Ichthyostega
2533565f83
...since all those metaprogramming techniques rely on SFINAE, but *instantiating* a template means to compile it, which is more than just substituate a type into the signature If forming the signature fails -> SFINAE, try next one If instantiating a template fails -> compile error, abort
150 lines
5 KiB
C++
150 lines
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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// 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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/** @file try.cpp
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** Metaprogramming: is it possible to distinguish a generic lambda from something not a function at all?
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** Answer: not really. We can only ever check for the function call operator.
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** Even worse: if we instantiate a templated function call operator with unsuitable parameter types,
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** the compilation as such fails. Whereas SFINAE is only limited to substituting a type signature.
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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/format-util.hpp"
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#include "lib/meta/function.hpp"
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#include "lib/test/test-helper.hpp"
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//#include <functional>
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#include <utility>
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#include <string>
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using lib::meta::No_t;
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using lib::meta::Yes_t;
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using lib::meta::_Fun;
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using lib::test::showSizeof;
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using std::function;
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using std::forward;
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using std::move;
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using std::string;
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namespace lib {
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namespace meta{
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template<class FUN, typename...ARGS>
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struct ProbeFunctionInvocation
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{
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using Ret = decltype(std::declval<FUN>() (std::declval<ARGS>()...));
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using Args = Types<ARGS...>;
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using Sig = Ret(ARGS...);
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};
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template<typename FUN, typename...ARGS>
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class can_Invoke
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{
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template<typename FF,
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typename SEL = decltype(std::declval<FF>() (std::declval<ARGS>()...))>
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struct Probe
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{ };
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template<class X>
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static Yes_t check(Probe<X> * );
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template<class>
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static No_t check(...);
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public:
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static const bool value = (sizeof(Yes_t)==sizeof(check<FUN>(0)));
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};
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template<class FUN, typename TYPES, typename SEL =void>
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struct ProbeFunctionArgument
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{
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static_assert(not sizeof(FUN), "Tilt");
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};
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template<class FUN, typename T, typename TYPES>
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struct ProbeFunctionArgument<FUN, Node<T,TYPES>, enable_if<can_Invoke<FUN,T>> >
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: ProbeFunctionInvocation<FUN,T>
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{ };
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template<class FUN, typename T, typename TYPES>
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struct ProbeFunctionArgument<FUN, Node<T,TYPES>, disable_if<can_Invoke<FUN,T>> >
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: ProbeFunctionArgument<FUN, TYPES>
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{
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};
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}}
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using lib::meta::Types;
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using lib::meta::can_Invoke;
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using lib::meta::ProbeFunctionArgument;
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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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int
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main (int, char**)
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{
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auto lamb1 = [](int i) { return double(i) / (i*i); };
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auto lamb2 = [](auto i) { return double(i) / (i*i); };
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SHOW_TYPE (decltype(lamb1));
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SHOW_TYPE (decltype(lamb2));
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SHOW_EXPR ((can_Invoke<decltype(lamb1), string>::value));
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SHOW_EXPR ((can_Invoke<decltype(lamb2), int>::value ));
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using InferredSIG = typename ProbeFunctionArgument<decltype(lamb1), typename Types<string,long,int>::List>::Sig;
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//NOTE does not work with the generic lamb2, because instantiating lab2(string) would be a *compile* error, not a substitution failure
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SHOW_TYPE (InferredSIG);
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cout << "\n.gulp.\n";
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return 0;
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}
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