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

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::test开发者_如何学Python1 works fine.

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

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.

You need to move the mutable logger outside of your class then all will work fine. Or you use something different which gives you an immutable logger object.