logging of calls and returns

#include <functional>
#include <iostream>
#include <string>

namespace cppx {
    using
        std::function,
        std::string,
        std::ostream, std::clog, std::endl;

    class Movable
    {
    private:
        Movable( const Movable& )                           = delete;
        auto operator=( const Movable ) -> Movable&         = delete;
    public:
        Movable() noexcept                                  = default;
        Movable( Movable&& ) noexcept                       = default;
        auto operator=( Movable&& ) noexcept -> Movable&    = default;
    };

    class Logger
    {
        using Func = function<void(ostream&)>;
        enum Direction{ entering, leaving };

        string      m_funcname;
        Func        m_custom_output;

        static auto nesting_level()
            -> int&
        {
            static int the_level = 0;
            return the_level;
        }

        static auto indent( const int level )
            -> string
        { return string( 4, ' ' ); }

        void log( const Direction dir, const int level ) const
        {
            clog << (dir == entering? "->" : "<-") << indent( level ) << m_funcname;
            if( m_custom_output ) {
                clog << " (";
                m_custom_output( clog );
                clog << ")";
            }
            clog << endl;
        }

    public:
        ~Logger()
        {
            log( leaving, nesting_level()-- );
        }

        Logger( string funcname, Func custom_output = {} ):
            m_funcname( funcname ),
            m_custom_output( custom_output )
        {
            log( entering, nesting_level()++ );
        }
    };

}  // namespace cppx

namespace app {
    using std::ostream, std::cout, std::endl;
    using cppx::Logger;

    auto gcd( const int a, const int b )
        -> int
    {
        Logger logger( __func__, [&]( ostream& stream )
        {
            stream << "a = " << a << ", b = " << b;
        } );

        if( b == 0 ) {
            return a;
        } else {
            return gcd( b, a% b );
        }
    }

    void run()
    {
        const int r = gcd( 42, 12 );
        cout << "gcd( 42, 12 ) = " << r << endl;
    }
}  // namespace app

auto main() -> int { app::run(); }