What's the difference between an object and a struct in OOP?

Solution 1:

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.

Solution 2:

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.

Solution 3:

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)

Solution 4:

Short answer: Structs are value types. Classes(Objects) are reference types.

Solution 5:

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)