I have two arrays:
struct Data {
all_objects: Vec<Rc<dyn Drawable>>;
selected_objects: Vec<Rc<dyn Drawable>>;
}
selected_objects is guarenteed to be a subset of all_objects. I want to be able to somehow be able to add or remove mutable references to selected objects.
I can add the objects easily enough to selected_objects:
Rc::get_mut(selected_object).unwrap().select(true);
self.selected_objects.push(selected_object.clone());
However, if I later try:
for obj in self.selected_objects.iter_mut() {
Rc::get_mut(obj).unwrap().select(false);
}
This gives a runtime error, which matches the documentation for get_mut: "Returns None otherwise, because it is not safe to mutate a shared value."
However, I really want to be able to access and call arbitrary methods on both arrays, so I can efficiently perform operations on the selection, while also being able to still perform operations for all objects.
It seems Rc does not support this, it seems RefMut is missing a Clone() that alows me to put it into multiple arrays, plus not actually supporting dyn types. Box is also missing a Clone(). So my question is, how do you store writable pointers in multiple arrays? Is there another type of smart pointer for this purpose? Do I need to nest them? Is there some other data structure more suitable? Is there a way to give up the writable reference?
Ok, it took me a bit of trial and error, but I have a ugly solution:
struct Data {
all_objects: Vec<Rc<RefCell<dyn Drawable>>>;
selected_objects: Vec<Rc<RefCell<dyn Drawable>>>;
}
The Rc allows you to store multiple references to an object. RefCell makes these references mutable. Now the only thing I have to do is call .borrow() every time I use a object.
While this seems to work and be reasonably versitle, I'm still open for cleaner solutions.
So I was reading glib manual Instantiable classed types: objects: GObject Reference Manual. I know that when I am creating my own class I should use GClassInitFunc to handle class initialization. Question is, when do I need to use GBaseInitFunc?
Not to be flip, but I think the answer is "extremely rarely." I've never had to use it.
This is a really good question though. I was curious so I did a code search. It looks like the majority of occurrences of GBaseInitFunc are just casting a null pointer to the correct function pointer type. Some others I don't understand what they are doing. Here's one that is used to keep track of how many classes are initialized from that base class (though the info doesn't seem to be used.)
I was explaining to a friend a few days ago the concept or inheritance and containers.
He has very little programming knowledge so it was really just a friendly chat.
During the conversation he came to me with a question that i just couldn't answer.
"Why cant you just have an array of the top level class, and add anything to it"
I know this is a bad idea having being told so before by someone far smarter but for the life of me i couldn't remember why.
I mean we do it all the time with inheritance.
Say we have class animal which is parent of cat and dog. If we need a container of both of these we make the array of type animal.
So lets say we didn't have that inheritance link, couldn't we just use the base object class and have everything in the one container.
No specific programming language.
Syntactically, there is no problem with this. By declaring an array of a specific type, you are giving implicit information about the contents of that array. You could well declare a contain of Object instances, but it means you lose all the type information of the original class at compile-time.
It also means that each time you get an object out of the array at runtime, the only field instances and methods you know exist are the fields/methods of Object (which arguably is a compile time problem). To use any of the fields and methods of more specific subclasses of the object, you'd have to cast.
Alternatively, to find out the specific class at runtime you'd have to use features like reflection which are overkill for the majority of cases.
When you take elements out of the container you want to have some guarantees as to what can be done with them. If all elements of the container are returned as instances of Animal (remember here that instances of Dog are also instances of Animal) then you know that they can do all the things that Animals can do (which is more things than what all Objects can do).
Maybe, we do it in programming for the same reason as in Biology? Reptiles and Whales are animals, but they are quite different.
It depends on the situation, but without context, it's definitely okay in most (if not all) object-oriented languages to have an array of a base type (that is, as long as they follow all the substitution principles) containing various instances of different derived types.
Object arrays exist in certain cases in most languages. The problem is that whenever you want to use them, you need to remember what type they were, and stay casting them or whatever.
It also makes the code very horrible to follow and even more horrible to extend, not to mention error prone.
Plant myplant = new Plant();
listOfAnimals.Add(myplant);
would work if the list is object, but you'd get a compile time error if it was Animal.
What distinguishes and object from a struct?
When and why do we use an object as opposed to a struct?
How does an array differ from both, and when and why would we use an array as opposed to an object or a struct?
I would like to get an idea of what each is intended for.
Obviously you can blur the distinctions according to your programming style, but generally a struct is a structured piece of data. An object is a sovereign entity that can perform some sort of task. In most systems, objects have some state and as a result have some structured data behind them. However, one of the primary functions of a well-designed class is data hiding — exactly how a class achieves whatever it does is opaque and irrelevant.
Since classes can be used to represent classic data structures such as arrays, hash maps, trees, etc, you often see them as the individual things within a block of structured data.
An array is a block of unstructured data. In many programming languages, every separate thing in an array must be of the same basic type (such as every one being an integer number, every one being a string, or similar) but that isn't true in many other languages.
As guidelines:
use an array as a place to put a large group of things with no other inherent structure or hierarchy, such as "all receipts from January" or "everything I bought in Denmark"
use structured data to compound several discrete bits of data into a single block, such as you might want to combine an x position and a y position to describe a point
use an object where there's a particular actor or thing that thinks or acts for itself
The implicit purpose of an object is therefore directly to associate tasks with the data on which they can operate and to bundle that all together so that no other part of the system can interfere. Obeying proper object-oriented design principles may require discipline at first but will ultimately massively improve your code structure and hence your ability to tackle larger projects and to work with others.
Generally speaking, objects bring the full object oriented functionality (methods, data, virtual functions, inheritance, etc, etc) whereas structs are just organized memory. Structs may or may not have support for methods / functions, but they generally won't support inheritance and other full OOP features.
Note that I said generally speaking ... individual languages are free to overload terminology however they want to.
Arrays have nothing to do with OO. Indeed, pretty much every language around support arrays. Arrays are just blocks of memory, generally containing a series of similar items, usually indexable somehow.
What distinguishes and object from a struct?
There is no notion of "struct" in OOP. The definition of structures depends on the language used. For example in C++ classes and structs are the same, but class members are private by defaults while struct members are public to maintain compatibility with C structs. In C# on the other hand, struct is used to create value types while class is for reference types. C has structs and is not object oriented.
When and why do we use an object as opposed to a struct?
Again this depends on the language used. Normally structures are used to represent PODs (Plain Old Data), meaning that they don't specify behavior that acts on the data and are mainly used to represent records and not objects. This is just a convention and is not enforced in C++.
How does an array differ from both,
and when and why would we use an
array as opposed to an object or a
struct?
An array is very different. An array is normally a homogeneous collection of elements indexed by an integer. A struct is a heterogeneous collection where elements are accessed by name. You'd use an array to represent a collection of objects of the same type (an array of colors for example) while you'd use a struct to represent a record containing data for a certain object (a single color which has red, green, and blue elements)
Short answer: Structs are value types. Classes(Objects) are reference types.
By their nature, an object has methods, a struct doesn't.
(nothing stops you from having an object without methods, jus as nothing stops you from, say, storing an integer in a float-typed variable)
When and why do we use an object as opposed to a struct?
This is a key question. I am using structs and procedural code modules to provide most of the benefits of OOP. Structs provide most of the data storage capability of objects (other than read only properties). Procedural modules provide code completion similar to that provided by objects. I can enter module.function in the IDE instead of object.method. The resulting code looks the same. Most of my functions now return stucts rather than single values. The effect on my code has been dramatic, with code readability going up and the number of lines being greatly reduced. I do not know why procedural programming that makes extensive use of structs is not more common. Why not just use OOP? Some of the languages that I use are only procedural (PureBasic) and the use of structs allows some of the benefits of OOP to be experienced. Others languages allow a choice of procedural or OOP (VBA and Python). I currently find it easier to use procedural programming and in my discipline (ecology) I find it very hard to define objects. When I can't figure out how to group data and functions together into objects in a philosophically coherent collection then I don't have a basis for creating classes/objects. With structs and functions, there is no need for defining a hierarchy of classes. I am free to shuffle functions between modules which helps me to improve the organisation of my code as I go. Perhaps this is a precursor to going OO.
Code written with structs has higher performance than OOP based code. OOP code has encapsulation, inheritance and polymorphism, however I think that struct/function based procedural code often shares these characteristics. A function returns a value only to its caller and only within scope, thereby achieving encapsulation. Likewise a function can be polymorphic. For example, I can write a function that calculates the time difference between two places with two internal algorithms, one that considers the international date line and one that does not. Inheritance usually refers to methods inheriting from a base class. There is inheritance of sorts with functions that call other functions and use structs for data transfer. A simple example is passing up an error message through a stack of nested functions. As the error message is passed up, it can be added to by the calling functions. The result is a stack trace with a very descriptive error message. In this case a message inherited through several levels. I don't know how to describe this bottom up inheritance, (event driven programming?) but it is a feature of using functions that return structs that is absent from procedural programming using simple return values. At this point in time I have not encountered any situations where OOP would be more productive than functions and structs. The surprising thing for me is that very little of the code available on the internet is written this way. It makes me wonder if there is any reason for this?
Arrays are ordered collection of items that (usually) are of the same types. Items can be accessed by index. Classic arrays allow integer indices only, however modern languages often provide so called associative arrays (dictionaries, hashes etc.) that allow use e.g. strings as indices.
Structure is a collection of named values (fields) which may be of 'different types' (e.g. field a stores integer values, field b - string values etc.). They (a) group together logically connected values and (b) simplify code change by hiding details (e.g. changing structure layout don't affect signature of function working with this structure). The latter is called 'encapsulation'.
Theroretically, object is an instance of structure that demonstrates some behavior in response to messages being sent (i.e., in most languages, having some methods). Thus, the very usefullness of object is in this behavior, not its fields.
Different objects can demonstrate different behavior in response to the same messages (the same methods being called), which is called 'polymorphism'.
In many (but not all) languages objects belong to some classes and classes can form hierarchies (which is called 'inheritance').
Since object methods can work with its fields directly, fields can be hidden from access by any code except for this methods (e.g. by marking them as private). Thus encapsulation level for objects can be higher than for structs.
Note that different languages add different semantics to this terms.
E.g.:
in CLR languages (C#, VB.NET etc) structs are allocated on stack/in registers and objects are created in heap.
in C++ structs have all fields public by default, and objects (instances of classes) have all fields private.
in some dynamic languages objects are just associative arrays which store values and methods.
I also think it's worth mentioning that the concept of a struct is very similar to an "object" in Javascript, which is defined very differently than objects in other languages. They are both referenced like "foo.bar" and the data is structured similarly.
As I see it an object at the basic level is a number of variables and a number of methods that manipulate those variables, while a struct on the other hand is only a number of variables.
I use an object when you want to include methods, I use a struct when I just want a collection of variables to pass around.
An array and a struct is kind of similar in principle, they're both a number of variables. Howoever it's more readable to write myStruct.myVar than myArray[4]. You could use an enum to specify the array indexes to get myArray[indexOfMyVar] and basically get the same functionality as a struct.
Of course you can use constants or something else instead of variables, I'm just trying to show the basic principles.
This answer may need the attention of a more experienced programmer but one of the differences between structs and objects is that structs have no capability for reflection whereas objects may. Reflection is the ability of an object to report the properties and methods that it has. This is how 'object explorer' can find and list new methods and properties created in user defined classes. In other words, reflection can be used to work out the interface of an object. With a structure, there is no way that I know of to iterate through the elements of the structure to find out what they are called, what type they are and what their values are.
If one is using structs as a replacement for objects, then one can use functions to provide the equivalent of methods. At least in my code, structs are often used for returning data from user defined functions in modules which contain the business logic. Structs and functions are as easy to use as objects but functions lack support for XML comments. This means that I constantly have to look at the comment block at the top of the function to see just what the function does. Often I have to read the function source code to see how edge cases are handled. When functions call other functions, I often have to chase something several levels deep and it becomes hard to figure things out. This leads to another benefit of OOP vs structs and functions. OOP has XML comments which show up as tool tips in the IDE (in most but not all OOP languages) and in OOP there are also defined interfaces and often an object diagram (if you choose to make them). It is becoming clear to me that the defining advantage of OOP is the capability of documenting the what code does what and how it relates to other code - the interface.
I have created an array in the implementation of my class loginController. Now I want to use this array (with its objects) in another class of my project. What is the right way to import it?
You really need to specify the language.
In general, if the array is a member variable in one class, it's considered bad form to directly use it from another class. This violates the "encapsulation" idea that is quite the thing in object-oriented programming.
The preferred thing to do is often to add methods, called "getters" and "setters", to the class owning the array, or make it available by some other more structural means, which depend on the exact semantics and usage of the array. It might, for instance, not be required that outside users even know that it is an array.
There is no right way given this information. What is located in the array, only integers or strings/objects etc. Do you store objects of pointers to objects?
Passing the array is the sameway as passing any other object to a function
The general answer would be: declare it as public
It is not very good thing to do but as a beginner, you can start with that.