14.1 — Function templates

The need for function templates

In previous chapters, you’ve learned how to write functions and classes that help make programs easier to write, safer, and more maintainable. While functions and classes are powerful and flexible tools for effective programming, in certain cases they can also be somewhat limiting because of C++’s requirement that you specify the type of all parameters.

For example, let’s say you wanted to write a function to calculate the maximum of two numbers. You might do so like this:

This function would work great -- for integers. What happens later when you realize your max() function needs to work with doubles? Traditionally, the answer would be to overload the max() function and create a new version that works with doubles:

Note that the code for the implementation of the double version of maximum() is exactly the same as for the int version of max()! In fact, this implementation would work for all sorts of different types: chars, ints, doubles, and if you’ve overloaded the > operator, even classes! However, because C++ requires you to make your variables specific types, you’re stuck writing one function for each type you wish to use.

Having to specify different “flavors” of the same function where the only thing that changes is the type of the parameters can become a severe maintenance headache and time-waster, and it also violates the general programming guideline that duplicate code should be minimized as much as possible. Wouldn’t it be nice if we could write one version of max() that was able to work with parameters of ANY type?

This is where function templates come in!

What is a function template?

If you were to look up the word “template” in the dictionary, you’d find a definition that was similar to the following: “a template is a model that serves as a pattern for creating similar objects”. One type of template that is very easy to understand is that of a stencil. A stencil is an object (eg. a piece of cardboard) with a shape cut out of it (eg. the letter J). By placing the stencil on top of another object, then spraying paint through the hole, you can very quickly produce stenciled patterns in many different colors! Note that you only need to create a given stencil once -- you can then use it as many times as you like to create stenciled patterns in whatever color(s) you like. Even better, you don’t have to decide the color of the stenciled pattern you want to create until you decide to actually use the stencil.

In C++, function templates are functions that serve as a pattern for creating other similar functions. The basic idea behind function templates is to create a function without having to specify the exact type(s) of some or all of the variables. Instead, we define the function using placeholder types, called template type parameters. Once we have created a function using these placeholder types, we have effectively created a “function stencil”.

It turns out that you can’t call a function template directly -- this is because the compiler doesn’t know how to handle placeholder types directly. Instead, when you call a template function, the compiler “stencils” out a copy of the template, replacing the placeholder types with the actual variable types in your function call! Using this methodology, the compiler can create multiple “flavors” of a function from one template! We’ll take a look at this process in more detail in the next lesson.

Creating function templates in C++

At this point, you’re probably wondering how to actually create function templates in C++. It turns out, it’s not all that difficult.

Let’s take a look at the int version of max() again:

Note that there are 3 places where specific types are used: parameters nX, nY, and the return value all specify that they must be integers. To create a function template, we’re going to replace these specific types with placeholder types. In this case, because we have only one type that needs replacing (int), we only need one template type parameter. Let’s call our this placeholder type “Type”. You can name your placeholder types almost anything you want, so long as it’s not a reserved word. Here’s our new function with a placeholder type:

(Note: I also changed the Hungarian notation on the variables to reflect that they are not necessarily integers any longer -- they are whatever type Type is!)

This is a good start -- however, it won’t compile because the compiler doesn’t know what “Type” means! In order to tell the compiler that Type is meant to be a placeholder type, we need to formally tell the compiler that Type is a template type parameter. This is done using what is called a template parameter declaration:

Believe it or not, we’re done! This will compile!

Now, let’s take a slightly closer look at the template parameter declaration. We start with the keyword template -- this tells the compiler that what follows is going to be a list of template parameters. We place all of our parameters inside angled brackets (<>). To create a template type parameter, use either the keyword typename or class. There is no difference between the two keywords in this context, and you will usually see people use the class keyword. However, we prefer the newer typename keyword, because it makes it clearer that the template type parameter doesn’t have to be a class. After the typename or class keyword, all that’s left is to pick a name for your placeholder type. Traditionally, with function that have only one template type parameter, the name “Type” (often shortened to “T”) is used. If the template function uses multiple template type parameter, they can be separated by commas:

Using function templates

Using a function template is extremely straightforward -- you can use it just like any other function:

Note that all three of these calls to max() have parameters of different types!

As you can see, template functions can save a lot of time, because you only need to write one function, and it will work with many different types. Once you get used to writing function templates, you’ll find they actually don’t take any longer to write than functions with actual types. Template functions reduce code maintenance, because duplicate code is reduced significantly. And finally, template functions can be safer, because there is no need to copy functions and change types by hand whenever you need the function to work with a new type!

Template functions do have a few drawbacks, and we would be remiss not to mention them. First, older compilers generally do not have very good template support. However, modern compilers are much better at supporting and implementing template functionality properly. Second, template functions produce crazy-looking error messages that are much harder to decipher than those of regular functions. However, these drawbacks are fairly minor compared with the power and flexibility templates bring to your programming toolkit!

Note: The standard library already comes with a templated max() function. If you use the statement “using namespace std;” the compiler will be unable to tell whether you want your version of max() or std::max().

14.2 -- Function template instances
13.7 -- Random file I/O

14 comments to 14.1 — Function templates

  • AJB

    I have a question. I have this function, using vector of integers, and like to make it a template, so I can use it also to print doubles. How to do this ?

    Need to include string, iostream, vector, algorithm !

    void PrintVector(std::vector &A)
    // go through the vector using an iterator
    std::vector::const_iterator it; // declare an read-only iterator
    it = A.begin(); // assign it to the start of the vector

    // print only first 25 elements..
    int nPrinted = 0;
    int nMaxPrint = 25;
    while (it != A.end()) // while it hasn't reach the end
    std::cout << *it << " "; // print the value of the element it points to
    it++; // and iterate to the next element
    if (nPrinted == nMaxPrint)
    std::cout << std::endl;
    std::cout << std::endl;

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