# C++/Templates 1

At times we find difficulties in programming, we want to make something work the same for different objects and we often have to design the same thing several times in order to make it work. Suppose you want to make a simple Vector class, as in std::vector. Practically, in C, there is only one way to do it: Keeping type information stored somewhere in your list. This is a very common problem in C, and several people came up with several solutions for that, but now, in C++, the solution to all type-insensitive (those who do not rely on class functions / members) classes and functions came up. If you are one of those interested in the concept of generics (same as type-insensitive), you are on the right place. Welcome to C++ templates.

## Introduction

Templates are the mechanism by which C++ implements the generics concept. The following example illustrates two non-generic (type-sensitive) functions for multiplying two numbers, x and y:
Two functions that do exactly the same thing, but cannot be defined as a single function because they use different data types.

## Function Templates

Templates were made to fullfil the need to design generic code, that work the same way in different situations. Many people get scared when they first look at a template, as the syntax looks too complex for a very simple thing. Although the syntax is indeed a little complex, it is just different, not harder, not easier than other syntax presented in C++.

### Syntax

To start a template, you must provide the following declaration:
or
The keywords class and typename have exactly the same meaning in this case, but some compilers may replace the word class for typename when giving you information about the class, as it's often better accepted by programmers.
Type is our generic data type's name, and when the template is to be used, it would be the same as if Type was a typedef for your datatype, and that's exactly how it behaves. Therefore, whenever you use the word Type, you are talking about the data type being used on your template. The following example now illustrates how the multiply function would be written using a template:
To use the function, however, the full syntax is a bit weird:
As a syntax sugar, C++ provides the option of hiding the type information, case in which the data type used on the function call is assumed. For instance:
Both yield the same result, as they are exactly the same. An important note to take from here is that this is the one of the very few syntax sugars that templates have.
Optionally, a template can have more type options, and the syntax is pretty simple. For a template with three types, called First, Second and Third, we have:
And it works exactly as the single-typed template. The template data types are order sensitive, of course, so doing:
Is not the same as doing:
As the order the types are assigned changed.

## Class Templates

As another powerful feature of C++, you can also make template classes, which are classes that can have members of the generic type.
The Standard Template Library (STL) relies heavily on the power of template classes for its many functionalities, as the containers std::vector and std::list, for example. Examples on the implementation of both containers will be presented later on, as well as some more.

### Syntax

The syntax for template classes is exactly the same as for template functions. A small difference is that you can never hide the type of a template class when you instantiate it.
Therefore, a small class Node, with three members, previous, value and next can be written as:
Notice one important thing: Although the data type for Node pointers previous and next is hidden, the type Type is assumed. The same declarations could be written as:
Also notice that the exact same syntax used for function calling is used for class instantiating. You can hide data types in specific cases, but for legibility and to establish a solid coding convention, you shouldn't, unless you are on the same class as the variable being declared / instantiated.
There is also the possibility of setting a default type, as in:
Or
Both syntax are equally valid, and you can use previous types to define next ones (as T1 is the template type for type T2). Note the space between the two closing >, which prevent the compile to read them as one token (left-shift operator).
Note it only works effectively when you have more than one type. When you have only one type, you would be lacking the template argument, causing the compiler to give you an error. For example:
Although the default type for the template class is valid, you can not lack the type list when instantiating, but it can be empty, as in:
Note: The default type setting is available only for template classes, functions do not present this feature, as they already set the defaults, as explained before.

### Implementation

Now let's get our hands to work on a simple yet useful class, which we'll call Vector. Not to worry with minor problems such as dynamic memory allocation, which is needed in this case (to make the Vector grow when needed and optionally make it shrink when needed), we'll use an imaginary pointer that we believe contains an allocated array for us to use as a storage.
Our Vector class will have two functions: push and pop. We will also define an interesting thing we find in STL's std::vector, the iterators (to walk through the list), together with iterating functions begin and end.
To reach this goal, we will rely on a template class and some new concepts which will be explained with the code. This is how it looks:
This illustrates how simple templates can be. Now get your hands to work and, if you know how iterators work, play a little bit with the class; if not, don't worry, a fast and objective explanation will be given soon. If you got curious about the typedef we used to define iterator, head to the section relative to it in this lesson.
A simple use of our vector class:
Which then prints numbers from 0 to 10 in new lines, sequentially. It also illustrates the use of iterators to walk trough the list.
As requested by HappyCamper, here is an example of a container template class called List, that uses the same concept of std::list, the linked list.
A new iterator class had to be designed in order to overload the operators (to be the same as our Vector class iterators, in practice). Also notice that, when the template class instantiated (inside another class) is not of the class type, it requires the argument list to be written.
Warning: Both of the classes, Vector and List, were written in request-time, e.g. they didn't go through exhaustive tests to assure its secure execution, it just illustrates the concept of template containers.
We can also easily design our counterpart of STL's std::pair, which we'll name Pair. It goes as follows:
A pair is a very simple structure that holds two values (hence the term pair) that may be of different types. For such achievement we use two arguments in our template. Function first returns a reference to the first value and, function second, a reference to the second one.
To be continued