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'.
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':
A C tutorial about 'Constants' If necessary, the qualifier can actually be used in both places - so a constant integer constant pointer, if you like, would be represented as const int. const ptrname. The final use of const which we're going to talk about in this tutorial is the use of the qualifier on class/struct member functions.To demonstrate this, let's firstly set up a basic class.
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:
/could-not-get-toolstationwood-from-tconstructtoolstinkertools.html. Crafting Guide gives step-by-step instructions for making anything in Minecraft or its many mods. Just say what you'd like to make, what you already have, it will do the rest, giving you a list of raw materials and instructions of which items to make in the proper order. If I recall, you can also achieve infinite durability I think on all tools if you use Magical Wood (Although the mining level will be quite low). If you make a tool entirely out of Magical Wood, you get 11 modifiers with the ability to get 14. 10 Obsidian Plates, and w/e you wanna use the last 4 modifiers for. Mar 17, 2014 Isn't the tool station the one you can make with wood? If so, there's your problem, you need a Tool Forge, since, uh, like 100 editions ago?Roughly the same time that the Smeltery was added you needed to use the Smeltery to craft metal parts, and you needed the tool forge to make T2 Tools like Hammers and Lumber Axes. Jul 14, 2014 I also have this issue, but cannot pinpoint which mod(s) are causing it. All mods I have installed are intended for 1.7.10. EDIT: The issue was because of a corrupt download of a couple of the ProjectRed modules, in case anybody else has this issue, re-download PR and see if that fixes things.
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.
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