using per class boost::log::sources::logger from const member function

Question

I want a log source per class like the basic_logger example from the docs

This works but I get a compiler error complaining about the constness when logging something from a const member function like so:

#include "boost/log/sources/channel_logger.hpp"
#include "boost/log/common.hpp"
#include "boost/log/utility/init/to_console.hpp"
#include "boost/log/utility/init/common_attributes.hpp"

class Test
{
    public:
        // this works, not a const function
        void test1()
        {
            BOOST_LOG(m_logger) << "Test 1";
        }

        // this will not compile, const function
        void test2() const            
        {
            BOOST_LOG(m_logger) << "Test 2";
        }
    private:
        boost::log::sources::channel_logger<std::string> m_logger;
};

int main()
{
    boost::log::add_common_attributes();
    boost::log::init_log_to_console();

    Test t;
    t.test1();
    t.test2();
    return 1;
}

In this example Test::test2 gives a compile error because its const and m_logger is not. Test::test1 works fine.

How can this be fixed in a clean way without const_cast, mutable etc.

Answer 1

If the log is stored in the class, then of course you cannot modify it in a const context.

I realise that you asked for something else, but I honestly think that mutable is the appropriate solution here. This is exactly the use case that is why mutable exists.