const char vs. const signed char in boost/archive/iterators/binary_from_base64.hpp

[Gernot Hillier <gernot.hillier@…>]

There seems to be a small bug in binary_from_base64.hpp:

     CharType operator()(CharType t) const{
        const char lookup_table[] = {
             -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
     [...]

However, it's not guaranteed that a "const char" accepts signed values, see e.g. ​http://www.network-theory.co.uk/docs/gccintro/gccintro_71.html. See also /usr/include/limits.h - depending whether __CHAR_UNSIGNED__ is set or not, the platform may not or may accept negative values for a const char.

Especially when enabling C++0x support, this leads to a compilation failure e.g. on PowerPC:

"powerpc-linux-gnu-g++"  -ftemplate-depth-128 -O3 -finline-functions -Wno-inline -Wall -pthread -fPIC -std=c++0x -DBOOST_ALL_NO_LIB=1 -DBOOST_SERIALIZATION_DYN_LINK=1 -DNDEBUG  -I"." -c -o "bin.v2/libs/serialization/build/gcc-powerpc/release/threading-multi/basic_text_iprimitive.o" "libs/serialization/src/basic_text_iprimitive.cpp"
[...]
In file included from ./boost/archive/impl/basic_text_iprimitive.ipp:30:0,
                 from libs/serialization/src/basic_text_iprimitive.cpp:19:
[...]
./boost/archive/iterators/binary_from_base64.hpp:51:9: error: narrowing conversion of '-0x00000000000000001' from 'int' to 'const char' inside { }

The following patch fixed the problem for us:

Index: boost_1_46_1/boost/archive/iterators/binary_from_base64.hpp
===================================================================
--- boost_1_46_1.orig/boost/archive/iterators/binary_from_base64.hpp
+++ boost_1_46_1/boost/archive/iterators/binary_from_base64.hpp
@@ -39,7 +39,7 @@ template<class CharType>
 struct to_6_bit {
     typedef CharType result_type;
     CharType operator()(CharType t) const{
-        const char lookup_table[] = {
+        const signed char lookup_table[] = {
             -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
             -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,
             -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,62,-1,-1,-1,63,

[Robert Ramey]

looks good to me, I'm rolling in your change. It will likely show up in 1.48