What is a resource in C++?

Question

In relation to this topic: What is the copy-and-swap idiom?

It states that one class should at most handle one resource. What is meant by resource?

EDIT: For example I have a class that handles the information for each monitor, and contains an array of the desktop pixels. Would the array and only the array be considered a resource? Would the array of monitors that hold the monitor information and array of the desktop pixels another resource, that would require another class? It is in another class, but is that what is meant?

I have searched here and Google but only find more information relating to the rule of three or resource files (*.rc and MSDN). Nothing that relates to a definition.

Answer 1

That question is referring to concepts in C++ in general. In this case, it is also using the general concept of a "resource": an object, usually external, provided by an outside source, that a block of code uses. It's the same as a "resource" in real life.

In the case of copy-and-swap, it is referring to what your C++ class represents. The idea is simple: each instance of your C++ class will provide access to a given resource. For instance, let's take a file on disk. The resource is the file; the operating system provides access to that file. Your C++ class would wrap around the operating system's API calls to read and write to that file.

The question that you linked is asking about how to mix these classes with C++'s initialization rules. How you follow these rules is going to be shaped by what accesses the outside source lets you use and what you personally need to do.

Of course, your program can also have its own resources. For example, a global array that maps one type of enumeration to another type of enumeration could be a resource in your program. So the answer to your question of "what should be a resource in my program" is "it really depends on how you wrote your program".

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Windows programs also have their own "resources", such as bitmaps, icons, dialog box control layouts, menu layouts, localized strings, and other stuff. To allow these things to be embedded in the output binary, Windows provides its own system, and also calls all these "resources". A .rc file is merely how you would list all of these Windows resources; you build that into your Windows program.

GLib (which GTK+ is built on top of), Qt, and the various Apple APIs have their own analogous systems, each of which also have "resource" in their names.

(You can write C++ classes that provide access to all these specific resources too; in this case, the resource is the resource.)

But it is very important when reading not to confuse he general term ("resource") for the specific technology (Windows resources) and vice versa, especially for such an abstract concept as a resource. Keep that in mind as you continue your programming journey.

Answer 2

Resources, are things like: raw pointers that need to be deleted (that's why we have smart pointers like std::unique_ptr, std::shared_ptr), files that need to be properly closed (that's why we have std::fstream), std::thread (to manage the raw handles that you would have to otherwise manage yourself in different operating systems). std::reference_wrapper manages references... and so on. As you may have noticed all of them manage a single resource, not more.

The idea is that you don't want a single unique_ptr to manage 2 pointers for you. You would create two unique_ptr classes for each raw pointer.

For your specific question which you have edited, generally what you wanna do is:

create a class MonitorsManager that manages all the monitors (that's his task)

create a class MonitorInfo (that manages a single monitor information and the pixels

You will notice things get easier if you have a good design, because in the future you will want to be able to easily edit and update those classes, and if you mix everything, it's gonna be really hard, trust me.

Also, your MonitorInfo class should not have any dependency on MonitorsManager because if people has to include the class "MonitorsManager" in their project in order to use your "MonitorInfo" class, then you have failed. Not everyone has multiple monitors, some may have a single one.