generic-toString(#985): define streamlined converter

...based on all the clean-up and reorganisation done thus far,
we're now able to rebuild the util::str in a more direct and
sane way, and thus to disentangle the header inclusion problem.

research/try.cpp

@@ -86,7 +86,6 @@ using lib::meta::is_StringLike;
 using lib::meta::can_lexical2string;
 using lib::meta::can_convertToString;
 using lib::meta::use_StringConversion4Stream;
-using lib::meta::CustomStringConv;
 using lib::meta::Strip;
 
 using std::string;

src/lib/format-cout.hpp

@@ -39,7 +39,6 @@
 #define LIB_FORMAT_COUT_H
 
 #include "lib/format-obj.hpp"
-//#include "lib/util.hpp"
 
 #include <string>
 #include <iostream>
@@ -51,20 +50,6 @@ using std::endl;
 
 
 
-namespace lib  {
-namespace meta {
-  
-  /** when to use custom string conversions for output streams */
-  template<typename X>
-  struct use_StringConversion4Stream
-    : __and_<std::is_class<typename Strip<X>::TypePlain>
-            ,__not_<std::is_pointer<X>>
-            ,__not_<can_lexical2string<X>>
-            >
-    { };
-}}
-
-
 
 namespace std {
   
@@ -80,7 +65,7 @@ namespace std {
   ostream&
   operator<< (ostream& os, X const& obj)
   {
-    return os << lib::meta::CustomStringConv<X>::invoke (obj);
+    return os << util::StringConv<X>::invoke (obj);
   }
   
   
@@ -93,7 +78,7 @@ namespace std {
   operator<< (ostream& os, X const* ptr)
   {
     if (ptr)
-      return util::showAddr(os, ptr) << " ↗" << lib::meta::CustomStringConv<X>::invoke (*ptr);
+      return util::showAddr(os, ptr) << " ↗" << util::StringConv<X>::invoke (*ptr);
     else
       return os << "⟂ «" << lib::meta::typeStr<X>() << "»";
   }

src/lib/format-obj.cpp

@@ -30,8 +30,8 @@
  ** tasks commonly used from debugging and diagnostics code, both to
  ** investigate object contents and show types and addresses. They
  ** are referred from our [lightweight string converter](\ref
- ** lib::meta::CustomStringConv), but also from the util::toString()
+ ** util::StringConv), but also from the util::toString() function
- ** function and more common [formatting utils](format-util.hpp).
+ ** and more common [formatting utils](format-util.hpp).
  ** 
  ** @see FormatHelper_test
  ** @see FormatString_test

src/lib/format-obj.hpp

@@ -43,10 +43,9 @@
 #define LIB_FORMAT_OBJ_H
 
 #include "lib/meta/trait.hpp"
-//#include "lib/util.hpp"
 
-#include <string>
 #include <boost/lexical_cast.hpp>
+#include <string>
 
 
 namespace std { // forward declaration to avoid including <iostream>
@@ -83,6 +82,10 @@ namespace meta {
 }}// namespace lib::meta
 
 
+
+
+
+
 namespace util {
   
   std::string showDouble (double) noexcept;
@@ -93,6 +96,70 @@ namespace util {
   std::ostream& showAddr (std::ostream&, void const* addr);
   
   
+  namespace {
+    /** toggle to prefer specialisation with direct lexical conversion */
+    template<typename X>
+    using enable_LexicalConversion = lib::meta::enable_if< lib::meta::use_LexicalConversion<X>>;
+  }
+  
+  
+  
+  /* === generalise the failsafe string conversion === */
+  
+  /** @note base case is defined in meta/util.hpp */
+  template<typename X>
+  struct StringConv<X,     enable_LexicalConversion<X>>
+    {
+      static std::string
+      invoke (X const& val) noexcept
+        try        { return boost::lexical_cast<std::string> (val); }
+        catch(...) { return "↯"; }
+    };
+  
+  /** explicit specialisation to control precision of double values.
+   * @note we set an explicit precision, since this is a diagnostic facility
+   *       and we typically do not want to see all digits, but, for test code,
+   *       we do want a predictable string representation of simple fractional
+   *       values like `0.1` (which can not be represented as binary floats)
+   */
+  template<>
+  struct StringConv<double>
+    {
+      static std::string
+      invoke (double val) noexcept
+      {
+        return util::showDouble (val);
+      }
+    };
+  template<>
+  struct StringConv<float>
+    {
+      static std::string
+      invoke (float val) noexcept
+      {
+        return util::showFloat (val);
+      }
+    };
+  
+  
+  
+  
+  /**
+   * get some string representation of any object, reliably.
+   * A custom string conversion operator is invoked, if applicable,
+   * while all lexically convertible types (numbers etc) are treated
+   * by boost::lexical_cast. For double or float values, hard wired
+   * rounding to a fixed number of digits will be performed, to yield
+   * a predictable display of printed unit-test results.
+   */
+  template<typename TY>
+  inline std::string
+  toString (TY const& val)  noexcept
+  {
+    return StringConv<TY>::invoke (val);
+  }
+  
+  
   
 } // namespace util
 #endif /*LIB_FORMAT_OBJ_H*/

src/lib/meta/trait.hpp

@@ -302,6 +302,22 @@ namespace meta {
            >
     { };
   
+  template<typename X>
+  struct use_LexicalConversion
+    : __and_<can_lexical2string<X>
+            ,__not_<can_convertToString<X>>
+            >
+    { };
+  
+  /** when to use custom string conversions for output streams */
+  template<typename X>
+  struct use_StringConversion4Stream
+    : __and_<std::is_class<typename Strip<X>::TypePlain>
+            ,__not_<std::is_pointer<X>>
+            ,__not_<can_lexical2string<X>>
+            >
+    { };
+  
   
   
   

src/lib/meta/util.hpp

@@ -114,10 +114,10 @@ namespace meta {
    *  Might fail in more tricky situations (references, const, volatile)
    * @see \ref format-obj.hpp more elaborate solution including lexical_cast
    */
-  template<typename T>
+  template<typename X>
   struct can_convertToString
     {
-      static T & probe();
+      static X & probe();
       
       static Yes_t check(std::string);
       static No_t  check(...);
@@ -126,6 +126,11 @@ namespace meta {
       static const bool value = (sizeof(Yes_t)==sizeof(check(probe())));
     };
   
+  /** toggle for explicit specialisations */
+  template<typename X>
+  using enable_CustomStringConversion = enable_if<can_convertToString<X>>;
+  
+  
   
   /** strip const from type: naive implementation */
   template<typename T>
@@ -220,6 +225,13 @@ namespace meta {
     return typeStr (&ref);
   }
   
+}}// namespace lib::meta
+
+
+
+namespace util {
+  
+  using lib::meta::typeStr;
   
   
   /** failsafe invocation of custom string conversion.
@@ -234,7 +246,7 @@ namespace meta {
    *    the returned string indicates "↯" in this case.
    */
   template<typename X, typename COND =void>
-  struct CustomStringConv
+  struct StringConv
     {
       static std::string
       invoke (X const& x)  noexcept
@@ -243,7 +255,7 @@ namespace meta {
     };
   
   template<typename X>
-  struct CustomStringConv<X,     enable_if<can_convertToString<X>> >
+  struct StringConv<X,     lib::meta::enable_CustomStringConversion<X>>
     {
       static std::string
       invoke (X const& val) noexcept
@@ -254,12 +266,9 @@ namespace meta {
   // NOTE: this is meant to be extensible;
   //       more specialisations are e.g. in format-obj.hpp
   
-}}// namespace lib::meta
   
   
   
-namespace util {
-  
   /** pretty-print a double in fixed-point format */
   std::string showDouble (double) noexcept;
   std::string showFloat (float)   noexcept;
@@ -280,7 +289,7 @@ namespace util {
   inline std::string
   showPtr (X* ptr =nullptr)
   {
-    return ptr? showAddr(ptr) + " ↗" + lib::meta::CustomStringConv<X>::invoke(*ptr)
+    return ptr? showAddr(ptr) + " ↗" + StringConv<X>::invoke(*ptr)
               : "⟂ «" + typeStr(ptr) + "»";
   }
   

src/lib/p.hpp

@@ -158,7 +158,7 @@ namespace lib {
   P<TAR,BASE>::operator std::string()  const noexcept
   try {
     if (BASE::get())
-      return meta::CustomStringConv<TAR>::invoke (this->operator*());
+      return util::StringConv<TAR>::invoke (this->operator*());
     else
       return "⟂ P<"+meta::typeStr(this->get())+">";
   }