c++ - Reducing number of template arguments for class -

i have method , 2 classes defined this:

template<template<class x> class t> void dosomething() {       t<int> x; } template <class t> class classwithonearg {     t t; };  template <class t1, class t2> class classwithtwoargs {     t1 t1;     t2 t2; }; 

i can

dosomething<classwithonearg>(); 

but cannot

dosomething<classwithtwoargs>(); 

however, i'd pass classwithtwoargs dosomething, t2 = double.

the method found create

template <class t1> class classwithtwoargs_child     : public classwithtwoargs<t1, double> { }; 

and then

dosomething<classwithtwoargs_child>(); 

this works, in concrete case classes require constructor argument , have create constructor argument in _child-class , pass base want avoid.

do have idea how that?

thanks lot!

indirection solution. instead of template template parameter pass "meta function" -- function maps 1 type in form of struct nested class template:

struct mf1 {   template<class arg1>   struct eval {     typedef classtemplatewithonearg<arg1> type;   }; };  template<class arg2> struct mf2 {   template<class arg1>   struct eval {     typedef classtemplatewithtwoargs<arg1,arg2> type;   }; };  template<class metafunc> void do_something() {   typedef typename metafunc::template eval<int>::type clazztype;   clazztype x; }  void foo() {   do_something<mf1>();   do_something<mf2<double> >(); } 

in c++0x reduced "template typedef":

template<class arg1> using newclasstemplate = classtemplatewithtwoargs<arg1,double>; 

which allows pass newclasstemplate template template argument accepts 1 template parameter.