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c++templatesoverloading
Seth Carnegie
Seth Carnegie

Reputation: 75130

Overloading template classes by template parameter number

Is it possible to have multiple versions of the same class which differ only in the number of template arguments they take?

For instance:

template<typename T>
class Blah {
public:
    void operator()(T);
};

template<typename T, typename T2>
class Blah {
public:
    void operator()(T, T2);
};

I'm trying to model functor type things which can take a variable number of arguments (up to the number of different templates that were written out).

Upvotes: 27

Views: 19004

Answers (3)

john
john

Reputation: 87959

This is untested code, I don't have a version of boost handy, but here goes anyway

#include "boost/tuple.h"

template <class T>
class Blah;

template <class T>
class Blah< boost::tuple<T> >
{
  void operator()(T arg);
};

template <class T, class U>
class Blah< boost::tuple<T, U> >
{
  void operator()(T arg1, U arg2);
};

etc. etc.

Upvotes: 0

Kerrek SB
Kerrek SB

Reputation: 477040

A template can have only one base definition. If you need a variable number of arguments and you don't want to use "null type" constructions as @awoodland suggests, and if you have a C++0x compiler, then you can use variadic templates:

template <typename ...Dummy> struct foo; // base case, never instantiated!

template <typename T> struct foo<T> { /*...*/ };  // partial spec. for one parameter
template <typename T, typename U> struct foo<T, U> { /*...*/ };  // ditto for two

Upvotes: 23

Flexo - Save the data dump
Flexo - Save the data dump

Reputation: 88711

The simplest answer would be to have just one template, with the maximum number you want to support and use void for a default type on all but the first type. Then you can use a partial specialization as needed:

template<typename T1, typename T2=void>
struct foo {
    void operator()(T1, T2);
};

template <typename T1>
struct foo<T1, void> {
     void operator()(T1);
};

int main() {
   foo<int> test1;
   foo<int,int> test2;
   test1(0);
   test2(1,1);
}

Upvotes: 28

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