Hmm, I think I found the issue. My signal object is const for some reason. I'm doing a lot of tricks to map enums to signal objects, I'll post all the code at the bottom of this message. My fusion::vector either must be containing the signal objects as const, or fusion::at_c must be returning const. I'm not really sure. namespace detail { template< InputSubscription t_sub > struct sub { const static InputSubscription value = t_sub; }; using boost::mpl::map; using boost::mpl::pair; typedef map< pair< sub, boost::signal >, pair< sub, boost::signal >, pair< sub, boost::signal >, pair< sub, boost::signal > > SubMap; } /// Obtains a boost::signal from a matching InputSubscription identifier. template< InputSubscription t_sub > struct GetSignal { typedef typename boost::mpl::at

::type signal; };

struct SignalBank { boost::fusion::vector< GetSignal::signal, GetSignal::signal, GetSignal::signal, GetSignal::signal > signals; }; template< InputSubscription t_sub > static void subscribe( SignalBank const& bank, typename GetSignal::signal::slot_type const& slot ) { using boost::fusion::at_c; const_cast::signal>( at_c( bank.signals ) ).connect( slot ); } void mouseCB( int, int ) { } void footest() { SignalBank bank; //boost::fusion::at_c<1>( bank.signals )( 3, 4 ); subscribe( bank, &mouseCB ); };