<<(a+b+c)<<endl;
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};
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using Hol = Holder<decltype(fun), int, int, int>;
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Hol holder(tup);
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holder.applyTuple (fun);
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cout << "\n.gulp.\n";
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return 0;
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}
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