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Using integral constant template parameters


Permalink 08:39:43 pm, by lano1106, 327 words, 4697 views   English (CA)
Categories: C++

Using integral constant template parameters

One usage of them commonly demonstrated in C++ Templates textbooks such as C++ Templates is when you want to control the maximum capacity of a container like this:

template< typename T, int MAXSIZE >
class stack
   T m_array[MAXSIZE];

I have found another very useful property of integral constant template parameters. You can define a class template with an integral constant template parameter and not use it at all in the declaration:

template<int i>
class Unique
  static int var;

template<int i>
int Unique<i>::var;

The property of this template is that a distinct var variable will be created for each instantiation of the template:

Unique<1234>::var = 1;
Unique<0>::var = 2;

and so on. By itself, it is more or less useful but imagine that you are dealing with a function registering a callback function that does not let you provide a parameter that will be passed to the callback function but your callback needs to access variables and you must provide more than 1 callback function and each of them needs to access its own set of variables. You could do it by hand by writing x different callbacks and create x sets of variables with different names but by doing that, you would be polluting your namespace and that would be a very long task for nothing. Instead, a class template with an integral constant parameter is the right tool for this situation:

template<int i>
class CB
  static void SetCBParams(int x1, int x2)
  { m_x1 = x1; m_x2 = x2; }
  static void CBFunc();
  static int m_x1;
  static int m_x2;

template<int i>
int CB<i>::m_x1;
template<int i>
int CB<i>::m_x2;

and then


Comments, Pingbacks:

Comment from: Emil [Visitor] Email
Very cool :)
PermalinkPermalink 08/27/07 @ 03:04

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Olivier Langlois's blog

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