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Library: metafunction to detect support for a specific extension point

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such a detector function can be used to enable some template specialisation
based on the fact that a target type exposes the desired extension point
This commit is contained in:
Fischlurch 2017-11-19 01:43:19 +01:00
parent 8f865183e3
commit fe3feee67a
5 changed files with 176 additions and 6 deletions
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10
research/try.cpp
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@ -38,7 +38,17 @@
/** @file try.cpp
** Metaprogramming: how to detect that a type in question exposes a free function extension point.
** Since such an extension point is typically injected alongside with the type exposing the extension
** point and intended to be picked up by ADL, all we have to check is if it is valid to invoke the
** extension point function with an instance of the target type.
**
** There are two difficulties to overcome, though
** - a function might return void. And while we may indeed pick up `void` from `decltype(expr)`,
** there is not much we can do with a void type. The remedy is just to use this type as template
** parameter on another template instantiation, which fails if this type can not legally be formed.
** - we do not know how to get a value of the type to probe, in order to feed it into the extension
** point function. Fortunately, the `std::declval<TYPE>()` function was included into the C++ language
** for this very purpose.
*/
typedef unsigned int uint;

29
src/lib/meta/duck-detector.hpp
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@ -168,6 +168,35 @@
/** Detector for support of a free-function extension point.
* Defines a metafunction (template), allowing to probe if the type
* in question supports a specific extension point function. Typically
* such functions are injected by some type in a way to be picked up by ADL.
* The detection test works by forming an expression to invoke the extension point,
* passing the type given as template parameter as function argument. If this expression
* type checks, the extension point is assumed to be supported.
* @warning beware of implicit type conversions
*/
#define META_DETECT_EXTENSION_POINT(_FUN_) \
template<typename TY> \
class HasExtensionPoint_##_FUN_ \
{ \
template<typename X, \
typename SEL = decltype( _FUN_(std::declval<X>()))>\
struct Probe \
{ }; \
\
template<class X> \
static Yes_t check(Probe<X> * ); \
template<class> \
static No_t check(...); \
\
public: \
static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0))); \
};
/** Detector for a dereferentiation operator. Works like member detection */
#define META_DETECT_OPERATOR_DEREF() \
template<typename TY> \

5
tests/12metaprogramming.tests
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@ -116,6 +116,11 @@ return: 0
END
TEST "Duck detect extension points" DuckDetectorExtension_test <<END
return: 0
END
TEST "functional closure" FunctionClosure_test <<END
out-lit: List1 :-<1>-<2>-<3>-
out-lit: List2 :-<5>-<6>-<7>-

129
tests/library/meta/duck-detector-extension-test.cpp Normal file
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@ -0,0 +1,129 @@
/*
DuckDetectorExtension(Test) - detecting support for extension points at compile time
Copyright (C) Lumiera.org
2017, Hermann Vosseler <Ichthyostega@web.de>
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.
This program 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 General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
* *****************************************************/
/** @file duck-detector-extension-test.cpp
** unit test \ref DuckDetectorExtension_test
*/
#include "lib/test/run.hpp"
#include "lib/meta/duck-detector.hpp"
#include "lib/util.hpp"
#include <string>
namespace lib {
namespace meta{
namespace test{
using std::string;
namespace { // test types and definitions to explore....
void
fun (long)
{ }
int
fun (string, long)
{
return 12;
}
void
fun ()
{ }
class Cheesy
{ };
class Fishy
{
/** @note type Fishy exposes an extension point `fun` */
friend void fun (Fishy&);
};
}//(End) test definitions
/***********************************************************************************//**
* @test detect the presence of a free function extension point at compile time.
* It is a common idiom in C++ to expose an extension point through a free function,
* which is expected to be picked up by ADL. To mention a prominent example, any type
* can offer the ability to be _iterated_ by injecting free functions `begin(TY)` and
* `end(TY)`, to yield a STL compatible iterator.
*
* Since such an extension point is used just by invoking the _unqualified_ function
* with the target type, we can build a meta predicate based on the fact if such an
* function invocation expression can be formed for the type in question.
*
* @note the test really hinges on the ability to form the extension point call.
* For this reason, some implicit conversions might be involved, and some
* other conversions won't work (like passing a value to an extension point
* taking a reference).
*/
class DuckDetectorExtension_test : public Test
{
META_DETECT_EXTENSION_POINT (funZ);
META_DETECT_EXTENSION_POINT (fun);
void
run (Arg)
{
fun ();
fun (23);
fun ("FUN", 45);
CHECK ( not HasExtensionPoint_funZ<long>::value );
CHECK ( HasExtensionPoint_fun<long> ::value );
CHECK ( HasExtensionPoint_fun<long&> ::value );
CHECK ( HasExtensionPoint_fun<long&&> ::value );
CHECK ( HasExtensionPoint_fun<char> ::value );
CHECK ( HasExtensionPoint_fun<char&> ::value );
CHECK ( HasExtensionPoint_fun<char&&> ::value );
CHECK ( not HasExtensionPoint_fun<string> ::value );
CHECK ( not HasExtensionPoint_fun<void> ::value );
CHECK ( not HasExtensionPoint_fun<Cheesy> ::value );
CHECK ( not HasExtensionPoint_fun<Fishy> ::value );
CHECK ( HasExtensionPoint_fun<Fishy&> ::value );
CHECK ( not HasExtensionPoint_fun<Fishy&&> ::value );
CHECK ( not HasExtensionPoint_fun<Fishy const&>::value );
}
};
/** Register this test class... */
LAUNCHER (DuckDetectorExtension_test, "unit meta");
}}} // namespace lib::meta::test

9
tests/library/meta/duck-detector-test.cpp
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@ -21,24 +21,21 @@
* *****************************************************/
/** @file duck-detector-test.cpp
** unit test \ref DuckDetector_test
** unit test \ref DuckDetector_test:
** duck typing through metaprogramming
*/
#include "lib/test/run.hpp"
#include "lib/meta/duck-detector.hpp"
#include "lib/format-cout.hpp"
#include "lib/util.hpp"
#include <iostream>
namespace lib {
namespace meta{
namespace test{
using std::cout;
using std::endl;
namespace { // some test ducks....