(C++) Binder

 

 

A binder is a type of adapter that allows a two-argument function object to be used as a single-argument function by binding one argument to a value [1]. Binders are useful when using algorithms.

 

There are two STL binders:

• std::bind1st
• std::bind2nd

 

There is one Boost binder:

• boost::bind

 

Using boost::bind results in easier to read and shorter code.

 

 

 

 

 

Replacing a for loop by an algorithm using std::bind2nd and boost::bind

 

---

#include <vector> struct Widget {   void DoItOften(const int n) const { /* do it n times */ } }; void DoItOften(const std::vector<Widget>& v, const int n) {   const std::size_t sz = v.size();   for (int i=0; i!=sz; ++i)   {     v[i].DoItOften(n);   } }

 

---

#include <algorithm> #include <numeric> #include <vector> struct Widget {   void DoItOften(const int n) const { /* do it n times */ } }; void DoItOften(const std::vector<Widget>& v, const int n) {   std::for_each(v.begin(),v.end(),     std::bind2nd(std::mem_fun_ref(&Widget::DoItOften),n)   ); }

 

---

#include <algorithm> #include <vector> #include <boost/bind.hpp> struct Widget {   void DoItOften(const int n) const { /* do it n times */ } }; void DoItOften(const std::vector<Widget>& v, const int n) {   std::for_each(v.begin(),v.end(),     boost::bind(&Widget::DoItOften, _1, n)   ); }

 

 

 

 

 

Advice

 

• Prefer Lambda expressions over binders [2]

 

 

 

 

 

References

 

1. Bjarne Stroustrup. The C++ Programming Language (3rd edition). ISBN: 0-201-88954-4. Chapter 18.4.4: 'A binder allows a two-argument function object to be used as a single-argument function by binding one argument to a value.'
2. Scott Meyers. C++ And Beyond 2012 session: 'Initial thoughts on Effective C++11'. 2012. 'Prefer Lambdas over Binders'