When exactly do I use a struct (Dont tell me when I want things to be allocated on a stack)

Question

A lot of times the answer is merely when I want things to be allocated on a stack instead of the heap.. assuming I dont know what the stack and the heap are (and please dont try to explain it here), when exactly should I be using structs instead of classes?

Here is the answer I've been giving out, but please tell me if I'm wrong or falling short of a better answer:

I create structs usually when I have enums that I want to add more data to. For instance, I might start with a simple enum:

public enum Colors { Blue, Green, Red }

Then if I need to store more aspects of this data I go to structs:

public struct Color
{
    string Name;
    int HexValue;
    string Description;
}

public class Colors
{
    public static Color Blue;
    public static Color Red;
    public static Color Green;
    static Colors()
    {
        Blue = new Color("Blue", 1234, "A light blue"
    }
}

The point is... similar to enums, I use structs just when I want to declare a bunch of types.

Answer 1

struct vs class in .NET

The real time to use a struct is when you want value type like properties. Value types behave very differently from reference types, and the difference can be shocking and bug inducing if you aren't aware. For example a struct is copied into (and out of) method calls.

The heap vs stack isn't a compelling argument to me. In a typical .NET app, how often do you care about where the object lives?

I very rarely use structs in .NET apps. The only place I've truly used them was in a game where I wanted value type like properties for objects like vectors and such.

struct vs class in C++

This is a much simpler question to answer. structs and classes in C++ are identical to each other with only one minor difference. Everything in a C++ struct is public by default, where as everything in a C++ class is private by default. That is the only difference.

Answer 2

Simplistically, structs are for data, classes are for data and the manipulations of those data.

Answer 3

well in C++ land, you can allocate a struct (or class) on the stack or the heap... I generally use a struct when the default public access to everything is useful and class when I want to encapsulate... I also use structs for functors that need state.

Answer 4

In C++, technically it doesn't matter. You could use struct for polymorphic object and class for PODs. The language wouldn't care, though your coworkers may plot a bloody revenge. Aside from default access, there's no difference between class and struct.

Ultimately, the most important consideration is that you pick a coding style, and apply it consitantly. Maybe that means everything is classes, or maybe PODs are structs. You need to decide for yourself, taking in to consideration any coding practices applied by whomever you work for.

As for myself, I only use structs if the object has only public members and no virtuals. They might have data only or data and methods. Typically I use structs for buckets of data that may or may not have simple operations associated with them, usually to convert from one type to another. Hence they may also have constructors.

Answer 5

You have this labelled with C#, C++ and "language-agnostic". The fact is though, the difference between structs and classes in C# and C++ are completely different, so this is not a language-agnostic question.

In C++ class is syntactic sugar for struct with different default visibility. The reason is that C had a struct and only had "public" visibility (indeed, it's not even a fully meaningful statement in C, there's no such thing as OO-style information hiding in C).

C++ wanted to be compatible with C so it had to keep struct default to everything visible. C++ also wanted to follow good OO rules, in which things are private by default, so it introduced class to do exactly the same, but with different default visibility.

Generally you use struct when you are closer to C-style use; no or relatively simple member functions (methods), simple construction, ability to change all fields from the outside ("Plain Old Data"). class is generally used for anything else, and hence more common.

---

In C# a struct has value semantics while a class has reference semantics (in C++ both classes and structs have value semantics, but you can also use types that access them with reference semantics). A struct, as a value-type is self-contained (the variable contains the actual value(s) directly) while a class, as a reference type refers to another value.

Some other differences are entailed by this. The fact that we can alias reference types directly (which has both good and bad effects) comes from this. So too do differences in what equality means:

A value type has a concept of equality based on the value contained, which can optionally be redefined (there are logical restrictions on how this redefinition can happen*). A reference type has a concept of identity that is meaningless with value types (as they cannot be directly aliased, so two such values cannot be identical) that can not be redefined, which is also gives the default for its concept of equality. By default, == deals with this value-based equality when it comes to value types†, but with identity when it comes to reference types. Also, even when a reference type is given a value-based concept of equality, and has it used for == it never loses the ability to be compared to another reference for identity.

Another difference entailed by this is that reference types can be null - a value that refers to another value allows for a value that doesn't refer to any value, which is what a null reference is.

Also, some of the advantages of keeping value-types small relate to this, since being based on value, they are copied by value when passed to functions.

Some other differences are implied but not entailed by this. That it's often a good idea to make value types immutable is implied but not entailed by the core difference because while there are advantages to be found without considering implementation matters, there are also advantages in doing so with reference types (indeed some relating to safety with aliases apply more immediately to reference types) and reasons why one may break this guideline - so it's not a hard and fast rule (with nested value types the risks involved are so heavily reduced that I would have few qualms in making a nested value type mutable, even though my style leans heavily to making even reference types immutable when at all practical).

Some further differences between value types and reference types are arguably implementation details. That a value type in a local variable has the value stored on the stack has been argued as an implementation detail; probably a pretty obvious one if your implementation has a stack, and certainly an important one in some cases, but not core to the definition. It's also often overstated (for a start, a reference type in a local variable also has the reference itself in the stack, for another there are plenty of times when a value type value is stored in the heap).

Some further advantages in value types being small relate to this.

Therefore in C# a struct when you are solely concerned with value-semantics and will not want to alias (string is an example of a case where value-semantics are very important, but you would want to alias, so it is a class to make it a reference-type). It's also a very good idea for such types to be immutable and an extremely good idea for such types to have fields that total to less than 16bytes - for a larger struct or a struct that needs to be mutable it may well be wise to use a class instead, even if the value-semantics make struct your first choice.

Answer 6

When to use struct...

• When you want object to behave as a value type
• When the required size of object is <=16 bytes roughly.

Answer 7

A struct is actually exactly the same thing as a class - with one difference: in a class, everything is private by default while in a struct, everything is public by default!

struct Color
{
    string Name;
private:
    int HexValue;
};

would be the same as

class Color
{
    int HexValue;
public:
    string Name;
};

Answer 8

I would say , use stack when your data is smaller in size and you don't want a few thoushands of this object because it can hurt ou back a lot because as already mentioned that value types are copied by nature so pasing few thoushands of objects which is copied by value is not a good idea also.

Second point , i would like to include is when you only want data for most of the time and data is numeric most of the time , you can use stack.