Question

I am trying to bind a template function during signaling. However, when I use a placeholder in the slot function, I get a “negative subscript” compiler error. I am using Visual Studio
2008. Thanks.

Code:

class My
{
private:
    boost::signals2::signal<void (int)> sig;

public:
    template < typename F, typename T, typename A1 >
    void proxy( F f, T t, A1 a1 )
    {
        boost::bind( f, t, a1 );    // this gets messaged elsewhere
    }

    int foo( int i )
    {
        return i-1;
    }

    int bar( int i )
    {
        return i+1;
    }

    template < typename F, typename T, typename A1 >
    boost::signals2::connection connect( F f, T t, A1 a1 )
    {
        return sig.connect( boost::bind( &My::proxy< F, T, A1 >, t, f, t, a1 ) );
    }

    void main()
    {
        boost::signals2::connection c1 = connect( &My::foo, this, 11 );     // ok
        boost::signals2::connection c2 = connect( &My::bar, this, _1 );     // negative subscript
    }
};

Error:

boost/bind/arg.hpp(37) : error C2118: negative subscript
boost/bind/bind.hpp(450) : see reference to function template instantiation 'boost::arg<I>::arg<A1>(const T &)' being compiled
with
[
    I=1,
    A1=int,
    T=int
]
boost/bind/bind_template.hpp(32) : see reference to function template instantiation 'void boost::_bi::list4<A1,A2,A3,A4>::operator ()<F,boost::_bi::list1<int &>>(boost::_bi::type<T>,F &,A &,int)' being compiled
with
[
    A1=boost::_bi::value<My *>,
    A2=boost::_bi::value<int (__thiscall My::* )(int)>,
    A3=boost::_bi::value<My *>,
    A4=boost::arg<1>,
    F=boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,
    T=void,
    A=boost::_bi::list1<int &>
]
boost/function/function_template.hpp(153) : see reference to function template instantiation 'void boost::_bi::bind_t<R,F,L>::operator ()<T0>(A1 &)' being compiled
with
[
    R=void,
    F=boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,
    L=boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>,
    T0=int,
    A1=int
]
boost/function/function_template.hpp(147) : while compiling class template member function 'void boost::detail::function::void_function_obj_invoker1<FunctionObj,R,T0>::invoke(boost::detail::function::function_buffer &,T0)'
with
[
    FunctionObj=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>,
    R=void,
    T0=int
]
boost/function/function_template.hpp(913) : see reference to class template instantiation 'boost::detail::function::void_function_obj_invoker1<FunctionObj,R,T0>' being compiled
with
[
    FunctionObj=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>,
    R=void,
    T0=int
]
boost/function/function_template.hpp(722) : see reference to function template instantiation 'void boost::function1<R,T0>::assign_to<Functor>(Functor)' being compiled
with
[
    R=void,
    T0=int,
    Functor=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>
]
boost/function/function_template.hpp(1065) : see reference to function template instantiation 'boost::function1<R,T0>::function1<F>(Functor,int)' being compiled
with
[
    R=void,
    T0=int,
    F=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>,
    Functor=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>
]
boost/function/function_template.hpp(1105) : see reference to function template instantiation 'boost::function<Signature>::function<F>(Functor,int)' being compiled
with
[
    Signature=void (int),
    F=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>,
    Functor=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>
]
boost/signals2/detail/slot_template.hpp(137) : see reference to function template instantiation 'boost::function<Signature> &boost::function<Signature>::operator =<F>(Functor)' being compiled
with
[
    Signature=void (int),
    F=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>,
    Functor=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>
]
boost/signals2/detail/slot_template.hpp(81) : see reference to function template instantiation 'void boost::signals2::slot1<R,T1,SlotFunction>::init_slot_function<F>(const F &)' being compiled
with
[
    R=void,
    T1=int,
    SlotFunction=boost::function<void (int)>,
    F=boost::_bi::bind_t<void,boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>>
]
test.cpp(179) : see reference to function template instantiation 'boost::signals2::slot1<R,T1,SlotFunction>::slot1<boost::_bi::bind_t<R,F,L>>(const boost::_bi::bind_t<R,F,L> &)' being compiled
with
[
    R=void,
    T1=int,
    SlotFunction=boost::function<void (int)>,
    F=boost::_mfi::mf3<void,My,int (__thiscall My::* )(int),My *,boost::arg<1>>,
    L=boost::_bi::list4<boost::_bi::value<My *>,boost::_bi::value<int (__thiscall My::* )(int)>,boost::_bi::value<My *>,boost::arg<1>>
]
test.cpp(196) : see reference to function template instantiation 'boost::signals2::connection My::connect<int(__thiscall My::* )(int),My*,boost::arg<I>>(F,T,A1)' being compiled
with
[
    I=1,
    F=int (__thiscall My::* )(int),
    T=My *,
    A1=boost::arg<1>
]

Answer 1

Possibly typo?
The member function &My::proxy< F, T, A1 > in the question takes the
placeholder _1‘s type itself instead of the actual parameter int.
So in this case, changing &My::proxy< F, T, A1 > to
&My::proxy< F, T, int > will resolve the error.
Here is a test on ideone.