Boost C++ Libraries: Ticket #11998: Static const variables cannot be logged

Andrey Semashev — Thu, 18 Feb 2016 10:02:04 GMT

I don't think this is a bug in the library.

The static constant members have to be defined in some (one) translation unit, which gives them storage. In C++11 and later this is required if a reference to the variable is taken, which happens when you call the operator<<. In pre-C++11 versions of the language, IIRC, the definition is required unconditionally.

The type conversion that you use as a workaround makes the operator<< bind the reference to a temporary, that's why it works. I'm not sure how it works differently in 1.59 since I don't see any changes that could lead to that but there were such changes since 1.58 - the operator<< overloads could have been chosen differently before, which would result in not taking the reference.

However, I can see that your use case could be useful, and I'll see if I can support it in the future.

Thu, 18 Feb 2016 10:34:06 GMT

The workaround is just to show that this behaviour relates to the static variable.

I compile with C++11 (but tested it also with C++98 and C++14). I did test it with the GCC (4.8.3) now aswell, same behaviour.

Wether it is a bug or not, it depends on the boost library version, which is strange.

o.z — Sun, 25 Sep 2016 12:42:31 GMT

I see similar problem using bitfields:

I took the example from http://www.boost.org/doc/libs/1_61_0/libs/log/example/doc/tutorial_trivial_flt.cpp and added a bitfield print:

#include <boost/log/core.hpp>
#include <boost/log/trivial.hpp>
#include <boost/log/expressions.hpp>
namespace logging = boost::log;
//[ example_tutorial_trivial_with_filtering
void init()
{
    logging::core::get()->set_filter
    (
        logging::trivial::severity >= logging::trivial::info
    );
}
struct BF {
                unsigned int b : 8;
                BF() : b(0) {}
};
int main(int, char*[])
{
    init();
    BF bf;
    BOOST_LOG_TRIVIAL(info) << "An informational severity message " << bf.b;
    return 0;
}
//]

With boost 1.60 I got a compilation error:

cannot bind bitfield 'bf.BF::b' to 'unsigned int&'

With boost 1.57 the code is compiled and run (prints: [2016-09-19 20:21:33.018112] [0x000007fd1d5be672] [info] An informational severity message 0)

status changed; resolution set

Andrey Semashev — Sat, 08 Oct 2016 19:31:09 GMT

status new → closed
resolution → fixed

Should be fixed in https://github.com/boostorg/log/commit/ea486c1468589b590a78ab88686dbc28ef2c91f5. Thanks for the report.