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(C++) std::unary_function

std::unary_function is deprecated [2,3].

std::unary_function is an empty class that serves as the base class of a unary functor. A unary functor defines operator(), where operator() takes one argument.

The advantage of using std::unary_function is its (two) typedefs, so that the derived functors fits into more algorithms. Make functors adaptable [1].

Example: MakeAbs


#include <algorithm>

#include <cmath>

#include <functional>

#include <vector>



template <class T> struct Abs : public std::unary_function<T,T>

{

  const T operator()(const T& x) const { return std::abs(x); }

};



void MakeAbs(std::vector<int>& v)

{

  std::transform(v.begin(),v.end(),v.begin(),Abs<int>());

}

Example definition of std::unary_function

Simplified from the GNU ISO C++ Library, version 4.7.2:


template<typename _Arg, typename _Result>

struct unary_function

{

  typedef _Arg argument_type;   

  typedef _Result result_type;  

};

References

Scott Meyers. Effective STL. ISBN: 0-201-74962-9. Item 40: 'Make functor classes adaptable'
Programming Language C++, Library Working Group. 2010-10-06. Document number: N3145=10-0135
Working Draft, Standard for Programming Language C++. 2014-08-22. N3936. Paragraph D.8.2. 'The class templates unary_function and binary_function are deprecated. A program shall not declare specializations of these templates.'

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