LUMIERA.clone/research/try.cpp

/* try.cpp  -  for trying out some language features....
 *             scons will create the binary bin/try
 *
 */

// 8/07  - how to control NOBUG??
//         execute with   NOBUG_LOG='ttt:TRACE' bin/try
// 1/08  - working out a static initialisation problem for Visitor (Tag creation)
// 1/08  - check 64bit longs
// 4/08  - comparison operators on shared_ptr<Asset>
// 4/08  - conversions on the value_type used for boost::any
// 5/08  - how to guard a downcasting access, so it is compiled in only if the involved types are convertible
// 7/08  - combining partial specialisation and subclasses
// 10/8  - abusing the STL containers to hold noncopyable values
// 6/09  - investigating how to build a mixin template providing an operator bool()
// 12/9  - tracking down a strange "warning: type qualifiers ignored on function return type"
// 1/10  - can we determine at compile time the presence of a certain function (for duck-typing)?
// 4/10  - pretty printing STL containers with python enabled GDB?
// 1/11  - exploring numeric limits
// 1/11  - integer floor and wrap operation(s)
// 1/11  - how to fetch the path of the own executable -- at least under Linux?
// 10/11 - simple demo using a pointer and a struct
// 11/11 - using the boost random number generator(s)
// 12/11 - how to detect if string conversion is possible?
// 1/12  - is partial application of member functions possible?
// 5/14  - c++11 transition: detect empty function object
// 7/14  - c++11 transition: std hash function vs. boost hash
// 9/14  - variadic templates and perfect forwarding
// 11/14 - pointer to member functions and name mangling
// 8/15  - Segfault when loading into GDB (on Debian/Jessie 64bit
// 8/15  - generalising the Variant::Visitor
// 1/16  - generic to-string conversion for ostream
// 1/16  - build tuple from runtime-typed variant container
// 3/17  - generic function signature traits, including support for Lambdas
// 9/17  - manipulate variadic templates to treat varargs in several chunks
// 11/17 - metaprogramming to detect the presence of extension points
// 11/17 - detect generic lambda
// 12/17 - investigate SFINAE failure. Reason was indirect use while in template instantiation


/** @file try.cpp
 ** Bug hunt: a typedef detecting metafunction failed mysteriously.
 ** As it turned out, the implemented check required a _complete definition_, since it applied `sizeof()`.
 ** But the Detection was invoked indirectly from a template while this was still in instantiation.
 ** This explains why the metafunction worked in a clean test setup, but failed when integrated
 ** in the actual code base. The tricky and insidious part of this story is the fact, that
 ** in a regular definition, this would cause a compilation failure. But since our detector
 ** relies on SFINAE, just the detection went wrong, and consequently an improper template
 ** specialisation was picked. These are the perils of metaprogramming with a language
 ** never really made for functional programming...
 ** 
 ** The solution or workaround is simple: use a detection technique able to work with incomplete types.
 ** 
 ** For context: lib::diff::Record defines various Iterators. And lib::dif::GenNode is a _recursive datatype_,
 ** which means, already the definition of GenNode requires the instantiation of `Record<GenNode>`. Now we added
 ** some detector magic to our Iterator Adapters, and this magic failed on the Iterators defined by `Record,`
 ** because `Record<GenNode>` was not fully instantiated at that point.
 */

typedef unsigned int uint;

#include "lib/format-cout.hpp"
#include "lib/meta/util.hpp"
#include "lib/util.hpp"

#include <vector>


  template<typename X, typename XX = typename X::value_type>
  struct Test_Incomplete
    { };
  
  template<typename X, int s = sizeof(typename X::value_type)>
  struct Test_Complete
    { };


using lib::meta::Yes_t;
using lib::meta::No_t;
  
  template<class X>
  static Yes_t detectComplete(Test_Complete<X> * );
  template<class>
  static No_t detectComplete(...);
  
  template<class X>
  static Yes_t detectIncomplete(Test_Incomplete<X> * );
  template<class>
  static No_t detectIncomplete(...);



  struct InStatuNascendi
    {
      using Iter = std::vector<InStatuNascendi>::iterator;
      
      static const auto detectedComplete   = (sizeof(Yes_t) == sizeof(detectComplete<Iter>(0)));    // will produce No, erroneously
      static const auto detectedIncomplete = (sizeof(Yes_t) == sizeof(detectIncomplete<Iter>(0)));  // will produce Yes
    };


#define SHOW_TYPE(_TY_) \
    cout << "typeof( " << STRINGIFY(_TY_) << " )= " << lib::meta::typeStr<_TY_>() <<endl;
#define SHOW_EXPR(_XX_) \
    cout << "Probe " << STRINGIFY(_XX_) << " ? = " << _XX_ <<endl;



int
main (int, char**)
  {
    SHOW_EXPR (InStatuNascendi::detectedComplete);
    SHOW_EXPR (InStatuNascendi::detectedIncomplete);
    
    cout <<  "\n.gulp.\n";
    
    return 0;
  }