Databases
StackOverflow2013
Postsdbo
POPossible to access private types in base classes via template indirection
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POPossible to access private types in base classes via template indirection
    primarykey
    data
    text
    <p>I'm trying to, at compile time, select a type to use depending on whether one is publicly available in a given scope. It's best to go straight to the code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;type_traits&gt; class Logger { std::string _p; public: Logger(std::string p): _p(p) { } void say(std::string message) { std::cout &lt;&lt; _p &lt;&lt; ' ' &lt;&lt; message &lt;&lt; std::endl; } }; struct Log { static Logger&amp; log() { static Logger _def("Default: "); return _def; } }; // 1. template &lt;typename P&gt; struct use_logger { static std::size_t test(P*); static char test(...); static const bool value = sizeof(test(reinterpret_cast&lt;P*&gt;(0))) == sizeof(std::size_t); }; class A { struct Log { static Logger&amp; log() { static Logger _def("A: "); return _def; } }; public: void say() { std::cout &lt;&lt; "A: " &lt;&lt; use_logger&lt;Log&gt;::value &lt;&lt; std::endl; std::conditional&lt;use_logger&lt;Log&gt;::value, Log, ::Log&gt;::type::log().say("From A"); } }; class B { public: void say() { std::cout &lt;&lt; "B: " &lt;&lt; use_logger&lt;Log&gt;::value &lt;&lt; std::endl; std::conditional&lt;use_logger&lt;Log&gt;::value, Log, ::Log&gt;::type::log().say("From B"); } }; class C : A { public: void say() { std::cout &lt;&lt; "C: " &lt;&lt; use_logger&lt;Log&gt;::value &lt;&lt; std::endl; //2. std::conditional&lt;use_logger&lt;Log&gt;::value, Log, ::Log&gt;::type::log().say("From C"); // Log::log().say("From C"); } }; class D : public A { public: void say() { // 2. std::cout &lt;&lt; "D: " &lt;&lt; use_logger&lt;Log&gt;::value &lt;&lt; std::endl; std::conditional&lt;use_logger&lt;Log&gt;::value, Log, ::Log&gt;::type::log().say("From D"); // Log::log().say("From C"); } }; int main(void) { { A i; i.say(); } { B i; i.say(); } { C i; i.say(); } { D i; i.say(); } } </code></pre> <p>My intention is that in <code>A</code>, there is a type <code>Log</code>, so that should be used rather than the global <code>::Log</code>, and in <code>B</code> where there is none, it should use the global <code>::Log</code>. Now both these work irrespective of <code>1.</code> (my incorrect test to see if the type is <em>private</em> in this scope..)</p> <p>The problem is in <code>C</code> and <code>D</code>, normally - without the test, <code>Log::log()</code> fails, because it's private in <code>A</code>. However if the <code>std::conditional&lt;&gt;</code> is used, there is no compilation error, and the output is incorrect as it is prefixed with <code>A:</code>. So, what have I missed (apart from the incorrect test - which I need to somehow fix...)? If nothing, then is this approach of exposing the private type in <code>A</code> using the <code>std::conditional</code> legal?</p> <p>EDIT: for sanity, I tested with the following:</p> <pre><code>std::conditional&lt;false, Log, ::Log&gt;::type::log("From C"); std::conditional&lt;false, Log, ::Log&gt;::type::log("From D"); </code></pre> <p>And it does use the global <code>::Log</code>, if it's true, it's somehow using the private <code>A::Log</code>.</p> <p>EDIT2: Infact this appears to be a more general condition, i.e. you can easily get access to some internal private types via a template indirection, e.g:</p> <pre><code>class F { struct Foo { void bar() { } }; }; template &lt;typename T&gt; struct ExposeInternal { typedef T type; }; int main(void) { { // We've got Foo! ExposeInternal&lt;F::Foo&gt;::type t; t.bar(); } { // Below fails F::Foo t; t.bar(); } } </code></pre> <p>EDIT 3: Okay - have confirmed, it is a reported GCC bug, nothing to do with <code>std::conditional</code>, not yet fixed in 4.7 or 4.8. <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47346" rel="nofollow">http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47346</a></p> <p>I will leave this question open for the moment.. will close it with the above later.</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USNim
    1. USNim
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. USAndreas Hehn
      1. VTFavorite
    3. VO
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. This table or related slice is empty.
      1. VTUpMod
    1. COPlease provide an SSCCE (and perhaps find a better title?!)
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. USstefan
    2. CO@stefan, it is - the code should compile, I left out two header inclusions for brevity...
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. USNim
    3. COstill lacking `#include <string>` and a semicolon after a struct. That's not an SSCCE.
      singulars
      1. POPossible to access private types in base classes via template indirection
      1. USstefan
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload