[lmi-commits] [lmi] master 29808d8 5/5: Resolve a marked defect [390]

[Greg Chicares]

branch: master
commit 29808d8e6539b9790a7fbb091b7a9ca97b70c7e3
Author: Gregory W. Chicares <address@hidden>
Commit: Gregory W. Chicares <address@hidden>

    Resolve a marked defect [390]
---
 value_cast_test.cpp | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/value_cast_test.cpp b/value_cast_test.cpp
index 3bebc75..5c29de1 100644
--- a/value_cast_test.cpp
+++ b/value_cast_test.cpp
@@ -368,12 +368,25 @@ int extra_tests0()
     double small = std::scalbn
         (1.0
         ,std::numeric_limits<double>::min_exponent
-// TODO ?? Why doesn't this work with '- 1' appended?
-//        ,static_cast<double>(std::numeric_limits<double>::min_exponent - 1)
         );
     BOOST_TEST_EQUAL( small, value_cast<double>(value_cast<std::string>( 
small)));
     BOOST_TEST_EQUAL(-small, 
value_cast<double>(value_cast<std::string>(-small)));
 
+    // Of course, the minimum normalized power of two is this:
+    double tiny = std::scalbn
+        (1.0
+        ,std::numeric_limits<double>::min_exponent - 1
+        );
+    BOOST_TEST_EQUAL(0x1p-1022, tiny);
+    // which converts to this:
+    BOOST_TEST_EQUAL(0x0.fffffffffffffp-1022, 
value_cast<double>(value_cast<std::string>(tiny)));
+    // which is the largest denormal.
+    //
+    // The round trip works for the values tested earlier only by
+    // accident: 17 digits are required for a binary64 round trip,
+    // but value_cast returns no more than 16 because the 17th is
+    // not known to be accurate.
+
     return 0;
 }