It's not uncommon to want some values to remain untouched and unchanged from certain processes. One of the easiest ways of doing this is to use the const qualifier. A qualifier, in C++, is simply something that modifies, or adds a quality to, what follows. These behave much like an adjective in a sentence: 'the weary fellow', 'the old hag', 'the constant integer'.
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The easiest way to make use of the const modifier is in a simple variable declaration. Take, for example, the following snippet of code which does not utilize this modifier:
The above code snippet should work fine, but let's pretend that the main snippet of code is actually part of a much bigger program. As such, we made a little typo before calculating and outputting the area in which we meant to set 'PINE', some other variable in our pretend big program, but accidentally set 'PI':
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The worst thing about the above code is that although 'Area' results will now always be incorrect, we may not have a clue that this is the case! Our compiler doesn't throw an error at us because what we're doing is technically correct, but we've wrecked the value of 'PI' which we may be using in a variety of places throughout the rest of the application code. Such errors are difficult to track down (Are we getting the input wrong? Is our formula incorrect?), and as such, we can use the const qualifier on the 'PI' variable to make the value a read-only value.
This means that the variable must be initialized as soon as it is declared so that it can possess a value at all, and from there the value cannot be modified -- the compiler will throw you an error if you try. The qualifier, in this case, should go before the to be a constant (using const), our compiler would luckily throw an error at us when we try to change its value:
The error should tell you that the variable is read-only or that you simply cannot assign to it, and as such you can jump in and fix or remove the line causing the problems. Phew!
One of the other extremely useful uses of the const qualifier is when passing to functions or member functions by reference. Despite the speed and memory advantages to doing so, it can be dangerous as the values we pass in could be directly modified by the function (by accident). Take for example the following where we accidentally change the value of 'value' via the function:
Again, the problems in the above may be difficult to isolate in a larger program, and as you may have guessed, const makes for a very good solution here. So instead of our function taking a regular integer reference, it can take a const int& v (constant integer reference)! This means that our function won't actually be able to modify the value that 'v' is aliasing ('value'), however we'll still be able to mess with 'v' in other situations (e.g. just in main). With the const modification, we'll get an error thrown for the attempted modification of a const parameter (Hurrah!):
The above means that you can get the speed and memory advantages of passing by reference, while preventing the danger of modifying variables that you don't want to be changed. You can also set const on 'regular' parameters too, however this often isn't as useful (although can be in some situations).
We should also talk a bit about constant pointers, as they can be a little bit more confusing. This confusion mainly comes from the fact that when dealing with pointers, there are two types of constant which you may wish to achieve. One is not being able to change what the pointer points to, and the other is not being able to change the value of the thing which the pointer points to. The latter can be accomplished easily by putting the const qualifier before thes firstly set up a basic class structure with some function prototypes (we're going to do it with a separate structure and implementation, simply for practice):
Before we even get on to the implementation of the 'square' function, we need to add the const qualifier to the prototype too. The qualifier can be added to a member function by very simply typing the const keyword after the parameter brackets (note that putting it before the as we've done previously - structuring and prototyping is more common in larger applications where separate header and source files come in to play):
Adding the qualifier in this context essentially just means that the member function can't modify any of the member variables (in this case, it can't modify 'n'). The other important thing about these constant member functions is that they can only call other constant member functions. If such a member function were to just call another member function to modify the value of a member variable, this would just defeat the whole point of the 'const' in this context, and as such, only other 'const' member functions can be called from these. In this case, it makes sense to build this functionality in as the 'square' member function shouldn't be modifying any member variables, and shouldn't call any other member functions that will (or even have the ability to) do so.
Variables are an extremely core concept to most object orientated programming languages. I like to visualize a variable much like a box. We can put things in the box, we can take things out of the box, and at any point we can see what is inside the box. Each box also has a name to which we can refer to it by, and in C++, each box can only hold a certain type of data.
When we create variables we call this the variable declaration, and then when we set them for the first time, we call this the initialization. To declare a variable in C++, we write the function. To declare a basic integer variable called 'age', we could write the following:
From this point we can then refer to the variable by its name, so in this case, we can just write 'age' whenever we want to refer to the variable. To initialise the variable we can write its name, followed by the equals sign, followed by the value we want to set the variable to (followed by a semicolon). The value we set it to can be a constant (a value that doesn't change), or another variable of the same type. An operator is a symbol which has a certain meaning in the programming language, in this case, the equals operator, represented by the = symbol, is an operator which sets whatever is on the left of the operator to whatever is on the right.
The constant value we set the variable to depends on the to 5 with something like the following:
We can actually combine the variable declaration and initialization into one more-compact line, like the following:
The 'age' variable now contains the number '5', and we can refer to this '5' by writing 'age' anywhere in our program. We can also change the value of the variable at any point by using the equals operator as we did for the first initialization:
Although this seems purely for convenience at the moment (as we could just write '5', '3', or '21' in place of 'age'), trust me when I say that these become extremely useful and powerful when you start dealing with dynamic logic and user input (the latter of which we'll be covering later in this tutorial).
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Just to give an example of accessing the contents of variables by using their names, we could create a new variable called 'age_two' which is set to the value of 'age', and then we can also try outputting one or both of these variables:
To be clear, all this code should be going into the basic program structure which we learnt how to create in the last tutorial. So we want our 'iostream' include for cout, cin, and some other stuff, we want the std namespace, and we want the majority of our code to be going in our 'main' function. So our full code to demonstrate variables so far, which you can compile and run at any point to test the functionality, is as follows:
Some number variables can handle positive and negative numbers, whereas 'unsigned' number variables can only handle positive numbers, although because of this restriction, can hold larger numbers. You can write the signed or unsigned keywords before the and 'short' - numbers with a decimal place in. Floats are accurate to around 6 or 7 digits and are declared using the float type. Float constants can be defined by simply writing a number with a decimal point followed by the 'f' notation. An example of a simple float declaration and initialization to a float constant is as follows:
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Care must be taken, however, with float (and other decimal) operations, as rounding and precision problems to do with how the numbers are stored can trip you up (we don't have infinite memory for recurring decimals like 1/3 for example) -- I recommend reading this article for more information on this if you're interested.
Doubles
The 'double' or 'e'. Character variables are declared by using the char type, and character constants are defined by using single quotes (apostrophes) around the character. An example of character declaration and initialization to a character constant is as follows:
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Strings
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The lastve talked about string variables in relation to cout before, and as such you should know that string constants are defined by using double quotes. String variables are declared by using the string type, however as strings aren't actually 'primitive' types in C++ (and are instead defined by the standard library of stuff that comes bundled with C++), you are required to #include <string> to use thist strings aren't massively useful, but this is just because we don't really know how to utilize all the functionality of different data-types yet - for example, we don't know how to perform simple mathematics on number types, or how to check the value of booleans to change the logic of the program. All will be revealed in future tutorials.