How to reduce a number of parameters of a template template parameter

Question

I need to adapt a two parameter template to a one parameter template.

I would like to bind the first parameter of a template:

template<class T1, class T2>
struct Two_Parameter_Template {
   // Two ctor's
   Two_Parameter_Template() { };
   template<class Param>
   Two_Parameter_Template(Param) { /* ... */ }
};

by using antoher (non intrusive) template mechanism:

//bind the first parameter of a template "bound", this template should work as 
//an adapter reducing the number of parameters required
template<class X, template<class, class> class bound>
struct bind_1st {
   template<class T> // this is the resulting one parameter template
   struct eval {
       eval () {
           bound<X, T>();
       }
       template<class T2>
       eval (T2 obj) {            
           bound<X, T>(obj);            
       }
   };
};

So that I could use this template later, as a paremeter for another template, with one less parameter of it's own (something like bellow):

template <template<class> class One_Parameter_Template>
struct SomeTemplate {
   //...
};

// Later in the code
typedef SomeTemplate<bind_1st<Bound_Param_class, Two_Parameter_Template> >::eval    ConcreteClass;

The question is - is there a syntax in C++ to support this.

Best Regards,
Marcin

In the example the Two_Parameter_Template is a checking policy which can throw an exception of type T1. I need to predefine the type of exception in advance (so that the number of "visible" parameters of this policy will be 1 instead of 2)and pass this policy to another template. — Marcin
– Marcin, Commented Oct 10, 2010 at 18:48

Vinzenz · Accepted Answer · 2010-10-10 19:22:35Z

1

You can use boost mpl bind for this

However it will not do exactly how you would like it to behave

Edit: I saw you made one little mistake in your code that it does not work as you expected:

typedef SomeTemplate< bind_1st<Bound_Param_class, Two_Parameter_Template>::eval > ConcreteClass;

If you put the eval inside it will work. This probably will be the easiest way to solve your issue. I hope I did this time get your problem better ;-)

edited Oct 10, 2010 at 19:22

answered Oct 10, 2010 at 18:20

Vinzenz

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3 Comments

Marcin Over a year ago

Thank you for precise answer. The difficult part is that I need to bind only the first parameter of the "Two_Parameter_Template" template, leaving the rest unspecified. The template cannot be "fully" concretized after binding. The second parameter has to be passed later.

Vinzenz Over a year ago

I just realize that your code almost works anyway. See edit above.

Marcin Over a year ago

Thats it! I can believe that I've posted this Question because of such a stupid typo :). The template's syntax can drive one nuts... Thanks a lot for spotting the error.

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