From 5d0c2b6d2cd4483e76e077d6f934fb08c1b9887e Mon Sep 17 00:00:00 2001
From: Ichthyostega <prg@ichthyostega.de>
Date: Fri, 30 Mar 2018 06:48:34 +0200
Subject: [PATCH] DI: special solution for singletons with private default ctor

...which declare DependencyFactory as friend.
Yes, we want to encourrage that usage pattern.

Problem is, std::is_constructible<X> gives a misleading result in that case.
We need to do the instantiation check within the scope of DependencyFactory
---
 src/lib/depend2.hpp                           | 20 +++++++++-
 .../library/dependency-configuration-test.cpp | 12 +++++-
 wiki/thinkPad.ichthyo.mm                      | 40 +++++++++++++------
 3 files changed, 57 insertions(+), 15 deletions(-)

diff --git a/src/lib/depend2.hpp b/src/lib/depend2.hpp
index 0277b7b54..4fef8ba27 100644
--- a/src/lib/depend2.hpp
+++ b/src/lib/depend2.hpp
@@ -187,9 +187,25 @@ namespace lib {
           atDestruction ([obj]{ delete obj; });
           return obj;
         }
+
+                                   // try to instantiate the default ctor
+      template<class X, typename = decltype(X())>
+      static std::true_type  __try_instantiate(int);
+      template<class>
+      static std::false_type __try_instantiate(...);
+      
+      /** metafunction: can we instantiate the desired object here?
+       * @remark need to perform the check right here in this scope, because
+       *  default ctor can be private with DependencyFactory declared friend
+       */
+      template<typename X>
+      struct canDefaultConstruct
+        : decltype(__try_instantiate<X>(0))
+        { };
+      
       
       template<class TAR>
-      static     meta::enable_if<std::is_constructible<TAR>,
+      static     meta::enable_if<canDefaultConstruct<TAR>,
       TAR*                      >
       buildInstance()
         {
@@ -204,7 +220,7 @@ namespace lib {
                              "Application architecture or lifecycle is seriously broken.");
         }
       template<class ABS>
-      static     meta::disable_if<std::__or_<std::is_abstract<ABS>,std::is_constructible<ABS>>,
+      static     meta::disable_if<std::__or_<std::is_abstract<ABS>,canDefaultConstruct<ABS>>,
       ABS*                       >
       buildInstance()
         {
diff --git a/tests/library/dependency-configuration-test.cpp b/tests/library/dependency-configuration-test.cpp
index 4bc4991b0..897122626 100644
--- a/tests/library/dependency-configuration-test.cpp
+++ b/tests/library/dependency-configuration-test.cpp
@@ -332,6 +332,11 @@ namespace test{
                   return magic_++;
                 }
             };
+
+          // NOTE: the following is rejected due to missing default ctor
+          DependInject<Dummy<9>>::useSingleton<Veryspecial>();
+          VERIFY_ERROR (FATAL, Depend<Dummy<9>>{}() );
+          
           
           int backdoor = 22;
           
@@ -358,6 +363,11 @@ namespace test{
           
           {////////////////////////////////////////////////////TEST-Scope
            
+////////////// NOTE: the following does not compile
+//          //       since Veryspecial has no default ctor...
+//          //
+//          DependInject<Dum>::Local<Dum> impossible;
+            
             DependInject<Dum>::Local<Dum> insidious ([&]{ return new Veryspecial{backdoor}; });
             
             CHECK ((1+5+7+9) == checksum );
@@ -378,7 +388,7 @@ namespace test{
             
             CHECK ((1+5+7+9+9) == checksum );
           }////////////////////////////////////////////////////(End)TEST-Scope
-           
+          
           CHECK ((1+5+7+9) == checksum );
           CHECK ((1+5+7+9)*7 == dumm().probe() );
           CHECK ( 0 == tricky().probe());
diff --git a/wiki/thinkPad.ichthyo.mm b/wiki/thinkPad.ichthyo.mm
index 9501c5a1a..024a8b9d0 100644
--- a/wiki/thinkPad.ichthyo.mm
+++ b/wiki/thinkPad.ichthyo.mm
@@ -27697,9 +27697,9 @@
 </node>
 </node>
 </node>
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522110414912" ID="ID_1861060212" MODIFIED="1522110495402" TEXT="Singleton-Erzeugung zusammenf&#xfc;hren">
+<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522110414912" ID="ID_1861060212" MODIFIED="1522384924858" TEXT="Singleton-Erzeugung zusammenf&#xfc;hren">
 <linktarget COLOR="#5f8d94" DESTINATION="ID_1861060212" ENDARROW="Default" ENDINCLINATION="-566;0;" ID="Arrow_ID_319700222" SOURCE="ID_1624172022" STARTARROW="None" STARTINCLINATION="205;-440;"/>
-<icon BUILTIN="flag-yellow"/>
+<icon BUILTIN="pencil"/>
 <node CREATED="1522110563499" ID="ID_470760423" MODIFIED="1522113397796" TEXT="stets ausf&#xfc;hrend">
 <icon BUILTIN="idea"/>
 <node CREATED="1522110568307" ID="ID_311971109" MODIFIED="1522110579069" TEXT="Depend&lt;SUB&gt;"/>
@@ -27772,8 +27772,8 @@
 </node>
 </node>
 </node>
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522200092935" ID="ID_1314237888" MODIFIED="1522382097329" TEXT="Deleter-Behandlung">
-<icon BUILTIN="pencil"/>
+<node COLOR="#338800" CREATED="1522200092935" ID="ID_1314237888" MODIFIED="1522384929202" TEXT="Deleter-Behandlung">
+<icon BUILTIN="button_ok"/>
 <node CREATED="1522200102534" ID="ID_284894079" MODIFIED="1522200105025" TEXT="bisher">
 <node CREATED="1522200106373" ID="ID_1981188993" MODIFIED="1522200226028" TEXT="was die Factory zur&#xfc;ckliefert, wird nicht gemanaged">
 <icon BUILTIN="broken-line"/>
@@ -27796,30 +27796,46 @@
 <node COLOR="#338800" CREATED="1522200189706" ID="ID_302642427" MODIFIED="1522382084962" TEXT="m&#xf6;glichst alle instance-Pointer zur&#xfc;cksetzen">
 <icon BUILTIN="button_ok"/>
 </node>
+<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522384942847" ID="ID_1152640572" MODIFIED="1522385141698" TEXT="Fehlermeldung beim Zugriff nach Zerst&#xf6;rung">
+<linktarget COLOR="#639d9e" DESTINATION="ID_1152640572" ENDARROW="Default" ENDINCLINATION="-89;0;" ID="Arrow_ID_1512382393" SOURCE="ID_1741501987" STARTARROW="None" STARTINCLINATION="28;-1;"/>
+<icon BUILTIN="flag-yellow"/>
+</node>
 <node COLOR="#338800" CREATED="1522200249626" ID="ID_215336836" MODIFIED="1522382087401" TEXT="Freunschafts-f&#xe4;hig">
 <icon BUILTIN="button_ok"/>
 </node>
 </node>
 </node>
+<node BACKGROUND_COLOR="#fdfdcf" COLOR="#ff0000" CREATED="1522384902500" ID="ID_1741501987" MODIFIED="1522385141698" TEXT="Zugriff beim Herunterfahren">
+<arrowlink COLOR="#639d9e" DESTINATION="ID_1152640572" ENDARROW="Default" ENDINCLINATION="-89;0;" ID="Arrow_ID_1512382393" STARTARROW="None" STARTINCLINATION="28;-1;"/>
+<icon BUILTIN="flag-pink"/>
 </node>
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522334796724" ID="ID_1080992734" MODIFIED="1522382015413" TEXT="Funktoren zusammenf&#xfc;hren">
-<icon BUILTIN="pencil"/>
-<node COLOR="#338800" CREATED="1522334843062" ID="ID_1526236017" MODIFIED="1522382043115" TEXT="DependencyFactory f&#xfc;r">
+</node>
+<node COLOR="#338800" CREATED="1522334796724" ID="ID_1080992734" MODIFIED="1522384890030" TEXT="Funktoren zusammenf&#xfc;hren">
 <icon BUILTIN="button_ok"/>
-<node CREATED="1522334805635" ID="ID_315782435" MODIFIED="1522334817813" TEXT="Erzeugungs-Funktor"/>
+<node COLOR="#338800" CREATED="1522334843062" FOLDED="true" ID="ID_1526236017" MODIFIED="1522385032004" TEXT="DependencyFactory f&#xfc;r">
+<icon BUILTIN="button_ok"/>
+<node CREATED="1522334805635" ID="ID_315782435" MODIFIED="1522384992864" TEXT="Erzeugungs-Funktor"/>
 <node CREATED="1522334818473" ID="ID_750129060" MODIFIED="1522334821036" TEXT="Deleter"/>
 <node CREATED="1522334822001" ID="ID_644059909" MODIFIED="1522334935421" TEXT="automatisches Instanz-Management"/>
 </node>
-<node BACKGROUND_COLOR="#eee5c3" COLOR="#990000" CREATED="1522334859291" ID="ID_1496044958" MODIFIED="1522382005045" TEXT="korrekt handhaben">
-<icon BUILTIN="pencil"/>
+<node COLOR="#338800" CREATED="1522334859291" ID="ID_1496044958" MODIFIED="1522384876245" TEXT="korrekt handhaben">
+<icon BUILTIN="button_ok"/>
 <node COLOR="#338800" CREATED="1522334896942" ID="ID_1215786079" MODIFIED="1522382047552" TEXT="default-Fall">
 <icon BUILTIN="button_ok"/>
 </node>
 <node COLOR="#338800" CREATED="1522334903741" ID="ID_1875541619" MODIFIED="1522382049855" TEXT="abstrakte Basisklasse">
 <icon BUILTIN="button_ok"/>
 </node>
-<node BACKGROUND_COLOR="#fdfdcf" COLOR="#ff0000" CREATED="1522381984647" ID="ID_136231234" MODIFIED="1522382051295" TEXT="Klasse ohne Default-ctor">
-<icon BUILTIN="flag-pink"/>
+<node COLOR="#338800" CREATED="1522381984647" ID="ID_136231234" MODIFIED="1522384873920" TEXT="Klasse ohne Default-ctor">
+<icon BUILTIN="button_ok"/>
+<node CREATED="1522384796267" ID="ID_1913154173" MODIFIED="1522384811849" TEXT="Problem">
+<icon BUILTIN="messagebox_warning"/>
+<node CREATED="1522384813129" ID="ID_502713353" MODIFIED="1522384821715" TEXT="private mit Freunschaft"/>
+<node CREATED="1522384822583" ID="ID_1870041254" MODIFIED="1522384845999" TEXT="std::is_constructible&lt;X&gt; kann das nicht unterscheiden"/>
+</node>
+<node COLOR="#338800" CREATED="1522384848011" ID="ID_834547191" MODIFIED="1522384870993" TEXT="L&#xf6;sung: duck-Detector im Rumpf von DependencyFactory">
+<icon BUILTIN="button_ok"/>
+</node>
 </node>
 <node COLOR="#338800" CREATED="1522334912412" ID="ID_1391948557" MODIFIED="1522382053286" TEXT="custom-Funktor">
 <icon BUILTIN="button_ok"/>