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Index » Programming » Tutorials » C & C++ » Advanced »

Templates

john\′s Photo
31 Oct 07, 5:45AM
Templates
Templates are one of the most powerful, yet seemingly unknown, programming techniques to arise in recent years.

Very simply, templates allow you to write functions that have variable parameter types. We are all used to the idea of creating function with one or more arguments, with each argument holding a range of possible value(s). With templates, your function can also have variable argument types as well.

Functions that use templates are distinct from regular functions, and are often called 'template functions'.

When Should They Be Used

Often people are introduced to templates, by designing the MAX macro as a template. I still use the MAX macro quite happily, so I've always felt a better example of template power is a 'swap' function, which will swap the values within two variables (passed by reference) depending on which is larger, i.e.
swap(10,11) => 10,11 
swap(11,5)  => 5,11

The function for doing this is simply:
void swap(int &a, int &b) 
{ 
  if(a>b) 
  { 
    int c=a; 
    a=b; 
    b=c; 
  } 
}
However, you'll invariably find that also want to 'swap' other variable types, i.e:
swap(10.2,45.3);                      // using doubles 
swap('a','b');                        // using chars 
swap(string("fred"),string("harry")); // using std::string

You can off course rewrite the function n times, overloading the function to account for every possible usage, remembering as you go that each argument must have the same type. To solve this, we use templates.
template <class T> 
swap(T &a, T &b) 
{ 
  if(a>b) 
  { 
    T c=a; 
    a=b; 
    b=c; 
  } 
}

Drop this function into a header file (.h) somewhere, and whenever you want to swap values if one is larger than the other, you can. The important pieces of this code are really just
template <class T>
when 'class', should be confused with objects or anything else is OOP, but is there to represent any type (like a variant in VB), such that 'T' is any type, but critically always the same type in each instansiation of the function. Effectively the compiler will identify the type T that is being passed to the function, and simple replace every instance of T with that type. As such our swap function will accept any type, so long as 'a' and 'b' are off the same type.

For example:
swap(10.2,45.3);                      // T become a float or double 
swap('a','b');                        // T become a char 
swap(string("fred"),string("harry")); // T becomes a std::string

Often you could write these functions using macros (#define's), however the great thing about templates over #defines is that do type checking. So it would be illegal to call out swap function with:
swap('a',45.6); 
swap(string("jim"), 20);

What Are The Performace Issue

Templates are translated into regular functions at compile time, so its as if you wrote a number of overloaded functions for each set of types you use to call the function. As such templates functions can actually be designed to be faster than their conventional cousins. However in general there is no performance bottle neck, aside from longer compile times (see below)

What Are The Nagatives

Template function can not be pre-compiled, since the compiler doesn't know what types should be used until its unwound the template in accordance with how you call it. For this reason templates can't be used in pre-compiled libraries that you may want to distribute to your customer
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