[boost] Proposal for improved versions of some std function objects

From: Matthias Troyer (troyer_at_[hidden])
Date: 2003-05-01 04:01:05

Dear Boosters,

Section 20.3.2 of the standard defines some arithmetic function objects 
like

template <class T> struct multiplies : binary_function<T,T,T> {
   T operator()(const T& x, const T& y) const { return x*y;}
};

We have several applications where we find it too restrictive that both 
arguments as well as the result tyoe have to be of the same type. I 
thus want to propose extended versions for Boost, like:

template <class Arg1, class Arg2=Arg1, class Res=Arg1> struct 
multiplies : binary_function<Arg1,Arg2,Res> {
   Res operator()(const Arg1& x, const Arg2& y) const { return x*y;}
};

Consider e.g. the case where I want to multiply a vector of valarrays 
with a factor:

std::vector<std::valarray<double> > arrays;
std::vector<std::valarray<double> > out;
std::vector<double> factors;

std::transform(arrays.begin(),arrays.end(),factors.begin(),out.begin(),
                 
multiplies<std::valarray<double>,double,std::valarray<double> >());

I could not do that with the std::multiplies function object, where all 
three types have to be the same. We have written such extended versions 
for all the function objects in Sec. 20.3.2 and Sec. 20.3.4 of the 
standard (I don't have any such need for the comparators in Sec. 
20.3.3), as well as the additional bit operation objects:

bit_and
bit_or
bit_xor
bit_not

I would like to propose to add those function objects to the 
functional.hpp header of Boost, since this header already contains 
other improvements to Sec. 20.3 of the standard. If Mark Rodgers and 
the Boosters agree with this proposal then I would prepare some 
documentation and submit the necessary patches.

Matthias