How to use bind1st and bind2nd? - c++

Question

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

You need to use a Copyable/Refrencable object, the following works:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
void print_i(int t, std::string separator)
{
std::cout << t << separator;
}
int main()
{
std::vector<int> elements;
std::string delim = "\n";
for_each(elements.begin(),
elements.end(),
std::bind2nd(std::ptr_fun(&print_i),delim));
return 0;
}
Normally you can get the same effect by simply doing the following:
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
int main()
{
std::vector<int> elements;
std::copy(elements.begin(),
elements.end(),
std::ostream_iterator<int>(std::cout,"\n"));
return 0;
}
Also assuming you have access to TR1 in the STL you're using, its always best to revise/replace any uses of bind1st and bind2nd with std::bind

Answer 2

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 std::bind which is probably the better choice anyway.

Answer 3

You need to do the following steps:
1. create a struct (or class) that inherits from std::binary_function
2. define your predicate function in the operator() member function of the struct created in step 1
3. use bind2nd to bind an appropriate value to the struct created in step 1
I have done all this in an example. You can read the article and download the complete code on the following link: bind and find

Answer 4

These functions are deprecated since C++11 and removed in C++17. As mentioned in one comment above, the better solution now is to use std::bind and the placeholders:
void callable(int a, int b);
auto f = std::bind1st(&callable, 42); // returns a 1-arg function
becomes:
// returns a 1-arg function
auto f = std::bind(&callable, 42, std::placeholders::_1);