160 lines
4.5 KiB
C++
160 lines
4.5 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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/** @file try.cpp
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** Investigation: pitfalls of "perfect forwarding".
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** Find out about the corner cases where chained argument forwarding
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** does not work as expected. The key point is to put close attention on the
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** template parameters we're passing on to the next lower layer. It is crucial
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** either to add the reference explicitly, or to have it included implicitly
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** by relying on the way how template params are matched on function calls:
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** - lvalue -> parameter becomes TY &
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** - rvalue -> parameter becomes TY
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**
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** In this final stage, the example shows what happens if we erroneously fail
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** to take the variadic arguments by '&&' on forwarding: we end-up with a
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** slicing copy to the base class.
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**
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*/
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#include "lib/test/test-helper.hpp"
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#include "lib/util.hpp"
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#include <cstddef>
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#include <utility>
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#include <iostream>
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#include <functional>
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#include <string>
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using std::string;
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using std::cout;
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using std::endl;
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class Interface
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{
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public:
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Interface(){}
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Interface(Interface const& o) { cout << "COPY.CT from "<<&o<<" !!!\n"; }
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Interface(Interface const&& o) { cout << "MOVE.CT from "<<&o<<" !!!\n"; }
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Interface&
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operator= (Interface const& o) { cout << "COPY= from "<<&o<<" !!!\n"; return *this; }
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Interface&
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operator= (Interface const&& o) { cout << "MOVE= from "<<&o<<" !!!\n"; return *this; }
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virtual ~Interface() { }
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virtual string op() const
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{
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return "happy SLICING";
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}
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};
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class Impl
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: public Interface
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{
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string s_;
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string
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op() const override
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{
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return s_;
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}
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public:
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Impl(string ss ="IMP")
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: s_(ss)
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{ }
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};
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template<typename X>
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string
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showRefRRefVal()
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{
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return std::is_lvalue_reference<X>::value? " by REF"
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: std::is_rvalue_reference<X>::value? " by MOVE": " VAL";
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}
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template<typename... XS>
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void
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diagnostics (string id, XS const&... xs)
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{
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cout << "--"<<id<<"--\n"
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<< lib::test::showVariadicTypes<XS...>(xs...)
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<< "\n"
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;
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}
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void
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invoke (Interface const& ref)
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{
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using Ty = Interface const&;
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diagnostics<Ty> ("Invoke", ref);
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cout << "instanceof Impl?" << bool(INSTANCEOF(Impl, &ref)) <<"\n";
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cout << "________________"
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<< ref.op()
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<< "____\n";
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}
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template<class FUN, typename...ARG>
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void
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indirect_1 (FUN fun, ARG... args) // NOTE: erroneously taking ARG as-is, which results in taking BY VALUE when used in the forwarding chain!
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{
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diagnostics<ARG...> ("Indirect-1", args...);
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fun (args...);
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}
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template<class FUN, typename...ARG>
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void
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indirect_2 (FUN fun, ARG&&... args)
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{
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diagnostics<ARG&&...> ("Indirect-2", args...);
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indirect_1 (fun, std::forward<ARG>(args)...);
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}
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int
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main (int, char**)
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{
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Impl obj;
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Interface const& ref = obj;
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cout << "before call. Address... "<<&ref<<"\n";
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std::function<void(Interface const&)> fun(invoke);
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indirect_2 (fun, ref);
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indirect_2 (fun, Impl("honk"));
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cout << "\n.gulp.\n";
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return 0;
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}
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