Hi all -- I'm getting a strange warning from . What's strange is not the warning, but the circumstances: if I use the boost headers installed in /usr/include, no warning. But if I make a copy of /usr/include/boost and tell the compiler to use the copy, I get a warning. I can reproduce this on CentOS 5 using our custom build of GCC 4.4.4 and boost 1.44.0, and on Ubuntu 10.04 (lucid) using Ubuntu's builds of GCC 4.4.3 and boost 1.40.0. It doesn't take much code to reproduce the problem: $ cat test.cpp #include First, a good build using Ubuntu /usr/bin/g++ (4.4.3) and /usr/include/boost (1.40.0) $ /usr/bin/g++ -Wall -Wconversion -c test.cpp -o test.o [no warnings] Now let's make a copy of the boost headers and use the copy: $ mkdir include $ cp -rp /usr/include/boost include/. $ /usr/bin/g++ -Wall -Wconversion -I include -c test.cpp -o test.o In file included from include/boost/functional/hash/detail/hash_float.hpp:30, from include/boost/functional/hash/hash.hpp:15, from include/boost/functional/hash.hpp:6, from test.cpp:1: include/boost/functional/hash/detail/hash_float_generic.hpp: In function ‘size_t boost::hash_detail::float_hash_impl2(T) [with T = float]’: include/boost/functional/hash/detail/hash_float_generic.hpp:84: instantiated from ‘size_t boost::hash_detail::float_hash_impl(T) [with T = float]’ include/boost/functional/hash/detail/hash_float.hpp:74: instantiated from ‘size_t boost::hash_detail::float_hash_value(T) [with T = float]’ include/boost/functional/hash/hash.hpp:282: instantiated from here include/boost/functional/hash/detail/hash_float_generic.hpp:59: warning: conversion to ‘float’ from ‘size_t’ may alter its value include/boost/functional/hash/detail/hash_float_generic.hpp:84: instantiated from ‘size_t boost::hash_detail::float_hash_impl(T) [with T = float]’ include/boost/functional/hash/detail/hash_float.hpp:74: instantiated from ‘size_t boost::hash_detail::float_hash_value(T) [with T = float]’ include/boost/functional/hash/hash.hpp:282: instantiated from here include/boost/functional/hash/detail/hash_float_generic.hpp:71: warning: conversion to ‘float’ from ‘size_t’ may alter its value I get the same behaviour if include/boost is a symlink to /usr/include/boost. Changing "-I include" to an absolute path ("-I `pwd`/include") makes no difference. If I repeat the experiment on my CentOS 5 machine with custom builds of more recent versions of GCC (4.4.4) and boost (1.44.0), I see the same thing. The line numbers are slightly different: In file included from include/boost/functional/hash/detail/hash_float.hpp:30, from include/boost/functional/hash/hash.hpp:15, from include/boost/functional/hash.hpp:6, from test.cpp:1: include/boost/functional/hash/detail/hash_float_generic.hpp: In function ‘size_t boost::hash_detail::float_hash_impl2(T) [with T = float]’: include/boost/functional/hash/detail/hash_float_generic.hpp:82: instantiated from ‘size_t boost::hash_detail::float_hash_impl(T) [with T = float]’ include/boost/functional/hash/detail/hash_float.hpp:74: instantiated from ‘size_t boost::hash_detail::float_hash_value(T) [with T = float]’ include/boost/functional/hash/hash.hpp:315: instantiated from here include/boost/functional/hash/detail/hash_float_generic.hpp:56: warning: conversion to ‘float’ from ‘size_t’ may alter its value include/boost/functional/hash/detail/hash_float_generic.hpp:82: instantiated from ‘size_t boost::hash_detail::float_hash_impl(T) [with T = float]’ include/boost/functional/hash/detail/hash_float.hpp:74: instantiated from ‘size_t boost::hash_detail::float_hash_value(T) [with T = float]’ include/boost/functional/hash/hash.hpp:315: instantiated from here include/boost/functional/hash/detail/hash_float_generic.hpp:69: warning: conversion to ‘float’ from ‘size_t’ may alter its value but obviously the trace through the header files is basically the same. Any clues? Google did not turn up any similar problems from anyone else. Oh yeah: if you're wondering why I want to use a copy of (or symlink to) the real boost installation, it's because we need to support building many different versions of our software, and they don't all use the same versions of boost or gcc (or various other 3rd-party components). So each build tree has a bunch of local symlinks pointing to exactly the right versions of gcc, boost, etc. Thanks! Greg