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Research: probing a generic lambda is not possible

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...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
This commit is contained in:
Fischlurch 2017-11-24 01:21:03 +01:00
parent 01937f9736
commit 2533565f83
2 changed files with 133 additions and 28 deletions
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92
research/try.cpp
View file

@ -39,6 +39,9 @@
/** @file try.cpp
** Metaprogramming: is it possible to distinguish a generic lambda from something not a function at all?
** Answer: not really. We can only ever check for the function call operator.
** Even worse: if we instantiate a templated function call operator with unsuitable parameter types,
** the compilation as such fails. Whereas SFINAE is only limited to substituting a type signature.
*/
typedef unsigned int uint;
@ -54,6 +57,7 @@ typedef unsigned int uint;
using lib::meta::No_t;
using lib::meta::Yes_t;
using lib::meta::_Fun;
using lib::test::showSizeof;
using std::function;
@ -62,6 +66,58 @@ using std::move;
using std::string;
namespace lib {
namespace meta{
template<class FUN, typename...ARGS>
struct ProbeFunctionInvocation
{
using Ret = decltype(std::declval<FUN>() (std::declval<ARGS>()...));
using Args = Types<ARGS...>;
using Sig = Ret(ARGS...);
};
template<typename FUN, typename...ARGS>
class can_Invoke
{
template<typename FF,
typename SEL = decltype(std::declval<FF>() (std::declval<ARGS>()...))>
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<FUN>(0)));
};
template<class FUN, typename TYPES, typename SEL =void>
struct ProbeFunctionArgument
{
static_assert(not sizeof(FUN), "Tilt");
};
template<class FUN, typename T, typename TYPES>
struct ProbeFunctionArgument<FUN, Node<T,TYPES>, enable_if<can_Invoke<FUN,T>> >
: ProbeFunctionInvocation<FUN,T>
{ };
template<class FUN, typename T, typename TYPES>
struct ProbeFunctionArgument<FUN, Node<T,TYPES>, disable_if<can_Invoke<FUN,T>> >
: ProbeFunctionArgument<FUN, TYPES>
{
};
}}
using lib::meta::Types;
using lib::meta::can_Invoke;
using lib::meta::ProbeFunctionArgument;
#define SHOW_TYPE(_TY_) \
@ -70,34 +126,6 @@ using std::string;
cout << "Probe " << STRINGIFY(_XX_) << " ? = " << _XX_ <<endl;
void
fun1 (long)
{
cout << "long FUN" <<endl;
}
int
fun1 (string, long)
{
cout << "string FUN" <<endl;
return 12;
}
void
fun1 ()
{
cout << "NO FUN" <<endl;
}
class Cheesy
{ };
class Fishy
{
friend void fun1 (Fishy&);
};
int
main (int, char**)
@ -108,6 +136,14 @@ main (int, char**)
SHOW_TYPE (decltype(lamb1));
SHOW_TYPE (decltype(lamb2));
SHOW_EXPR ((can_Invoke<decltype(lamb1), string>::value));
SHOW_EXPR ((can_Invoke<decltype(lamb2), int>::value ));
using InferredSIG = typename ProbeFunctionArgument<decltype(lamb1), typename Types<string,long,int>::List>::Sig;
//NOTE does not work with the generic lamb2, because instantiating lab2(string) would be a *compile* error, not a substitution failure
SHOW_TYPE (InferredSIG);
cout << "\n.gulp.\n";
return 0;

69
wiki/thinkPad.ichthyo.mm
View file

@ -5238,6 +5238,9 @@
<icon BUILTIN="pencil"/>
<node CREATED="1511455644829" ID="ID_1545300823" MODIFIED="1511455655796" TEXT="ist generisches Labmda detektierbar?">
<icon BUILTIN="help"/>
<node CREATED="1511482195487" ID="ID_954120351" MODIFIED="1511482198474" TEXT="jein"/>
<node CREATED="1511482200510" ID="ID_714363558" MODIFIED="1511482216359" TEXT="man kann nur einen Funktionsoperator erkennen"/>
<node CREATED="1511482216971" ID="ID_1806534330" MODIFIED="1511482240548" TEXT="nicht aber, wenn ein Solcher mehrere Overloads hat oder ein Template ist"/>
</node>
<node CREATED="1511455664043" ID="ID_1966980711" MODIFIED="1511455676078" TEXT="kann man Fallback-Mechanismus bauen?">
<icon BUILTIN="help"/>
@ -5247,6 +5250,72 @@
<node CREATED="1511455711525" ID="ID_1584168712" MODIFIED="1511455721127" TEXT="(a) mit dem Value-Type ins Argument"/>
<node CREATED="1511455721907" ID="ID_312038081" MODIFIED="1511455730558" TEXT="(b) mit dem Iterator-Typ ins Argument"/>
</node>
<node CREATED="1511482247535" ID="ID_487969118" MODIFIED="1511482309885" TEXT="geht nicht">
<icon BUILTIN="broken-line"/>
<node CREATED="1511482254927" ID="ID_1687625403" MODIFIED="1511482263673" TEXT="zwar funktionieren die Positiv-F&#xe4;lle"/>
<node CREATED="1511482264405" ID="ID_1003171577" MODIFIED="1511482304307">
<richcontent TYPE="NODE"><html>
<head>
</head>
<body>
<p>
aber eine falsche Template-Instantiierung
</p>
<p>
ist ein <i>Compile</i>-Fehler, kein <i>Substitutions</i>-Fehler
</p>
</body>
</html>
</richcontent>
</node>
</node>
</node>
</node>
<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1511482367184" ID="ID_1569720883" MODIFIED="1511482519599" TEXT="bleibt nur...">
<icon BUILTIN="pencil"/>
<node CREATED="1511482374415" ID="ID_554184880" MODIFIED="1511482441927" TEXT="maximal ein Versuch">
<icon BUILTIN="yes"/>
<node CREATED="1511482446317" ID="ID_951810826" MODIFIED="1511482457247" TEXT="entweder der Typ der Funktion ist bestimmbar"/>
<node CREATED="1511482457978" ID="ID_564239157" MODIFIED="1511482493455">
<richcontent TYPE="NODE"><html>
<head>
</head>
<body>
<p>
oder man f&#228;llt auf <i>eine</i>&#160;m&#246;gliche Substitution zur&#252;ck
</p>
</body>
</html>
</richcontent>
</node>
<node CREATED="1511482495719" ID="ID_327135583" MODIFIED="1511482510962">
<richcontent TYPE="NODE"><html>
<head>
</head>
<body>
<p>
...und wenn die Scheitert, ist das ein <b>compile</b>-Fehler
</p>
</body>
</html>
</richcontent>
</node>
</node>
<node CREATED="1511482393172" ID="ID_1538766965" MODIFIED="1511482428415" TEXT="das w&#xe4;re dann: den Iterator zu verwenden">
<richcontent TYPE="NOTE"><html>
<head>
</head>
<body>
<p>
denn das ist der sinnvollste Fall f&#252;r ein generisches Lambda
</p>
</body>
</html>
</richcontent>
</node>
</node>
</node>