How to use bind1st and bind2nd ?

Question

Hi,

I would like to learn how to use binding functions. Here is the idea: I have this function which takes to parameters:

void print_i(int t, std::string separator)
{
        std::cout << t << separator;
}

And I would like to do:

std::vector<int> elements;
// ...
for_each(elements.begin(), elements.end(), std::bind2nd(print_i, '\n'));

But it does not work !

Here is what I get:

/usr/include/c++/4.3/backward/binders.h: In instantiation of ‘std::binder2nd<void ()(int, std::string)>’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/backward/binders.h:138: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:141: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:145: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:149: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:155: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/backward/binders.h:140: error: field ‘std::binder2nd<void ()(int, std::string)>::op’ invalidly declared function type
/usr/include/c++/4.3/backward/binders.h: In function ‘std::binder2nd<_Operation> std::bind2nd(const _Operation&, const _Tp&) [with _Operation = void ()(int, std::string), _Tp = char]’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/backward/binders.h:164: error: ‘void ()(int, std::string)’ is not a class, struct, or union type
/usr/include/c++/4.3/bits/stl_algo.h: In function ‘_Funct std::for_each(_IIter, _IIter, _Funct) [with _IIter = __gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, _Funct = std::binder2nd<void ()(int, std::string)>]’:
main.cpp:72:   instantiated from here
/usr/include/c++/4.3/bits/stl_algo.h:3791: error: no match for call to ‘(std::binder2nd<void ()(int, std::string)>) (int&)’
make: *** [all] Error 1

I could use functor, but it is quicker to use binding.

Thanks!

Answer 1

The argument to bind2nd must be an AdaptableBinaryFunction. A plain binary function does not fulfill this requirement (an adaptable function required typedefs for its return and argument types, a plain function type does not provide any typedefs). You could use boost::bind which is probably the better choice anyway.

Answer 2

Oh gosh, don't use the old bind1st and bind2nd. You'd do much better to use the improved boost bind which will be in C++0x.