c++ - 如何编写 boost::lambda 表达式作为函数回调参数

Question

假设我有以下代码:

    #include <boost/lambda/lambda.hpp>
    #include <boost/lambda/bind.hpp>
    #include <boost/function.hpp>
    #include <list>                                                                                                                                                         

    class MyListChild
    {
    };

    struct Filter
    {
      bool matches(MyListChild& child){ return true;}
    };


    class MyList 
    {
      public:
        typedef boost::function<bool(MyListChild&)> FilterCallback;
        // A common function that using any callback
        void filterChild(FilterCallback filter, std::list<MyListChild> list)
        {
          //... loop in children element
          MyListChild child;
          //filtering the child
          if(filter(child)) list.push_back(child);
        }
        // A specific function that using a custom filter
        void filterChild(const Filter& filter, std::list<MyListChild> list)
        {
          using namespace boost::lambda;
          //Trying to use the common function above
          filterChild(bind(&Filter::matches, filter, _1), list); //This will not compile.
        }
    };
    int main()
    {

      return 0;
    }

编译错误是:

    || /usr/include/boost/lambda/detail/actions.hpp: In instantiation of ‘static RET boost::lambda::function_action<3, T>::apply(A1&, A2&, A3&) [with RET = bool; A1 = bool (Filter::* const)(MyListChild&); A2 = const Filter; A3 = MyListChild; T = boost::lambda::detail::unspecified]’:
    /usr/include/boost/lambda/detail/lambda_functor_base.hpp|441 col 5| required from ‘RET boost::lambda::lambda_functor_base<boost::lambda::action<3, Act>, Args>::        call(A&, B&, C&, Env&) const [with RET = bool; A = MyListChild; B = const boost::tuples::null_type; C = const boost::tuples::null_type; Env = const boost::tuples::null_type;   Act = boost::lambda::function_action<3>; Args = boost::tuples::tuple<bool (Filter::* const)(MyListChild&), const Filter, const boost::lambda::lambda_functor<boost::lambda::    placeholder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type,   boost::tuples::null_type>]’
    /usr/include/boost/lambda/detail/lambda_functors.hpp|195 col 50| required from ‘typename boost::lambda::lambda_functor<Base>::inherited::sig<boost::tuples::tuple<A&>   >::type boost::lambda::lambda_functor<Base>::operator()(A&) const [with A = MyListChild; T = boost::lambda::lambda_functor_base<boost::lambda::action<3, boost::lambda::        function_action<3> >, boost::tuples::tuple<bool (Filter::* const)(MyListChild&), const Filter, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::     tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>  >; typename boost::lambda::lambda_functor<Base>::inherited::sig<boost::tuples::tuple<A&> >::type = bool]’                                                                       
    /usr/include/boost/function/function_template.hpp|132 col 42| required from ‘static R boost::detail::function::function_obj_invoker1<FunctionObj, R, T0>::invoke(boost::detail::function::function_buffer&, T0) [with FunctionObj = boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::action<3, boost::lambda::           function_action<3> >, boost::tuples::tuple<bool (Filter::* const)(MyListChild&), const Filter, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::     tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type>  > >; R = bool; T0 = MyListChild&]’
    /usr/include/boost/function/function_template.hpp|934 col 38| required from ‘void boost::function1<R, T1>::assign_to(Functor) [with Functor = boost::lambda::           lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::action<3, boost::lambda::function_action<3> >, boost::tuples::tuple<bool (Filter::* const)(MyListChild&),      const Filter, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >; R = bool; T0 = MyListChild&]’
    /usr/include/boost/function/function_template.hpp|722 col 7| required from ‘boost::function1<R, T1>::function1(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::action<3,       boost::lambda::function_action<3> >, boost::tuples::tuple<bool (Filter::* const)(MyListChild&), const Filter, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::  null_type> > >; R = bool; T0 = MyListChild&; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
    /usr/include/boost/function/function_template.hpp|1069 col 16| required from ‘boost::function<R(T0)>::function(Functor, typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type) [with Functor = boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambda::action<3,       boost::lambda::function_action<3> >, boost::tuples::tuple<bool (Filter::* const)(MyListChild&), const Filter, const boost::lambda::lambda_functor<boost::lambda::placeholder<1> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::  null_type> > >; R = bool; T0 = MyListChild&; typename boost::enable_if_c<boost::type_traits::ice_not<boost::is_integral<Functor>::value>::value, int>::type = int]’
    test.cc|33 col 59| required from here
    /usr/include/boost/lambda/detail/actions.hpp|96 col 37| error: no matching function for call to ‘boost::lambda::function_adaptor<bool (Filter::*)(MyListChild&)>::      apply(bool (Filter::* const&)(MyListChild&), const Filter&, MyListChild&)’
    ||        template apply<RET>(a1, a2, a3);
    ||                                      ^
    /usr/include/boost/lambda/detail/actions.hpp|96 col 37| note: candidates are:
    /usr/include/boost/lambda/detail/function_adaptors.hpp|394 col 17| note: template<class RET, class A1> static Result boost::lambda::function_adaptor<Result (Object::*  )(Arg1)>::apply(Result (Object::*)(Arg1), Object*, A1&) [with RET = RET; A1 = A1; Object = Filter; Arg1 = MyListChild&; Result = bool]
    ||    static Result apply( Result (Object::*func)(Arg1), Object* o, A1& a1) {
    ||                  ^
    /usr/include/boost/lambda/detail/function_adaptors.hpp|394 col 17| note:   template argument deduction/substitution failed:
    /usr/include/boost/lambda/detail/actions.hpp|96 col 37| note:   cannot convert ‘a2’ (type ‘const Filter’) to type ‘Filter*’
    ||        template apply<RET>(a1, a2, a3);
    ||                                      ^
    /usr/include/boost/lambda/detail/function_adaptors.hpp|398 col 17| note: template<class RET, class A1> static Result boost::lambda::function_adaptor<Result (Object::*  )(Arg1)>::apply(Result (Object::*)(Arg1), Object&, A1&) [with RET = RET; A1 = A1; Object = Filter; Arg1 = MyListChild&; Result = bool]
    ||    static Result apply( Result (Object::*func)(Arg1), Object& o, A1& a1) {
    ||                  ^
    /usr/include/boost/lambda/detail/function_adaptors.hpp|398 col 17| note:   template argument deduction/substitution failed:
    /usr/include/boost/lambda/detail/actions.hpp|96 col 37| note:   cannot convert ‘a2’ (type ‘const Filter’) to type ‘Filter&’
    ||        template apply<RET>(a1, a2, a3);

在我的案例中，编写 boost lambda 表达式的正确方法是什么？

Answer 1

问题解决了。只需将“const”添加到 Filter::matches。

struct Filter
{
  bool matches(MyListChild& child) const { return true;}
};