Lionel B wrote:
> I have a function which takes a functor argument. I which to call it for a
> functor which is actually a class member; this works fine, using
> the mem_fun_ref and bind1st functions (see listing 1 below). Or, rather,
> it works fine as long as my member functor is const. The problem comes
> when I wish to use it for a *non*-const functor (see listing 2 below):
>
> *** Start listing 1 ************************************************** *
>
> // test1.cpp
>
> #include <iostream>
>
> // function taking (ref. to) functor
> template<typename F> double bar(const F& f, double x, double y)
> {
> return f(x)+y;
> }
>
> // class with member functor
> class Object
> {
> private:
> double a, b;
>
> public:
> Object(double a_, double b_) : a(a_), b(b_) {}
>
> // the (const) member functor
> double foo(double x) const {return a*x+b;}
> };
>
> int main(int argc, char* argv[])
> {
> using namespace std;
>
> // create object
> Object object(2.0,3.0);
>
> // create functor ref from member functor "foo", bind to object
> // and pass to function "bar"
> double z = bar(bind1st(mem_fun_ref(&Object::foo),object),1.0, 2.0);
>
> cout << "z = " << z << '\n';
>
> return 0;
> }
>
> *** End listing 1 ************************************************** ***
>
> $ g++ test1.cpp -o test1
> $ ./test1
> z = 7
>
> So far so good. Now I try to make the member functor "foo" non-const:
>
> *** Start listing 2 ************************************************** *
>
> // test2.cpp
>
> #include <iostream>
>
> // function taking (ref. to) functor
> template<typename F> double bar(F& f, double x, double y)
> {
> return f(x)+y;
> }
>
> // class with member functor
> class Object
> {
> private:
> double a, b;
>
> public:
> Object(double a_, double b_) : a(a_), b(b_) {}
>
> // the (non-const) member functor
> double foo(double x) {return a*x+b;}
> };
>
> int main(int argc, char* argv[])
> {
> using namespace std;
>
> // create object
> Object object(2.0,3.0);
>
> // create functor ref from member functor "foo", bind to object
> // and pass to function "bar"
> double z = bar(bind1st(mem_fun_ref(&Object::foo),object),1.0, 2.0);
>
> cout << "z = " << z << '\n';
>
> return 0;
> }
>
> *** End listing 2 ************************************************** ***
>
> Note that I have made the first argument of bar now a *non* const ref:
Switch both bind1st and mem_fun_ref over to boost and it works with
non-const member function.
However, it remains to be seen if you get what you seek. bar's first
arguement must be const.
#include <boost/functional.hpp>
replace
using namepsace std;
with
using boost::binder1st;
using boost::mem_fun_ref;
>
> $ g++ test2.cpp -o test2
> test2.cpp: In function 'int main(int, char**)':
> test2.cpp:33: error: no matching function for call to 'bar(std::binder1st<std::mem_fun1_ref_t<double, Object, double> >, double, double)'
> test2.cpp:6: note: candidates are: double bar(F&, double, double) [with F = std::binder1st<std::mem_fun1_ref_t<double, Object, double> >]
>
> I don't understand this, as I would have thought that there'd be sensible
> non-const overloads for the binder and mem_fun routines...
>
> Any help much appreciated.
>
> Using gcc 4.1.1 on RHEL linux on x86_64
>
> --
> Lionel B
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